OpenSprinkler Firmware Update Program 2.0 (with Video Tutorial)

We’ve just released a new OpenSprinkler Firmware Update program, with a video tutorial to walk you through the steps of how to upgrade your firmware. Hopefully this will make it easy for users to transition to the upcoming Firmware 2.1.0, which has a number of significant new features and improvements.

The new update program is written in Qt, and does not rely on Java any more. It’s cross-platform just like before. It also supports downloading the latest firmwares from the OpenSprinkler Github repository, and auto-detect of your OpenSprinkler hardware version. If you are a Windows user (especially Windows 8 and 8.1), you will still have to go through the hassle of installing driver. The video tutorial shows you a step-by-step guide of how to install driver.

For those who are interested in modifying the OpenSprinkler firmware code, I am experimenting with, which is a cloud-based Arduino platform. It’s really convenient in that it’s essentially a web-based Arduino IDE that runs in a browser; it also make it easy for people to share their code and modifications. I think its convenience will likely lower the barrier of programming, and motivate more users to modify OpenSprinkler firmware code to add custom functionality. I’ve made requests to add OpenSprinkler to their list of supported boards. Hopefully I will hear back from them soon!

Understanding 24VAC Sprinkler Valves

Note: check out our OpenSprinkler DC-powered version, which uses an innovative circuit design that drives sprinkler solenoids using DC-only voltage.

I often get questions about sprinkler valves, so in this post I will explain the basic electric properties of sprinkler valves. If you are designing a sprinkler controller circuit, understanding these properties can be helpful. It’s a common mistake to assume sprinkler valves work with DC voltage. While most valves indeed CAN be powered by DC voltage (see below), they are designed to work with AC voltage in the range of 22VAC to 28VAC. That’s why if you look at a standard sprinkler transformer, the output is usually AC.

The electric part of a sprinkler valve is the solenoid — it’s a cylindrical-shaped thing screwed into the valve. At the center of the solenoid is a rod supported by a spring. The solenoid has two wires connected to its internal coil. Applying 24VAC on the two wires energizes the coil, and causes the rod to contract into the solenoid. This releases the internal water pressure thus opening the valve, allowing water to flow through the valve. Removing the voltage causes the rod to revert back to its original position. This allows the water pressure to build up internally hence stopping the flow. Because closing the valve relies on internal pressure build-up, it usually takes a few seconds to completely stop the water flow. This also means if your water pressure is too low you may not be able to completely stop the water flow.


Let’s start by measuring the resistance of the sprinkler solenoid. I have two example solenoids, one made by Orbit and one made by Hunter. According to the multimeter, one measures 32.3 ohm, and the other measures 24.1 ohm. So the resistances are pretty low. If you think about it for a while, you might realize something is not quite right here: if we apply 24V on the solenoid, wouldn’t that produce a 24 V / 24.1 ohm = 1 amp current draw? That’s quite steep. In fact, my sprinkler transformer is only rated 750 mA output current, so it can’t provide enough current to drive even one solenoid?!

The catch is exactly in the fact that sprinkler solenoids are powered by AC voltage. Because the solenoid is made of a coil, it not only has coil resistance but also inductance. When operated on AC power, the inductance produces significant reactance which cannot be ignored. You can read the Wikipage to find out how reactance is calculated, but basically it has to do with the frequency of the input voltage, and the inductance of the coil. Because inductors ‘prevent’ current from changing rapidly, it behaves like a ‘resistor’ under changing current (i.e. AC). The higher the frequency, the higher the ‘resistance’ (i.e. reactance).

With an LCR meter, I measured the inductance of the two solenoids:
One reads 63.57 mH, the other 132.45 mH. So if we power the solenoids by 24VAC, 60Hz, which is the standard output of a sprinkler transformer, we will get a reactance of:

This, plus the resistance, gives a total impedance of:

Note that the reactance counts into the imaginary part of the impedance. Using complex numbers is just a convenient way of denoting not only the magnitude but also the phase. For example, when you apply a sinusoid voltage on the inductor, the corresponding current is also a sinusoid wave, but with a different phase. Using complex numbers, the calculation can be carried out quite easily.

