Using 24V AC Relays with OpenSprinkler

Back in early December last year, we introduced a new feature in OpenSprinkler Firmware 2.1.1 that allows OpenSprinkler to directly talk to remote power sockets. With this feature, you can use OpenSprinkler to not only switch sprinkler valves, but also switch powerline devices such as light, pump, heater, fan. While it’s a powerful feature, it is after all a wireless solution so it’s not the most reliable — sending wireless signals to remote power sockets is prone to interference and is limited by distance and barriers (e.g. walls, floors) in between.

If you still prefer using relays, whether because it’s more reliable, or because it’s the classic way, there are plenty of choices. I’ve often received questions about how to use OpenSprinkler with a relay. The easiest solution is to get a 24V AC relay. Then you can wire it up as if it’s a sprinkler valve — when OpenSprinkler turns on the corresponding station, the relay is activated, and that turns on the device connected to the relay. Because AC relays are not as common as DC relays, they tend to be more expensive. Here I list three choices I’ve found:

1. Omron G7L-2A-TUB-J-CB-AC24: this is available on Amazon. This is actually commonly used in sprinkler pump start relays — if you have a pump start relay, you can open it up and check if it has a 24V AC relay in side. This particular one has a contact rating of 25A @ 220VAC, which is sufficient for most applications. You can also find similar ones with different specifications: if you search for ‘Omron G7L-‘ you will find many choices, look for the ones ending with AC24, which are the 24V AC versions.

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I am quite curious how such 24V AC relays work. It probably is constructed somewhat differently from a DC relay. So I ventured to open it up. As you can see in the picture on the right above, it has a big coil, a contact piece connected via a spring. When voltage is applied on the coil, the contact piece gets attracted and therefore connecting two contact pins together. That’s how a basic relay works.

Something strange I noticed is that when I try to measure the resistance on the coil, it gives me a very large value — several mega-ohms. The coil resistance can’t be that large. There is probably additional circuitry inside the relay. I then measured the forward voltage drop on the coil, at either polarity it gives about 1.25V drop, which strongly indicates there is a bridge (i.e. full-wave) rectifier inside. This makes sense, because a bridge rectifier is a standard way to convert AC to DC. So the additional ‘construction’ inside the AC relay (compared to DC) is probably the rectifier.

To verify it, I had to open it up even further, which involves cutting some pieces of the plastic. Eventually I was able to open it up completely. Check the bottom section of the relay:



Yup, there is a bridge rectifier and a MOV. Previously I was measuring the resistance through the bridge rectifier, which explained why the value I got was incorrect. Now I can measure the real resistance of the coil, which turns out to be about 294 ohm. That’s about right — under 24V AC, it will draw an average of about 80 mA AC current, which matches the specification.

So in case you are looking for a 24V AC relay to work with OpenSprinkler in order to switch high-voltage devices, this is an inexpensive (

2. Schrack RT314524: This is a very small 24V AC relay that you can buy from Mouser or Digikey. It has a contact rating of 16A @ 250VAC, which is also plenty for common applications.



3. Other choices: There are some other choices which were brought up on the forum, including a open-style panel mount 24V AC relay, and even solid-state relays (SSRs).

In any case, if you are looking for a relay to work with OpenSprinkler, the above are the ones worth considering.

OpenSprinkler Firmware 2.1.1 New Feature: Control Remote Power Sockets

In the past I’ve written several blog posts about how to use Arduino to interface with remote power sockets. For home automation involving powerline devices (e.g. lights, heaters, pumps, fans), this is my favorite solution, because it’s low-cost (remote power sockets are widely available at cheap price) and convenient (no messing around with relays and powerline wires). Also, one Arduino plus transmitter can simultaneously talk to many power sockets, making this a scalable solution too.

With the just released OpenSprinkler firmware 2.1.1, support for interfacing with remote power sockets has finally arrived. So you can now use OpenSprinkler not only to control sprinkler valves, but also powerline devices. Trying to find a programmable way to control your Christmas lights? Look no further! With OpenSprinkler’s easy-to-use web interface and flexible programming capability, you can enable automated control of lights, heaters, pumps, fans — anything that can be plugged into wall outlets.

Here is a quick video tour on how to get started:

Below are detailed instructions.

Required Parts:

How does this work?
Let me briefly explain how the whole thing works. First, common remote power sockets operate in the 433MHz radio frequency band. When you press a button on the remote, it sends out a signal to the power socket, which gets decoded and acted upon. If we can sniff the signal, we can use a microcontroller plus a 433MHz transmitter to replicate the signal, thus be able to directly control the power socket in software. The RFToy is a gadget that I’ve designed to easily decode signals from common remote power sockets. Once we have the code, we can use OpenSprinkler to simulate the code, thus be able to control remote devices.

Heads-up: the following steps require a small amount of soldering. The estimate time for modification is 15 to 20 minutes.

