Recently through a customer I learned about a product called WireSprout, and it only works with the AC-powered OpenSprinkler but not the DC-powered version. Out of curiosity, I looked into why this is happening. So what is WireSprout? Simply speaking, it allows individually controlling 2 zones using a single zone wire. This is useful in situations where some of your zone wires are broken and it’s too much hassle to repair the broken wires. Let’s say you have 2 zones, but only 1 good zone wire. Using WireSprout, you to control both zones using the single good zone wire. This works on any sprinkler controller (well, as you will see later, as long as it’s an AC sprinkler controller). A single WireSprout pack contains a pair of two ‘sprouts’. Each sprout is a tiny little circuit wrapped in heat shrink tubing, and has 3 wires: 1 blue and 2 green wires. Below is the diagram that shows how to connect it to a sprinkler controller:

To be fair it’s not adding more zones — to control 2 zones you still need to take 2 zone ports on the sprinkler controller. Also it requires the Common (COM) wire to be a good (i.e. non-broken) wire. But what it helps with is to reduce the number of zone wires. Note that it can only go with a pair of 2 zones. For example, if you want to control 4 zones, you need another good zone wire and another pack of sprouts. It unfortunately cannot allow you to control 4 zones with a single good zone wire.

Each sprout is very small, so likely it only contains a few electronic components. Also, it’s very general — it can work with any AC sprinkler controller, so the circuit doesn’t rely on the knowledge of any specific controller. It also works only for 2 zones at a time. Finally according to the customer, it doesn’t work with DC-powered OpenSprinkler, only works with AC-powered version. So it must rely on the property of AC to work. I googled similar products, and after a bit of research, it became clear to me that the circuit is indeed extremely simple. Each sprout is essentially two diodes in series, where the two ends are the green wires, and the center (between the two diodes) is the blue wire. Below is what I believe each sprout contains internally:

So how does it work all together? Here is the diagram:

Because the output voltage is AC, it has positive and negative cycles. As you can see, on the positive cycle of the COM wire, if Zone 2 port is on, the two diodes circled green will turn on. So the current flows from COM to Solenoid 1 through the Common wire, then through the zone wire to Zone 2 port. The other two diodes are reverse biased therefore solenoid 2 cannot turn on even if Zone 1 port is on. Conversely, on the negative cycle of the COM wire, the situations with all diodes are flipped, so only Solenoid 2 can turn on (assuming Zone port 1 is on). In this particular arrangement, Zone port 1 controls Solenoid 2, and Zone port 2 controls Solenoid 1. If you want them to correspond to each other (i.e. 1 -> 1 and 2 -> 2), just horizontally flip one of the sprouts.

In short, the WireSprout works by leveraging the fact that AC waves have positive and negative cycles. By using diodes, it can cleverly block half of the AC waves, therefore Solenoid 1 can only turn on during the positive cycles, and Solenoid 2 can only turn on during the negative cycles, or vice versa. Thus these two zones can be individually controlled.

Now it’s obvious why the DC-powered OpenSprinkler can’t work with WireSprout: DC-powered OpenSprinkler outputs DC-only voltage, there are no positive or negative cycles — there is only positive voltage. Therefore WireSprout can’t leverage the negative cycles to disable one of the solenoids therefore it cannot achieve individual control of 2 solenoids using a single zone wire.

There is possibly a downside of this method: each solenoid only get half of the AC waves as opposed to the full wave normally. Would this cause any reliability issues? I am not sure, but it seems there hasn’t been any reported issue so far.

Finally, we can also explain why WireSprout always works in pairs of 2 and not more than that: if you want to control, say 4 solenoids with a single zone wire, that would require counting the parity of the AC waves, which would be much more complex and may require an active circuit.