Now we can calculate the operating current under 24VAC (rms). We only care about the magnitude, so the current (rms) would be:

OK, so this is getting closer to the reality. But 0.6 amp current still sounds high. What is missing? Well, remember that when the solenoid is activated, the rod will be attracted into the solenoid, and that can change the inductance significantly. So let’s re-measure the inductance with the rod pushed in:


Indeed the inductance jumped from 63.57 mH and 132.45 mH previously to 194.4 mH and 199.6 mH respectively. OK, now if we redo the calculations, we will find out that the correct reactance is:

and current (rms) os:

The resulting current is about the same on the Hunter solenoid. So this roughly matches the electric specification of a typical sprinkler valve. It’s actually still a bit off: if we measure the actual AC current flowing through the solenoid:

The readings are about 0.2 amp. I suspect the difference comes from the measurement of the inductance. On my LCR meter, which can measure inductance at two frequency levels: 120 Hz and 1 kHz, I find that the inductance is measured differently under the two frequencies. Because the actual operating frequency of the solenoid is 60 Hz, and my meter cannot measure 60 Hz, the inductance I am getting is probably somewhat off. That should explain the difference between the calculation and the actual current reading.

The 0.6 amp current we calculated above probably explains the inrush current: when the solenoid is just energized, there is an impulse current that’s typically higher than the holding current. This is because the rod is still out, and hence the reactance is lower, causing a higher current than when the rod is attracted in.

Operating Sprinkler Valves Under DC

Note: check out our OpenSprinkler DC-powered version, which uses an innovative circuit design that drives sprinkler solenoids using DC-only voltage.

From the calculations above, it’s obvious that the coil inductance is important at limiting the operating current when the valve is powered under AC. What about if we power the valve under DC? Obviously we shouldn’t use 24VDC, because that would draw too much current (0.75 to 1 amp). If you search online, you will find plenty of posts talking about powering sprinkler valves using 12VDC. This actually works well in general. Using 12VDC has advantages in that 12VDC power adapters are cheaper and much easier to find; the circuit design is simpler, and you can use the same circuit to interface with other DC devices like relays and motors. In contrast, 24VAC power circuits are more complex and you can’t use the same circuit to directly interface with DC devices.

However, you should be aware that because the sprinkler solenoid’s resistance is pretty low, the operating current under 12VDC will be relatively high, around 400 to 500 mA. This more than doubles the 200 mA operating current (rms) under 24VAC. Also, the coil will heat up more, and this potentially shortens its life. For example, under 12VDC, the Orbit valve above will dissipate 12 * 12 / 32.3 = 4.5 Watt; whereas under 24VAC, the same valve only dissipates 0.2 * 0.2 * 32.3 = 1.3 Watt (note that only the resistive portion dissipate power, inductive portion does’t).

Again, the issue is that under DC there is no reactance, so the coil’s inductance plays no effect at limiting the current. What if we reduce the voltage further to 9VDC, in order to reduce the operating current? After all, the solenoid only needs 200mA holding current to remain activated. Unfortunately that won’t work: I’ve tried powering solenoids with 9V, and I can’t get the valve to reliably energize. The problem is that 9V is not sufficient to provide the required inrush current, so the rod cannot get fully attracted in. However, if the rod is already in, 9V is sufficient to hold the solenoid activated. So if you really want to make it work with 9V, you need a circuit that can provide a high impulse voltage; then once the solenoid is activated, you can lower the voltage to reduce the current (hence power) consumption. A possible solution is to use a boot converter (very much similar to circuits for latching solenoids) to provide an impulse high voltage, but this comes at the cost of increased circuit and software complexity.

Announcing OpenSprinkler Bee (OSBee) Arduino Shield v1.0

Update: check out our new standalone OpenSprinkler Bee (OSBee) 2.0 with built-in WiFi and OLED display.

Two months ago, I wrote a blog post about the preview of OpenSprinkler Bee, which is an open-source arduino-based controller for battery-operated sprinkler valves. While that’s still in the development stage, today I am glad to announce that an Arduino shield version of OpenSprinkler Bee is completed and immediately available for purchase at the Rayshobby Shop.

Update: check out our new standalone OpenSprinkler Bee (OSBee) 2.0 with built-in WiFi and OLED display.


So what is this Arduino shield version, and how is this different from other OpenSprinkler prodcuts that we carry? Well, an Arduino shield is a circuit board that you plug into an existing Arduino — it does not have a microcontroller chip itself, but contains additional circuitry that extends the basic functionality of an Arduino. So to use the shield, you will need to provide an existing Arduino board.