Step 1: Decode Remote Power Sockets
Take out the RFToy, plug in a 433MHz receiver (making sure the VCC and GND pins on the receiver match the +5V and GND pins on the RFToy). Follow the on-screen instructions to record the on/off signal of a power socket. Once decoded, the signal will be converted to a 16-character hexademical code.

To test if the code works, take out the 433MHz transmitter, and solder a 17cm (6.7inch) long wire antenna to the ANT pin. Then plug it into the RFToy (making sure the DATA and GND pins on the transmitter match the DATA and GND pins on the RFToy). Bend the pins as necessary. Now click button S3 or S1 on the RFToy, the power socket should be toggle on or off just like when you press the buttons on the remote. Keep in mind that although most remote power sockets work in the 433MHz band, there are some that work in the 315MHz band. In that case, just use a 315MHz transmitter-receiver pair.

Step 2: Install RF Transmitter to OpenSprinkler
Remove the OpenSprinkler enclosure, and locate the RF transmitter pinouts (marked A3 VIN GND). The pinouts are located either close to the top of the PCB, or next to the Ethernet jack. Plug in the transmitter to the pinouts, making sure the DATA-VCC-GND pins on the transmitter match the A3-VIN-GND pins on the circuit board. Then solder the three pins at the back of the circuit board, and clip as necessary. Carefully arrange the wire antenna around the LCD and re-install the enclosure.

It’s important to use a wire antenna of sufficient length, otherwise the transmission range will be severely limited.

Step 3: Final Testing
Make sure your OpenSprinkler is running firmware 2.1.1 or above. If not, please follow the firmware instructions to upgrade your firmware first. Then go to Edit Stations, select the station you’d like to use as an RF station, and change its name to the 16-character hexademical code recorded on the RFToy. Any station with a name of this form will be automatically recognized as an RF station. When the station is turned on, the controller will automatically send out the signal through the installed RF transmitter, thus turning on the corresponding power socket (and vice versa for turning off the station).

Three quick notes:

  • The normal station function still works (i.e. if there is a sprinkler valve connected to that station, it will be switched on/off accordingly).
  • Most likely you want to turn off the ‘sequential’ flag for RF stations. This is because unlike sprinkler stations, you probably don’t want RF stations to be serialized with other stations.
  • If you are short of stations, just increase the number of expansion boards. You don’t need to have the physical expansion boards (think of RF stations as virtual sprinkler valves). Firmware 2.1.1 supports up to 48 stations in total.

With this feature, you can now use OpenSprinkler to programmatically switch a large number of powerline devices, such as Christmas lights, landscape lights, water pumps, heaters, fans.

Keep in mind that because this is still an experimental feature, don’t use it on anything critical (i.e. those that can cause damages if accidentally left on). Depending on the distance and obstacles between OpenSprinkler and remote power sockets, it might not reliably switch on/off power sockets. So take time to do plenty of testing before you finalize the setup.


That’s it. We encourage you to try out firmware 2.1.1 and let us know your comments / suggestions / feedback. Don’t forget to post pictures of your projects. We would greatly appreciate your efforts. Thanks!

Dead-Simple Driver Installation for USBasp and USBtiny on Windows

Today I came across a surprisingly simple approach to installing USBasp and USBtiny drivers for all versions of Windows — XP, 7, 8, 8.1, whether 32-bit or 64-bit, all inclusive! As you may know, installing open-source drivers such as USBasp and USBtiny have been a great pain on some of the recent Windows OS, due to the enforcement of signed drivers. The typical solution involves rebooting Windows into a mode that disables driver signature enforcement. Even after you’ve done it once, if you boot into the normal mode next time, it may fail to recognize the driver again (reporting it’s not digitally signed). A huge source of frustration.

Anyways, while searching for ‘fully signed USBasp driver’, I came across this tool called Zadig, which can be used to install libusb drivers on all versions of Windows, and it’s digitally signed. Since USBasp and USBtiny are both based on libusb, could it be the right solution? To my great surprise it worked really well — I was able to install both drivers on Windows XP, 7 (32-bit and 64-bit), 8, and 8.1 instantly, without messing with driver signature enforcement at all. I was mostly surprised such a great solution wasn’t documented more widely online.

  • Go to and download the software (note that Windows XP has a separate link).
  • Plug in your USBasp or USBtiny device. In case your microcontroller uses a USBasp or USBtiny bootloader, enter bootloading mode, and let Windows detect the device (it will report driver not found). If a window pops up asking to search for driver, just close it or click on Cancel.
  • At this point, run Zadig, it should detect the USBasp or USBtiny, or any libusb device that you have. Then in the selection box (see below), choose libusb-win32 (v1.2.6.0), and click on Install Driver, and wait for the installation to complete.

That’s it! Because the drivers are digitally signed, there is no hassle installing it in Windows 7 64-bit and Windows 8.1.

I will be updating the driver installation instructions for OpenSprinkler 2.1 and SquareWear right away, as they both use USBasp bootloader. Users have often complained that it’s frustrating to install USBasp driver for Windows 7 64-bit and Windows 8.1. Those days are now past!

