Driver for Gardena watering valve

In my weather station project, which is still in development, there is a need to control the watering valve. The plan is to use the valve from Gardena C 1030 plus watering timer.

Timer electronic is connected to the valve via RCA connector, as can be seen when timer is detached:

This valve is designed to work on a single 9V battery, and to withstand whole year of continous operation. This means power consumption is very low, in fact power is consumed only when valve is swiched ON/OFF, othervise power is not consumed.

To figure out the impulses which switch the valve ON/OFF I connected an oscilloscope to RCA connector and here are the impulses:

So polarity needs to be reversed when changing valve state. It also should be noticed that impedance of valve coil is around 50 Ohms, so electronics which will drive this valve must be able to provide enough power.

After I designed and tested the driver circuit, I found other solution of the same problem here.
We both manage to drive the valve, yet soultions are totally different.

Now let's say something about the circuit. The whole circuit powers from 9V supply. To switch the valve ON negative impulse of 6,5V is needed, for switching OFF a 2,5V impulse, which in total gives 9 V. The important part is a 2,5V reference, which must be able to source and sink current. Most linear voltage regulators can only source current, but I used LT1118 2,5, which can both source and sink.

Output from LT1118 is a reference for output to valve solenoid. Other parts on schematics are NPN and PNP transistors used as switches, and an optocoupler to drive PNP transistor.

Switching can be controlled directly from a microcontroller providing 5V pulses. Based on measurement with completed module, the length of impulses is not important, they should be just longer than 250 ms for open and 65 ms for close.

Transistors and voltage regulator must be able to supply enough current, due to relatively low impedance of valve solenoid. I used medium power 2N2219 and BC 160 transistors.

Here's the schematics:

Realization of the driver was on a pre-drilled prototype board, and it turnd out pretty compact. It will be a nice little module to add to my weather station.

Hope you found this useful..
FB