Today we’ve officially released OpenSprinkler firmware 2.2.0(1). As you may have noticed on this forum post, this firmware is a major revision from the previous firmware 2.1.9. Most of the new features are thanks to Valeriy Soltan’s excellent work almost 2 years ago (I know, time flies during the pandemic), including support for sequential groups, setting a date range for each program, support for pausing and resuming stations, shifting zones forward, more flexible master on/off adjustment times etc. In addition, we’ve added a device name parameter, which is included in all IFTTT notifications; and a new weather adjustment method called ‘Monthly’ adjustment is added to set a manual watering percentage per month. For OpenSprinkler 3.x only — one major step forward is the support of OpenThings Cloud (OTC) token, which allows remote access via the OTC cloud server. This eliminates the need for settings up port forwarding, which has been a pain for manual users. OTC has been enabled for our OpenGarage product for a while, and it seems to be working pretty well, so this is a good time to enable OTC for OpenSprinkler as well. A major benefit of OTC is that the same web interface / UI is available both locally and remotely, and OTC server serves as a real-time ‘relay’ between the firmware and user, it doesn’t actually store any date on the cloud server. Another step forward for OpenSprinkler 3.x is upgrading ESP8266 core to version 3.0.2 (the latest ESP8266 core for Arduino). In important benefit of this is that the WiFi and wired Ethernet are unified, through the lwip library available in this core version. As a result, wired Ethernet and WiFi use exactly the same interface functions, so all WiFi features (including OTA update) are also supported by wired Ethernet. Moreover, this allows OTC to be supported by both WiFi and wired Ethernet as well.

We’ve release a test version of firmware 2.2.0 a month ago and since then have been working on fixing bugs both in the firmware and the UI. At this point, the firmware is ready to be released to the public, and we are really excited. The full list of new features can be found in the Github release notes. As usual, firmware update instructions can be found on our support website at support.opensprinkler.com Since this firmware is very new, bugs and issues are probably inevitable. If you encounter any problem, please be patient and file a support ticket at support.opensprinkler.com We will try to handle them as quickly as we can.

It’s hard to believe that two years have gone by since my last post. Lots of things happened during these two years: on the good side, we have our first baby born during a year of pandemic, and he is bringing joy to the family every day. On the bad side, there is a pandemic, which brought so many challenges, from supply chain issues to shipping delays and to the difficulty of finding available employees to hire. The pandemic has also taken a huge roll on my mental health, significantly limiting my productivity and creativity.

This post is my attempt to resume regular blogging, a habit that I’ve always enjoyed in the past but was lost during the two terrible years. I am hoping this will motivate me to continue learning and sharing new knowledge about electronics, and continue to provide new passion in my life.

In this post, I will briefly summarize I/O expander choices I’ve considered when designing the OpenSprinkler circuits, and the pros and cons of each choice. I/O expanders are often necessary when the microcontroller’s I/O pins are insufficient for the application. For example, a sprinkler controller may need a large number of output pins in order to drive many zones independently. This is the reason that from the very first version of OpenSprinkler, I’ve decided to use an output expander, to allows the number of zones to be scalable and not limited by the available I/O pins on the microcontroller itself.

74HC595 Shift Register

The most common choice for increasing output pins is to use a 74HC595 shift register, or more precisely: serial-in, parallel-out shift register. 74HC595 is cheap, widely available, and quite simple to connect to a microcontroller like Arduino. Each chip adds 8 output pins, and you can daisy chain them to almost any number, limited only by the potential signal degradation/distortion when cascading too many of them. At the minimum, a microcontroller only needs to use 3 pins to interface with any number of daisy chained shift registers. These pins are named LATCH, CLOCK, and DATA, which are essentially CS, CLOCK, and MOSI pins in SPI terms. You can begin the data transfer by setting LATCH pin low, then at the rising edge of each CLOCK cycle, the HIGH or LOW signal presented on the DATA pin is transferred to the first storage register, while the data already transferred in previous clock cycles are shifted down to the next storage registers respectively, hence the name ‘shift register’. After 8 clock cycles, 8 bits of data are shifted in. Finally set the LATCH pin high, upon which the values in the storage registers are transferred and presented on the output buffers. You can bit bang the pin values, use Arduino’s built-in shiftOut function, or if CLOCK and DATA pins happen to be connected to the microcontroller’s SPI CLK and MOSI pins, you can use SPI functions for even faster operation.