How is the OpenSprinkler Bee (OSBee) different from the other OpenSprinkler products? The main difference is that OSBee is designed to work with battery-operated sprinkler valves. These valves internally use a latching solenoid, which only draws power when you open or close the valve, and does not draw power if it remains in the same state. So it’s very efficient and suitable for battery-operated controllers. The other OpenSprinkler products, such as OpenSprinkler 2.1s, DIY 2.1u, OSPi 1.4, OSBo 1.0, are all designed for 24V AC sprinkler valves, which operate on 24V AC and require a power adapter / transformer.

While OSBee shield itself does not have built-in wireless modules, you can stack it with other Arduino shields, such as RF, WiFi, Ethernet shields, to provide web connectivity. The OSBee Arduino library has one example of using the Arduino Ethernet shield with OSBee shield to create a web interface for sprinkler control.

Is there any easy way to tell latching solenoid valves from 24V AC valves? Yes. Latching solenoid valves usually come with a special plug, and the two wires are usually colored differently because the solenoid has polarity. 24V AC valves usually come with just two wires colored in the same way (because AC voltage has no polarity). Here are some examples of latching solenoid valves. Note the special plugs and/or different wire colors.


For valves that come with stripped wires, simply attach them to the screw terminal blocks on OSBee shield. For valves with special plugs, you can cut and strip two pieces of wire (20 to 24 AWG): insert one end of the wire to the plug, and the other end to the screw terminal block.


How to open or close the latching solenoid valve? Electrically, latching solenoid valves have quite low coil resistance (a few ohms). To open the valve, you apply a momentary positive voltage on the coil. The specific voltage depends on the valve specification, but it typically varies between 9V to 22V. To close the valve, just reverse the voltage polarity. The important thing to keep in mind is that the voltage is applied as a pulse — usually 25 to 100 milliseconds. Because the coil resistance is so low, the instantaneous current is very high, up to a few amps. So you can’t apply the voltage continuously (or it will smoke the coil or the power supply!) In addition, it’s better to first build up the voltage into a capacitor, and then dump the charge to the valve from the capaictor.

How to generate such a high voltage from Arduino’s 5V or 3.3V pin? It’s by using a neat circuitry called ‘boost converter‘. The Wikipedia has plenty of information about how it works, but the basic principle is to use a MOSFET switch, an inductor, a diode, and a PWM signal to build up the charge into a capacitor. This way you can generate a high voltage from a low-voltage source such as AA batteries.

How to apply a voltage in both polarities? This is by using another neat circuitry called ‘H-Bridge‘. The H-Bridge is made of four MOSFET switches. By closing the pair of switches in each diagonal direction, you can apply voltage in either positive or negative polarity. Because you also need a state where no voltage is applied on the solenoid, that’s three states in total and hence two microcontroller pins are required to produce three states. Wait, why not directly use two microcontroller pins to apply the voltage? Well, microcontroller pins can neither handle high voltage nor provide high current, so you need MOSFETs to help switch high voltage and high current using only logic signals from the microcontroller.

With all the technical concepts explained, here is the diagram of the various components on the OSBee Sheild:

The shield can switch 4 independent valves / zones. The boosting voltage is software adjustable — anywhere from 9V to 24V. An Arduino library with three demo programs are provided in the OSBee Github repository. For details, please refer to:

OSBee Shield v1.0 is now available for purchase at the Rayshobby Shop. Thanks!

At Bay Area Maker Faire 2014

We are at the Bay Area Maker Faire at San Mateo Event Center. This year we got assigned to Station 5 (HomeGrown Village), which is a bit surprising because the past two years we’ve always been at Station 2 with the Arduino and Raspberry Pi gadgets. But this is probably an interesting change as we will be neighbors to other makers working on smart watering and home grown food. If you are at the faire, make sure to come by Station 5 and take a look at our gadgets. See you there!


The Story of OpenSprinkler: an Open-Source Web-Based Sprinkler Controller

Last week I wrote a short story on OpenSprinkler for the Make Magazine blog. It’s about how learning Arduino inspired me to invent the OpenSprinkler. Check out the blog post at the link below:

Thanks to everyone who helped and contributed to this project!

This year we will be having a booth at the Bay Area Maker Faire again. If you are going there too, make sure to drop by our booth and check out OpenSprinkler, SquareWear, AASaver, and the upcoming goodies. Thanks!