Announcing OpenSprinkler Firmware 2.1.0 (Major Upgrade)

I am excited to announce that OpenSprinkler Firmware 2.1.0 is officially release. This is a major upgrade that includes a number of new features, including:

  • Automatic Weather-based Water Time Adjustment using real-time weather data obtained from Wunderground (thanks to Rich Zimmerman who introduced the method, the adjustment method is named after him).
  • Improved Program Settings including per-station water time, flexible start times, custom name, per-program weather adjustment control, and up to 14 different programs.
  • Automatic Timezone and DST Detection based on your location. No need to select time zone and mess with DST any more — once you set your location, the firmware can automatically determine your time zone and DST.
  • Improved Station Attributes and Scheduler including station ‘disable’ attribute, ‘activate relay’ attribute, test station feature (replacing the previous manual mode), automatic serialization of overlapping schedules, and the ability to manually start a program on the controller using buttons.
  • Numerous UI Improvements (thanks to Samer’s hard work) including unified mobile interface, export / import configurations, improved visualization of logging data, and the number of supported languages has expanded to 17 (thanks to all who contributed)!

This is a pretty major milestone as it not only addresses the previous limitations but also introduced critical new features including weather-based control. Furthermore, consider all these are implemented on a small microcontroller with only 64KB flash memory and 4KB RAM 🙂 These significant changes are worth making a new video for. So here is the video tutorial for firmware 2.1.0 (it’s a bit long, but gives you a comprehensive overview of the main features);


With this firmware I’ve also written a more detailed user manual, and API documentation. These are available on the Support page of our new website In addition, there are a total of 4 tutorial videos that walk you through the hardware installation, WiFi connection, firmware features, and upgrading firmware. Be sure to check them out first.

Upgrade to Firmware 2.1.0

All OpenSprinkler 2.x devices (including 2.0, 2.1, and 2.2) are eligible to upgrade to firmware 2.1.0. Please check the ‘Firmware Update’ instructions on the support page to download and run the firmware updater. OpenSprinkler 2.0 and 2.2 are the easiest as drivers are pretty straightforward to install, and there is no bootloading procedure; OpenSprinkler 2.1 is tricky because the driver installation is more involved, and there is a bootloading procedure you need to follow. In any case, the firmware upgrade tutorial video gives you a quick walk-through of all the steps.

To use the weather feature, you need to apply for a Wunderground API key. Again, instructions can be found on the support page.

Firmware 2.1.0 has gone through internal alpha testing and external beta testing, so it should be pretty stable. For issues and suggestions, please use the forum, or the support page to submit support tickets.


When I say ‘all these are implemented on a small microcontroller with only 64KB flash memory and 4KB RAM’, it’s not entirely true — the weather feature and timezone / DST detection are actually implemented using Python scripts hosted at Why? Because these require fairly heavy processing power that’s simply beyond the capability of a small microcontroller. So they are implemented by using Python scripts that serve as the ‘middle man’ — retrieving data from weather websites, perform the necessary parsing and computation, and produce the final results to send back to OpenSprinkler. This way the heavy computation is done in the cloud, and OpenSprinkler only needs to poll the server once in a while to update the results. If you are interested in customizing the scripts, you can download the Python scripts from OpenSprinkler Github repository, modify them and host them on your own server. But for most people the default provided script should work pretty well.

Upcoming Features

As this firmware has been rolled out, we are getting excited to decide on the new features for the next round. Some planned features include:

  • Additional station attributes including soil type, slope type, serial group.
  • Support to store programs and station settings onto the microSD card (effectively allowing unlimited programs).
  • Adding firmware support to interface with remote power sockets, so you can use OpenSprinkler to control power line devices like heaters, fan, Christmas lights etc.
  • Support to use sunset and sunrise times for program start times (the sunset and sunrise times are already being detected using the timezone / DST script).
  • Support for flow sensor to monitor water consumption.
  • Cloud support: no more messing with port forwarding.

Suggestions and comments are welcome. Please post them below, or on the forum. Thanks!


OpenSprinkler Firmware Update Program 2.0

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!

Rayshobby Products at Micro Center

They have a pretty large ‘DIY Electronics’ section with all sorts of electronic goodies. This is what I pictured RadioShack should be.

After searching around a few times, I found the AASaver, SquareWear, and OpenSprinkler on one shelf. Very exciting! I do wish they were better organized, because as is, there are hundreds of kits and gadgets spread everywhere like baby toys at a daycare center. Seeing them in store does give me a good idea how to package the products more properly in the future.

So if you are interested in OpenSprinkler (DIY kit), AASaver, and SquareWear, and if you have a nearby Micro Center store, go and check them out. They also take online orders. I wonder if RadioShack could have been saved if they had taken the lead in offering a wider variety of DIY electronics products. Alas, it may be too late.

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