One advantage of 74HC595 is that it has a separate set of storage registers vs. output latches (aka output buffers). During data transfer, this allows the output values remain stable and not affected by the data that’s being shifted in. The output values only update upon the rising edge of the LATCH pin. This compares favorably to other shift registers like 74HC164 that are even cheaper but don’t have output latches (thus the output values may flicker during data transfer and such the flickering would be problematic for output devices like sprinkler solenoids).

Pros:

• Very cheap (a couple of cents in bulk pricing) and widely available.
• Can be daisy chained to enable a large number of output pins
• Relatively small number of microcontroller pins required to interface with it.

While the list of pros makes it sound like this is the perfect choice for almost any application, there are also a number of cons which are somewhat subtle but important for sprinkler controllers.

Cons:

• 74HC595 is output only, it does not support input pins (for that there are dedicated parallel-in, serial out shift registers, basically the opposite of 74HC595).
• Maximum output current is small — according to the datasheet, each output pin can only source or sink up to 4~7mA, depending on the supply voltage. If the output device is a transistor or MOSFET, this is more than sufficient. But on AC-powered OpenSprinkler, the output device is a traic, such as MAC97 or Z0103MN, which require relatively large gate current, and 4~7mA is only barely enough.
• Upon powering up, the output states are not deterministic — this means if LEDs are connected as output devices, they may have a brief moment of flickering upon powering up; similarly if the output devices are solenoid drivers like traics, some solenoids may be momentarily activated until the microcontroller clears out the output buffer. This problem can be alleviated by using an additional pin — the output enable pin — to disable the output latches upon powering up, but the downside is this requires another microcontroller pin.
• The communication between microcontroller and shift register is one-way, so it’s difficult for the microcontroller to detect or enumerate how many shift registers are connected. On OpenSprinkler, I had to use an analog pin in conjunction with a parallel resistor per shift register to be able to detect/enumerate the number of shift registers. This is a downside, particularly if the microcontroller is short of analog pins.

I2C I/O Expanders (such as PCF8574/8575/PCA9555/9535)

Due to the advantages of 74HC595, I used it on the legacy versions of OpenSprinkler (1.x and 2.x) for many years, until I had to move on to switch the microcontroller to ESP8266, in order to support built-in WiFi. Suddenly the cons of 74HC595 became a show stopper, partly because ESP8266 has a much smaller number of GPIO pins, so I could not afford to spare many GPIO pins to interface with 74HC595; and partly because ESP8266 has only one analog pin, which has to be used for solenoid current sensing, thus cannot be used to detect/enumerate the number of expanders.

This is where I discovered there are a large family of I2C I/O expanders. In fact NXP has a document that nicely summarizes the various choices. Most of them differ by the number of I/O pins available, output type, maximum output current, the availability of pull-up resistors on each pin. The output type is quite interesting: some of them use push-pull (i.e. Totem-poll) which means they can both source and sink a large amount of current (i.e. strong current sources and sinks); some of them (e.g. Quasi-output) can sink a large amount of current but only source a small amount of current (i.e. strong sinks but weak sources); some of them (e.g. open-drain) can only sink current but not source current at all.

Most of the pros and cons below are exactly the counterparts of the cons and pros of 74HC595 respectively.