Announcing OpenSprinkler Zone Expansion Board v2.0 (16 stations)

A new day, a new post. Today’s post is about the new OpenSprinkler Zone Expansion Board v2.0, with injection-molded enclosure and 16-station capability. The injection-molded enclosure provides a coherent look with the OpenSprinkler main controller; and the 16-station capability adds twice as many stations as before, without taking extra space.


As I briefly talked about in a previous post, this new version 2.0 is only slightly larger than the previous version 1.1, but can handle twice as many stations. You can think of it as basically two 1.1 boards (8 stations) squeezed together onto a single board that’s roughly the same size as before. What made this possible is a clever design that makes use of the same four support pillars to both attach the top and bottom pieces together, as well as secure the PCB inside the enclosure. This saves some space normally reserved for the PCB screws. The design was proposed by Xavier at SeeedStudio — the principal industrial designer for the project — and it follows the same design adopted for the OpenSprinkler enclosure.

Also, version 2.0 has included per-station bidirectional transient voltage suppressor (TVS) for added electric protection. These are essentially diodes can clamp the voltage if it exceeds a threshold in either direction. This can reduce the chance that a high transient voltage or lightening damages the circuit or the sprinkler solenoid.


In terms of usage, version 2.0 is the same as 1.1. It works for all of OpenSprinkler, OSPi and OSBo. Use the 2×4 extension cable to link the OUT port of the main controller to the IN port of the expansion board. Additional expansion boards can be daisy-chained in the same way — by following the OUT -> IN links. Because 2.0 and 1.1 have the same cable wiring, they can be mixed and used together.

The firmware currently does not automatically detect the number of stations. So you need to keep in mind that each 2.0 counts as 2 expansion boards (2x 8) in the option settings.

Because of the difficulty in stocking both versions, we have discontinued 1.1 and are only offering 2.0 now. Price-wise, it is a little more expensive than 1.1. So if you only need 8 extra stations, sorry, you have to pay the extra to get 2.0. But if you need a larger number of extra stations, this is a great deal because it’s considerably cheaper and saves a lot of space too.

OpenSprinkler Zone Expansion Board v2.0 is available for purchase at the Rayshobby Shop. We offer bundled discount if you are buying it together with any OpenSprinkler main controller, including OpenSprinkler v2.1s, DIY v2.1u, OSPi and OSBo. Be sure to check the “Add Zone Expansion Board Options” when you purchase. Thank you!

Announcing the OpenSprinkler Mobile App — Native Version (all platforms)

I am very excited to announce that the OpenSprinkler native mobile app is now available on all platforms (iOS, Android, Windows Mobile). A big shout-out to Samer Albahra for his diligent and absolutely fantastic work in developing this app!

Different from the previous OpenSprinkler web app, the native app can be installed as a standard app on your mobile device. This not only enables some new features (such as automatic scanning device, multi-language support) right away, but opens doors for more exciting features in the future, such as push notification and local storage (e.g. representing each station with a photo icon). The app is free, so go ahead and give it a try right now!



Let me briely go through the requirements for using the app:

  • Hardware: to use the app you need to have either an OpenSprinkler (v2.0 and above), or OpenSprinkler Pi (OSPi, any version), or OpenSprinkler Beagle (OSBo, any version). Even if you don’t have the hardware, you can set up a software simulation (Dan’s interval_program) on RPi or BeagleBone to try out the firmware and app. OpenSprinkler hardware v1.x is currently not supported. Sorry!
  • Firmware: the app requires OpenSprinkler firmware 2.0.4. If you don’t have this version, please follow the firmware update instructions to upgrade your firmware.
  • App Installation: search for ‘opensprinkler’ in iOS App Store, Android Play Store, or Windows Phone Store, and you should be able to find and install the app.
    • If you like the app, please give a +1 to show your appreciation of Samer’s work!
  • Port Forwarding (optional): unlike the previous web app, you don’t need to set up port forwarding any more if you are accessing the controller within your home network. However, if you want to access the controller outside of your home network (say, when you are traveling), you still need to set up port forwarding on your router. This requires knowing your OpenSprinkler’s IP address and port number (default is 80). Please refer to your router’s user instructions. You can also use dynamic DNS service (such as, freeDNS etc.) to set up an easy-to-remember DNS name for your router. Most routers support dynamic DNS service as well.

