# sending a single pulse when circuit is powered, and inverse single pulse when power is cut

#### Dongiulio

Joined May 12, 2021
4
Hello,

I'm trying to work my way around an automated irrigation system I'd like to build, I bought a bunch of VHM-014 (12 v) circuits so theoretically I should have everything I need for it.

Except that I'm struggling to find the right valves to actually open/shut the water flow.

Initially I bought some 12v solenoid valves, but quickly realised that with water pressure they won't open just on battery power (8 AA), and I tried with a 2A 12v transformer, it wouldn't open either (it does open when no/little pressure is applied).

I disassembled an old Gardena T14 which works with a 9v battery and figured that this device uses a latching 9v valve, that opens with an impulse in one direction and closes with an inverted impulse (i.e. switching + a -).

My VHM-014 are simpler than that and simply open a relay when they figure we need watering and close it when they figure they watered enough.

So I thought I need to explore converting this VHM-014 output so that when the relay closes and my circuit is energised (12v) it needs to send out a single pulse to the latching valve with a given + and - orientation.

Then when the circuit loses power it needs to send another pulse with + and - inverted, maybe using a capacitor?

technically I could use the same 12v source to power the circuit so to have power to send the closing pulse. Ideally I'd like this system to use the least possible current, so that I won't have to replace batteries too often.

something like this:

I need to fill the gap between D1 and D2 (through the solenoid) and equally through D3 and D4 (also through the solenoid)

U4 would be activated by the VHM-014 when watering is needed

can anyone help me out here?

thanks,

UPDATE: I've just learned of the H bridge, with relays, that can invert polarity from a power source to a consumer (ex. a latching valve) building one of these I would just need some kind of system that when current is applied only a pulse is sent, then stop.

with two relays and two of these systems I could complete my setup.

any ideas about how to build such a device to just send a pulse?

#### Tonyr1084

Joined Sep 24, 2015
7,886
Welcome to AAC.
I disassembled an old Gardena T14 which works with a 9v battery and figured that this device uses a latching 9v valve, that opens with an impulse in one direction and closes with an inverted impulse (i.e. switching + a -).
The reason for this is to preserve battery life. If the battery was used to "HOLD" the valve open then the battery would drain and die quickly. So a short "ON" pulse to turn the water on and a short - reversed polarity - "OFF" pulse to turn it off means the battery can last the whole season. Possibly longer. Exactly as you stated.

However, you mentioned water pressure. What is that pressure? Most electric water valves use a small spring to hold the valve shut. When pressure is equalized on both sides of the diaphragm the spring is all that is needed to shut the flow off. However, if you have a very low pressure then even when commanded to be "ON" the water pressure alone might not be enough to open the valve. So what is the pressure we're talking about?
when the circuit loses power it needs to send another pulse with + and - inverted, maybe using a capacitor?
If the battery loses power the capacitor will likely also lose power. Unless you use a diode to prevent back feeding the battery from the capacitor. But even then it's likely the capacitor might not have sufficient power by the time the battery has lost its power.

As for using the H bridge - yes, that can be done. But if you want low current draw you should consider using MOSFET's. They don't use current they use voltage to switch on and off. Relays are going to drain your battery faster than just using the valve and a momentary pulse.

There are plenty of battery powered sprinkler timers and valves on the market. You should be able to find one and copy its design. However, I don't understand why you're trying to re-invent the sprinkler control system when so many others have done it for you. But if it's your wish to do so - I can't say "Don't". However, if you're thinking about commercial applications (making a profit) you could be violating copyrights laws.

#### Dongiulio

Joined May 12, 2021
4

However, you mentioned water pressure. What is that pressure? Most electric water valves use a small spring to hold the valve shut. When pressure is equalized on both sides of the diaphragm the spring is all that is needed to shut the flow off. However, if you have a very low pressure then even when commanded to be "ON" the water pressure alone might not be enough to open the valve. So what is the pressure we're talking about?
The water pressure is about 3/4 bar, which should be well within the range of my valve, still for some reasons it won't open. I tried it using a long garden hose so I was able with a trick to gradually reduce the pressure applied to the valve, and found that it will open on battery power when very little (non zero) pressure is present. I didn't replicate the experiment with the transformer.

If the battery loses power the capacitor will likely also lose power. Unless you use a diode to prevent back feeding the battery from the capacitor. But even then it's likely the capacitor might not have sufficient power by the time the battery has lost its power.
it's not the battery losing power, it's the VHM-014 opening the relay, so it should be full power, then after a moment no power.