Pros:

• Each pin can be configured as either input or output, so with a single I/O expander chip, some of the pins can be configured as outputs to drive triacs or MOSFETs; while others can be configured as inputs to read sensor values or button statues.
• All of them interface with the microcontroller through I2C, therefore they require only the SDA and SCL pins from the microcontroller, and the same two pins can be shared with other I2C devices such as real-time clock, OLED display etc. This is particularly suitable for ESP8266, on which the number of GPIO pins are very limited.
• Using I2C almost means the communication is bidirectional — the microcontroller can detect and enumerate the number of I/O expanders connected to it.
• Another advantage of I2C is that each I/O expander has a unique I2C address, so by carefully allocating the address of each expander, the microcontroller can detect which type of OpenSprinkler (AC-powered, DC-powered, or Latch) by reading the I2C address.
• The I/O pins on these expanders can source or sink a large amount of current (up to 25mA), which is more than sufficient to drive triacs. Well, not all of them though: when designing OpenSprinkler 3.0, I was only aware of PCF8574 and PCF8575, which are weak current sources, so they have to be combined with PNP transistors or P-ch MOSFETs to source a large number of current. By the time I started designing OpenSprinkler 3.1 and 3.2, I became aware of PCA9555 and PCA9535 — they are strong current sources so there is no longer need for additional transistors or MOSFETs.
• Upon powering up, the output states are deterministic — in fact, the outputs are always high upon powering up, with weak pull-up resistors. This makes it possible to overcome the flickering issue at powering up.

Cons:

• They are relatively expansive (a dollar or so in bulk pricing). While this may not seem much, it does stand out quite significantly when compared to the pricing of 74HC595. Also, the chip shortage caused by the pandemic has made them even more expensive and frequently out of stock. In contrast, it hasn’t affected the pricing or availability of 74HC595 much.
• Because each expander must have a unique I2C address, and each chip (e.g. PCA9555) only has 3 bits of address pins, this means you can only connect up to 8 expanders chips. This is also a disadvantage compared to 74HC595 where you can daisy chain virtually unlimited number of expanders. Despite this limitation, I decided this is a worthwhile compromise to make for OpenSprinkler as the total number of zones per controller is usually not that much.

Among all the I2C expander chips, PCF8574/8575/PCA9555/9535 are relatively more common and available. The differences between them are:

• PCF8574 has 8 I/O pins, and PCF8575 has 16. Both of them are of Quasi-output type, so they can sink but not source a large amount of current. They were used for OpenSprinkler 3.0 controller and zone expanders.
• PCA9555 and PCA9535 both have 16 I/O pins, and both use Totem-poll output, so they can both sink and source a large amount of current. They are used in the current OpenSprinkler 3.2 controller and zone expanders. The only difference between the two is that PCA9555 has built-in pull-up resistor while PCA9535 does not. Thus for any pin configured as input, PCA9535 requires an external pull-up resistor.

CH423s I2C I/O expander

Recently I discovered a very low-cost I2C I/O expander CH423s. It’s made by a Chinese company QinHeng which is famous for making the low-cost USB-serial chip CH340. CH423s has a bulk pricing of about 20 cents, and it has 16 output only pins with 8 additional input/output pins. So it’s a quite capable and versatile chip. However, after reading its datasheet and understanding the sample programs, I found it has a big downside, that is it takes over too many I2C addresses. This is a quite strange design of the chip — unlike PCA9555/PCA9535 (each of which only takes over one I2C address, and you can configure the input/output of each pin by settings configuration registers), CH423s uses multiple I2C addresses to eliminate the need of configuration registers. While it is relatively easy to program, the large span of I2C address space makes it infeasible when the same I2C bus has to be shared with other devices like real-time clock and OLED display, which may conflict with its address space. Also, there are not configuration bits for the I2C address, so it’s not possible to use it on zone expanders. Nonetheless, this is an interesting choice to consider for some applications, if the I2C address space is not a problem.