NOTE to keep in mind: the app provides an alternative front end to the OpenSprinkler controller. The default front end, which is the controller’s webpage (accessed by typing in the controller’s IP in a browser), is still available and functioning. Any operation you apply through the app will also be reflected in the default front end.


The app has a number of notable new features:

  • Scan For Device: on first-use of the app, you need to input information about your OpenSprinkler device, normally the IP address and device password. Samer found a very clever way to quickly scan the local network and automatically find your OpenSprinkler device. This saves your work of having to find the IP address yourself.
  • Multi-Device Support: the app supports multiple device configurations. This is very useful if you have multiple OpenSprinklers and want to access each individual within the same app.
  • Multi-Language Support: this is another exciting new feature — the app provides language localization. There are currently six supported languages: English, Spanish, Hungarian, German, Italian, and Chinese. Big thank-you to Balazs for starting this feature and Samer for continued development. The translations are mostly provided by the users — for those who have contributed: thank you!


These are just some of the highlights I would like to bring to your attention. There are plenty of other features that really make the app stand out, such as automatic count-down timer, easy editing of station names and programs, intuitive program preview, instant launch of any existing program (not just the run-once program), import/export configurations. You’ve got to try it out yourself to find out.

To find out additional details, please go to Samer’s website. Discussions of ongoing development are available at the Rayshobby Forum. Samer is continuously supporting the app development. Please kindly leave your comments and suggestions, either in the comments section below, or on the forum. Thanks!

Funky Design of the New OpenSprinkler Pi (OSPi) v1.4

Curious what the next version of OpenSprinkler Pi (OSPi) is going to look like? Here is a sneakpeak preview of version 1.4:


Wait a minute, what is this strange looking board?! And the Pi is installed at an angle? What’s happening here? Well, the new design is all centered around one simple goal: to fit OSPi and RPi into the existing OpenSprinkler injection-molded enclosure. There are many good reasons to do so. The first is cost reduction: the current OSPi uses the Serpac WM032 enclosure with custom cutouts. This is quite expensive to make. On the other hand, the microcontroller-based OpenSprinkler already has an injection-molded enclosure (and I paid a good amount of money for the mold!), so it makes sense to consolidate the design to use the same enclosure. This will bring down the cost quite a bit. Using the same enclosure also gives both products a consistent look. In fact, in the near future OpenSprinkler Beagle will also adopt the same enclosure, and hence all three will have the same exterior look.

Design Story

The ‘why’ part is easy to explain. But the ‘how’ part proves to a non-trivial engineering challenge. When I was first fiddling with the idea I didn’t think it was at all possible. After all, the injection-molded enclosure was made before OSPi came into place, naturally it was not designed with RPi in mind. It turns out that, due to the positions of the support pillars, the enclosure is just a little bit too narrow to fit RPi at a straight angle. Ouch! But after staring at it for a while like a geometric puzzle, I was delighted to find out that if you rotate RPi by 3 to 4 degrees, it fits perfectly! This gave me inspiration to further develop the idea. It also explains why the Pi has to be installed at an angle, as the picture at the top shows.


In fact, by carefully positioning the Pi, the Ethernet cable can also fit, albeit through the USB cutout instead of the RJ45 cutout. It’s a bit tricky to figure out the position precisely, but I am glad that after two rounds of prototypes I finally got it right 🙂


Of course since OSPi doesn’t have buttons and LCD, some of the cutouts are useless. I am not completely sure what to do with the LCD cutout. If I leave it alone, it looks quite ugly (see picture on the left below). So my temporary solution is to just put a sticky label at the back, thus covering the big empty hole (see picture on the right below). Eventually I think it’s best to put some graphic design here. I am undecided what to put here. It shouldn’t be the OpenSprinkler logo as the enclosure already has a printed logo. If you have suggestions, please leave them in the comments section. I would greatly appreciate it!


Next, because the injection-molded enclosure is not high enough, I cannot continue using the current design where the Pi faces up and connects to OSPi through ribbon cables. It’s necessary to flip the Pi and plug it into OSPi facing down. This will reduce the height of the overall assembly. However, there is one additional complication: the USB, Ethernet, and composite video connectors on the Pi are all quite tall, so it’s necessary to make cutouts on the OSPi PCB to allow these connectors to sink below the board. The picture on the left below shows the back of the PCB, and you can see where the connectors go through the board.


