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Picture above shown an automatic watering timer using solenoid. The timer inside this device will turn on dc motor forward/ reverse for a short period to trigger the solenoid mechanism based on time set. Now, Im trying to using the timer from this device to trigger a relay to turn on a dc water pump instead of solenoid from the same output (black and red wire). Im guessing that this can be achieve with something that can trigger ON or OFF relay when voltage applied.

While searching on the internet, I found this PNP circuit but this circuit only turn on relay forawhile only.

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So, my question is, how can I turn on dc pump by removing the motor shown in the picture and use the voltage at black/red wire to trigger a relay?

Something like this:

When voltage detected Turn ON relay. Stay relay turn ON until next detection (based on timer setting)

Voltage detect again then Turn OFF relay this time. Stay OFF until next trigger.

  • \$\begingroup\$ What is your question? \$\endgroup\$ Apr 29, 2020 at 11:45

1 Answer 1


Your approach is correct, but the circuit will not work. With the Q2 emitter at +12 V, the base has to be at least +11.7 V to turn off the transistor, but your control signal has a max amplitude of +5 V. There are two solutions.

First, can you change the control signal logic polarity? That is, can whatever is generating it be +5 V = ON and 0 V = OFF? If so, then changing the transistor to an NPN type solved the problem.

Second, add another transistor to invert the signal going to the Q1 base. I'll try to edit this post with a schematic later.


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