1-Wire I/O expanders (DS2413/DS2408)

Wouldn’t it be nice if the microcontroller only needs one-pin to communicate with I/O expanders? It turns out such an option does exist! Take a look at the 1-Wire I/O expander chips: DS2413 supports two I/O pins, and DS2408 supports 8 I/O pins. They interface with a microcontroller through a single data wire, hence the name 1-wire. Probably the most well-known 1-wire device is the DS18B20 temperature sensor. While this may not seem a lot of I/O pins, each chip actually has a globally unique address, so you can connect virtually unlimited number of these chips, all sharing a single data line! These chips were brought to my attention by Patrick Morse — he proposed this as a solution to implement Hunter’s EZ Decode system. In fact, before his email, I’ve from time to time received requests to develop a 2-wire decoder system — similar to I2C, 2-wire decoders use a clock line and data line to transfer signals between the microcontroller and the solenoid driver. This is an attractive solution for applications where it’s a hassle to install a large number of long copper wires between the sprinkler controller and each solenoid. 2-wire decoders solve the problem by connecting all solenoid serially using only 2 data lines. Each decoder would have a unique address just like in the case of I2C. The microcontroller sends commands to discover the unique address of each decoder connected on the bus, and consequently can switch each solenoid valve independently. The 1-wire decoder would be a further simplification by reducing one data wire. The commercially available 2-wire decoders generally use proprietary communication protocols that are not open-sourced. The existence of DS2413 and DS2408 means we can easily implement a 1-wire decoder using well-documented 1-wire protocol. That’s great!

Pros:

• Using one single data line, suitable for implementing 1-wire decoders that can significantly save the amount of copper wires required in a sprinkler system.
• Each chip has a globally unique 1-wire address, making it possible to connect a large number of chips all sharing the same data line. By detecting the 1-wire address, the microcontroller can detect and enumerate each chip.

Cons:

• Relatively expensive: DS2413 has a bulk pricing of close to 2 dollars, and it only supports 2 I/O pins. Also they are less commonly used so more prone to chip shortage issues.

While it’s possible to interface with these devices by directly using a microcontroller pin (through the open-source 1-wire library), it may be better to use a 1-wire master chip, which interface with the microcontroller through I2C and it can talk to 1-wire devices using a more robust data line. The 1-wire master chip is particularly useful for ESP8266, which doesn’t have many GPIOs, so it may be necessary to delegate the communication task to a separate chip.

To conclude, this posts summarizes some of the I/O expander options I’ve considered when designing OpenSprinkler circuits, and the pros and cons of each. In the case of OpenSprinkler 3.x, the design decisions were largely driven by the limited number of pins on ESP8266. With other microcontrollers that are not short of GPIO pins, 74HC595 may still be the most attractive choice due to its many advantages and the significantly lower cost.

Update: the technique described in this article is possibly no longer necessary for the current version of OSBee, as it now supports setting a different opening vs. closing voltage.

Recently when helping a customer, I came across an interesting case of how to control Gardena 1251 latching solenoid valve using OpenSprinkler Bee. This valve is mostly sold in the European market and isn’t very popular in the US market. On spec, it’s operated using a single 9V battery, and to use this valve you need to buy a Gardena 1250 controller unit. The whole assembly including the valve and controller unit are quite pricy (close to 100 bucks), so it’s not a very cost-effective solution compared to other brands. Nonetheless, it’s an interesting case that helped me understand how these latching solenoids work.

Measure the Control Voltages

The initial request to look into this valve was due to the fact that OSBee can’t seem to operate this valve correctly: it can open the valve but never manages to close the valve. This was reported by a German customer, and it caught my curiosity. To figure out the issue, the first thing I did was to check how the control unit (1250) is sending out control voltages to the valve. It’s pretty common that when operating latching solenoid valves, the control circuit sends an impulse voltage to open the valve, and another impulse voltage in the reverse polarity to close the valve. On most solenoid valves I’ve seen, the two impulse voltage (of opposite polarity) are roughly the same, and that’s also how the OSBee circuit works.