As the Pi is now facing down, it’s not easy to access the GPIO pins directly. Therefore I’ve mapped out all the 2×13 pins on the Pi to the pinout area, seen at the top-left corner of the PCB.

The last bit of the puzzle is the SD card. Since the enclosure has very limited space, it’s not possible to fit a full-size SD card, without making a cutout on the side. So we need a low-profile SD card. There are several options, one is this microSD to SD adapter. It’s basically a microSD card slot soldered onto a PCB shaped like an SD card but half the size. This and other similar adapters are readily available online and they work reasonably well.


While Googling ‘low-profile SD card’, I found an surprising solution that’s dead simple. In case you don’t know this already: many full-size SD cards are actually half empty. What this means is that the useful stuff (namely the chip and connectors) only take half the space, while the other half has nothing in it! As a result, you can safely cut half of the card away, thus making a low-profile SD card without any adapter. This also has the advantage of preserving the high performance of a full-size SD card (while the microSD card is considerably slower). The downside though, is that if you ever need to insert the SD card to your computer’s SD card slot, you will have to tape the other half back, so that it extends to the original length. Otherwise it will be too short to push in and pop out.


Anyways, this is the end of the story. I admit the new funky design feels a bit forced: it’s born out of the need to re-use the injection-molded enclosure, which wasn’t designed with OSPi in mind. But I am quite excited that I figured out all the pieces of the puzzle. Eventually I may pay a sizable chunk of money to order a new mold dedicated to OSPi, but before that happens we have to stick with the existing resources 🙂

The hardware components remain largely the same with version 1.3. The only difference is that the relay has been upgraded to a slightly bigger, 250V / 3A type. I am also considering adding a pin header for the nRF24L01 transceiver, to make it possible to communicate with our upcoming products such as OpenSprinkler Bee. Pending a few minor tweaks, things should be finalized within a month or two. Please leave your comments and suggestions, as this will be the last chance to influence the final design of 1.4. Thanks!

Preview of OpenSprinkler Bee (OSBee) First Prototype

Update: check out our new standalone OpenSprinkler Bee (OSBee) 2.0 with built-in WiFi and OLED display.

It has been a month since my last post. Apologies for being silent for a month. In the meantime a lot of new exciting projects are happening. Today I am going to show you the first prototype of OpenSprinkler Bee (OSBee) — a programmable sprinkler timer for battery-operated valves.


Like the classic OpenSprinkler, OSBee is an open-source Arduino-based sprinkler timer. The main difference is that OSBee is designed to work with battery-operated sprinkler valves. It’s powered by two AA batteries. In contrast, the classic OpenSprinkler is powered by 24V AC wall adapters. Also, OSBee does not have built-in Ethernet interface, but it has a 2.4G RF transceiver (nRF24L01) and I am considering leaving spaces to add additional modules such as XBee and Bluetooth. This way OSBee can communicate with a base station or a smart phone, for creating web or smartphone-based user interface.

The name ‘Bee’ comes from the abbreviation Battery-Enabled Extension. It’s co-incidence that it sounded more like XBee. It may very well have an XBee slot in the end, but the name was not intended to imply XBee. Also, ‘Bee’ reminds me of the garden, and gardens need to be watered, so, there is the connection. Different from the classic OpenSprinkler, OSBee now comes with a water-proof enclosure, which means it can be left outdoors, such as in a garden, and will do its work diligently, like a Bee 🙂


The history of OpenSprinkler Bee traces all the way back to my very first Arduino-based sprinkler timer project — the Minty Water Valve Controller. It is a wireless sprinkler timer that is small enough to fit inside a mint tin. That was summer 2010 and I was shopping for a sprinkler controller for my new installed lawn. I was starting to learn about Arduino and had this great inspiration to design and make my own sprinkler controller using an Arduino 🙂 There I made it, and I posted a video about it (see below).

It was soon spotted by Chris Anderson, currently the CEO of 3D Robotics. We then started collaborating on the OpenSprinkler project. After some market research, we figured out that most common household sprinkler systems (particularly underground sprinkler systems) use the 24V AC sprinkler valves. That’s why instead of focusing on battery-operated valves, we started designing the classic OpenSprinkler for 24V AC valves. But battery-operated sprinkler valves are also very useful — they are power-efficient, most of them can be hooked up with garden hoses so they are very mobile — you can move them anywhere you have watering need. So far forward four years to now, after releasing the classic OpenSprinkler and its variants OSPi and OSBo, I’ve finally decided to finish up what I started a long time ago, that is, the OSBee!