I'll study MOSFETs I'm not familiar with them.

There are plenty of battery powered sprinkler timers and valves on the market. You should be able to find one and copy its design. However, I don't understand why you're trying to re-invent the sprinkler control system when so many others have done it for you. But if it's your wish to do so - I can't say "Don't". However, if you're thinking about commercial applications (making a profit) you could be violating copyrights laws.
I know of many battery powered sprinkler systems, all work with a timer though.

Because water is a precious resource with a time only dependent approach, I'll be likely to waste it.

This is because it will be really hard to get the perfect timing, and the amount of watering will be dependent on the weather, on rainy days (wetter) or windy days (dryer) with a timer based approach I will be inevitably overwatering or under-watering respectively, wasting water or ruining my garden which is a precious resource too (to me at least).

What I'm trying to achieve is to water my garden solely depending on soil moisture, and that's what the VHM-014 is designed for. It wasn't probably designed for being operated on batteries (that's why it works with 12v, and not 9v where batteries are easier to find).

I don't know of any watering system that works with a soil moisture sensor.

I'm very curious but very new too to electronics, and I'm looking for a convenient way to set this up. It is turning out to be way harder than I thought initially.

#### Tonyr1084

Joined Sep 24, 2015
7,886
I'm certainly not the expert here but I would imagine that monitoring soil moisture will also use battery power. My first inkling to approach soil monitoring would be a comparator circuit where one leg monitors the voltage passed through the soil and compares it to a reference voltage you set. When the reference voltage goes higher then the comparator output can go high (or low if designed that way) and trigger the water valve. Again, I'm not the expert on such things. But if battery power source is unlimited (solar for instance) then I might approach it that way. MIGHT!

3/4 bar is just below 11 PSI. Typical water pressure is 45 to 90 PSI. Water valves are designed for such circumstances.

#### Dongiulio

Joined May 12, 2021
4

I'm certainly not the expert here but I would imagine that monitoring soil moisture will also use battery power. My first inkling to approach soil monitoring would be a comparator circuit where one leg monitors the voltage passed through the soil and compares it to a reference voltage you set. When the reference voltage goes higher then the comparator output can go high (or low if designed that way) and trigger the water valve. Again, I'm not the expert on such things. But if battery power source is unlimited (solar for instance) then I might approach it that way. MIGHT!
The moisture measure is done via a forked sensor that I believe measures resistance between the two sides, infinite resistance = super dry, low resistance = super wet. so yeah uses battery power, but I don't think it uses a lot of it.

I already have the circuit that triggers the valve when watering is needed, but this just works by powering a relay (i.e. closing the circuit to energise the solenoid valve) to start watering, keeping it powered until the soil is moist enough, then just reopening the circuit in the relay. I can't change this cirquit, but because this configuration is made for normal solenoid valves it uses a lot of battery power, I'd like to modify it so that it can use latching valves, to do so I'm thinking about adding an extra circuit which I need to produce, that:

• when the relay closes the circuit (i.e. start watering signal) I just send a quick impulse to the latching valve (to open the water flow) and then stop energy flow almost right away, no need to waste battery power, the latching valve is already open and doesn't need additional current)
• when the relay opens the circuit (i.e. watered enough signal), my circuit sees a cut in energy and sends another quick impulse (with polarity switched + becomes - and - becomes +) that will shit the latching valve (stopping the water flow), then stop passing current to the latching valve, it has already switched, doesn't require any additional power

Another idea I could use the relay as a switch, when closed send a pulse, when open send another pulse.
I could even unsolder the relay from the VHM-014 and hack my circuit in, so that when there's tension between the two ends of where the solenoid used to be I could send a pulse and another when that tension goes off.

3/4 bar is just below 11 PSI. Typical water pressure is 45 to 90 PSI. Water valves are designed for such circumstances.
My calculator says that 3/4 bars is between 43 and 59 PSI

#### Ya’akov

Joined Jan 27, 2019
9,117
Traditionally, this sort of automation would be done with time delay relays of various sorts, possibly mounted on a DIN rail. All sensors and signals you need are available in this form, and with the cheap Chinese options, they won't cost much.

Take a look at DIN mount time delay relays, in particular single shot types that will generate a pulse on getting power. DIN mount power supplies are also cheap and readily available.