Upon connecting the control unit to an oscilloscope, I noticed something strange: no matter how I press the on/off button, it’s only sending a very short (a few milliseconds) pulse, which cannot possibly operate the valve. Then it became clear to me that the controller is in fact actively sensing the existence of the valve, and would not send control voltages if the valve is not detected. I measured the resistance of the solenoid valve, which is about 35 ohm. So I connected a 33 ohm resistor to the controller as a dummy load, and there you go, now we can observe the control voltages and pulse lengths.

It’s pretty easy to notice the asymmetry here: while opening the valve requires a pulse of 250 ms and -7.84 voltage (this is roughly the battery voltage since my 9V battery isn’t fully charged), closing the valve only requires a very short pulse of 62 ms and very low voltage — 2.5V. This is quite strange to me: how come closing the valve only requires such a short pulse and such low voltage?

How Does This Latching Valve Work?

In order to figure out what’s going on here, I un-tightened a bunch of screws and opened up the valve.

At the bottom of the valve is a pressure chamber with a spring. This is very similar to other valves I’ve seen.

The top section contains, supposedly, a coil and magnet inside, and a small cone-shaped metal piece that can be attracted to the magnet or released. It’s quite easy to observe that when opening the valve, the metal piece gets attracted (left picture above), this supposedly releases the pressure in the bottom chamber, thus allowing the water to flow through the valve. Conversely, when closing the valve, the metal piece is released and dropped to block the hole at the bottom, this supposedly allows water pressure to build up in the bottom chamber, thus stopping the water flow.

The key in making this latching is that an impulse voltage can permanently magnetize the core, thus permanently attracting the metal piece. This makes it possible for the valve to remain in the ‘open’ status without contiguously drawing current from the power source (which is unlike non-latching valves like 24VAC valves).

Observing this mechanism, it became clear to me that ‘closing’ the valve basically requires de-magnetizing the core, and that requires just a short pulse of low voltage in the opposite polarity. If you apply the same voltage and strength as before, it will start magnetizing the core the other way, thus the magnetic pole changes direction but the metal piece will still be permanently attracted!

Anyways, this is very interesting to me because previously I had no idea how latching solenoid valves work internally. Now at least I know understand this particular valve works, and this understanding will help me figure out how to get OpenSprinkler Bee to control this valve.

Use OSBee to Control the Gardena 1251 Valve

There are several issues that make OSBee incompatible with the Gardena 1251 valve, but it turns out they can all be solved without too much difficulties. The first is that OSBee by default boost the input voltage (which is 5V from USB) up to 22VDC, which is significantly higher than 9V required by the valve. This is reasonably easy to solve — by fine tuning the boosting time, I can find the sweet spot where the boosted voltage is just around 9V. This boosting time turns out to be around 80 to 100ms.

Second, OSBee by default uses 100ms pulse in both directions (i.e. both opening and closing the valve). This is very easy to change to 250ms and 62ms respectively to match the Gardena controller.

The last issue deserves more thinking, that is, opening the valve requires 9V, but closing the valve requires only 2.5V. Because the input voltage is from USB and it’s 5V, there is no obvious way to step that down to 2.5V since the boost converter can only bump up the voltage and never reduce the voltage. How do we create this asymmetric voltage in opposite polarities? Turns out that you can do so by making use of a diode connected in parallel with a 100 ohm resistor. Why? The diode is a one-way gate: when positively biased, it turns on almost fully (except the 0.7V voltage drop across it, which can be ignored here); but when reversely biased, it turns off, thus current has to flow through the resistor (connected in parallel to the diode), and that resistor will divide the voltage, ensuring that only about 2.5V falls on the valve.

I’ve attached here a diagram showing the connection. The diode can be almost any general-purpose rectifier, like 1N4148, 1N4001 and so on. I’ve also included two photos showing the actual components connected to OSBee and the valve. With this modification, OSBee can now both open and close the Gardena 1251 valve successfully. Mission accomplished!