Technical Details

For the technical minded, here are some details about the prototype design:

  • ATmega328 microcontroller, classic Arduino stuff 🙂
  • Powered by two AA batteries, which are boosted to 3.3V VCC using Microchip’s MCP1640 (same design with first-generation AASaver). MCP1640 has a bypass mode, allowing it to turn off boosting when microcontroller is sleeping. This way it can maximally save power.
  • nRF24L01 2.4G RF transceiver. This is one of the most popular and lowest-cost RF transceivers. This chip has excellent operating voltage range (1.9V to 3.6V) and reasonable transmission range (up to a couple hundred meters?). It also has a version with amplification which can extend the transmission range to a kilometer!
  • SMT buzzer, push-button, 16KB EEPROM (borrowed from SquareWear 2,0 and Mini design)
  • PWM-driven boost converter with software defined boost voltage, and an H-bridge to drive sprinkler solenoid. This allows OSBee to interface with many types of valves at different operating voltages, such as 9V, 12V, 24V.
  • Soil moisture sensor using a capacitive soil moisture probe and a Schmidt trigger oscillator to measure capacitance. This will provide OSBee with the capability of soil moisture based control.
  • Serpac 121 enclosure with water-proof perimeter seal.

One tricky part with preserving the water-proof design is how to make the connection from the controller to the sprinkler valve. To lower the cost, I have to stick with off-the-shelf enclosures, which means minimal customization. The current solution I figured is to simply drill two small holes and insert two wires through the holes. Then you can seal the holes with some hot glue or water-proof spray. Not the most elegant solution, but simple.

Also, the choice of AA batteries (as opposed to rechargeable Lithium batteries) is due to the consideration that the controller may be left outdoor during hot summer days. In this case the temperature can potentially rise up to 60 degrees Celsius, which is very unhealthy to Lithium batteries.


So there you go, the first prototype of OpenSprinkler Bee. I am in the process of finalizing the design. Comments and suggestions are much appreciated!

Announcing OpenSprinkler Hardware 2.1s (assembled), Firmware 2.0.4, and a GUI-based Firmware Upgrade Tool

This is an official announcement of several recent OpenSprinkler updates: the fully assembled OpenSprinkler 2.1s, a new firmware revision 2.0.4, and a GUI-based firmware update tool.

First, the hardware:


So what’s new in OpenSprinkler 2.1s? Here are the main updates:

  • ATmega644 mcu running at 12MHz with USBasp bootloader.
  • Added per-station transient voltage suppressor (TVS).
  • Added TVS and 2A fuse on the 24V AC line.
  • 24V AC terminal block is changed to orange-colored with 3.96mm spacing.
  • Added on-board 120V/2A mini-relay.

These pretty much follow the same updates we made in the DIY kit 2.1u. With these updates, it brings the fully assembled 2.1s to the same page, in terms of hardware features, with all other OpenSprinkler variants (e.g. OSPi, OSBo). OpenSprinkler v2.1s is now available for purchase at the Rayshobby Shop.

Next, the firmware 2.0.4:

This is a minor revision, but with important changes in preparation for Samer Albahra’s awesome new OpenSprinkler Mobile App. This new app is snappier and even more polished than the current version. It will have native apps on all mobile platforms. and it will also have cool new features such as language localization (yay!) and automatic discovery of OpenSprinkler devices. I am very grateful for Samer’s help and continued contributions to this project, and I’d also like to thank Balazs for contributing to the weather and the language localization features.

So technically what have changed in this firmware? Since I have never blogged about firmware 2.0.2 and 2.0.3, I ‘ve included the accumulated changes below:

  1. Added support for JSON and fixed several bugs.
  2. Added support to change Javascript URL.
  3. Stores controller operation enable bit, manual mode bit, rain delay time in EEPROM.
  4. More options are made editable through the web interface.
  5. Added support to change time manually (enabled when NTP sync is turned off).
  6. Added LCD auto-dimming feature (after 30 seconds of inactivity, the LCD brightness will lower down to the LCD Dimming value set in options).

About the first bullet, let me explain with more details. Support for JSON output has been added since firmware 2.0.2. This is very useful for integrating OpenSprinkler with mobile apps, external control programs, and jQuery and AJAX in the future. The specific JSON outputs are:

  • http://x.x.x.x/jo: returns options.
  • http://x.x.x.x/jc: returns controller variables.
  • http://x.x.x.x/jp: returns program data.
  • http://x.x.x.x/jn: returns station names and data.
  • http://x.x.x.x/js: returns station status bits.

where x.x.x.x is the OpenSprinkler’s IP address. Right now the JSON code co-exists with code that spits out HTML pages, so there is a considerable amount of redundancy. This will soon change and the code will be significantly simplified in the near future.

About the second bullet, the firmware now supports Javascript files stored anywhere, including on a microSD card inserted in the controller itself. These Javascript files are required to render the controller webpages. They are usually too big to store in the microcontroller internally, but can be stored on an external path. The default path is To change it, use http://x.x.x.x/su, where x.x.x.x is your OpenSprinkler’s IP address. To host the Javascript locally on the microSD card, copy the necessary Javascript files to a microSD card (2GB or less, formatted to FAT), and insert to OpenSprinkler. Then change the Javascript path to . (i.e. a dot), indicating the files are served locally. With these changes, the controller can be truly independent, without the need to reference the Internet. So you can access the controller even if your local network is not connected to the Internet.

Firmware 2.0.4 is compatible with OpenSprinkler 2nd generation (including 2.0s and 2.1s/u). The source code is available for download at the OpenSprinkler Github repository. Please note that 2.0 and 2.1 use different microcontroller frequencies (8MHz and 12MHz respectively), therefore the compiled firmware files are different for them. If you’d like to upgrade to this firmware, please read on.

Finally, the GUI-based Firmware Update Tool:

Update: the information below is outdated. Please check the new OpenSprinkler Firmware Updater 2.0.

As OpenSprinkler has gone through many different hardware and firmware revisions, updating firmware has become more and more confusing. That’s why I’ve decided to make a GUI-based firmware update tool. This doesn’t completely get rid of the technical challenges, but it certainly makes the process less confusing 🙂

The updater is written in Processing, so it’s naturally cross-platform. The usage is pretty simple: you select your OpenSprinkler hardware version, and the updater provides a list of available firmwares. You select which firmware you want (defaults to the latest version), and click on Upload. That’s it. The GUI provides some basic descriptions of the hardware (in case you forget which hardware you have), and instructions to enter bootloading mode.


The firmware information is all stored in a file named os_firmware_info.txt. The avrdude program is packaged together with the tool so you do not need to install it separately (except in Linux, where you can easily install avrdude by sudo apt-get install avrdude).

There is one remaining piece, though — if you are a Windows user, you need to install USBtinyISP driver (for OpenSprinkler 1st-gen and OpenSprinkler 2.0) or USBasp driver (for OpenSprinkler 2.1u/s). Please refer to the Firmware Update instructions for additional details. This is probably the biggest technical hurdle and we are trying to find a more user-friendly solution. Basically, if you are using Windows XP, Vista, or 7 users, it’s not too bad; but if you are using Windows 8, it gets really messy — you need to disable driver enforcement before you can install the drivers. If you are lost, try to google ‘Windows 8 USBtinyISP’, or ‘Windows 8 USBasp’ and you should be able to find successful solutions.

If for the life of yours you simply cannot get the driver installed, you are resort to the Rayshobby pre-configured VirtualBox image — is creates a Virtual Linux in your host Windows OS, and passes the USB programmer directly to the Virtual OS, thus bypassing the Windows driver issue.

The Firmware Updater Tool can be downloaded following the links below:

Please run the ‘osFirmwareUpdater’ program in the folder that corresponds to your operating system.

Note: if you see messages such as avrdude: warning: cannot set sck period. Please check for usbasp firmware update., and avrdude: error: usbasp_transmit: usbasp_control_msg: sending control message failed, these are normal and can be safely ignored. These warnings / errors have to do with the behavior of the bootloader upon reset. They do NOT mean the firmware upload has failed.

Mac Users: if you encounter an error Application is damaged and can’t be opened, you need to temporarily change a Security and Privacy setting in System Preferences. See the details in this work-around.

This is a first step towards making the update process more user friendly. Comments and suggestions are welcome!