I am installing a "Hot Water Bypass" system in my motor-home to conserve water.Instead of running cold water from the hot tap down the drain I have installed a solenoid to return it to the water-tank for a set time using a 555 timer circuit.
On its own the 555 timer (off a 12 v DC supply) drives a little relay and works fine with a small test buzzer across the relay contacts, BUT connect 24vdc to the relay contacts to drive the solenoid and the timer gets false triggering to keep it on for a random number of cycles. The relay coil and contacts are electrically isolated.
I guess there is induction happening causing the circuit to re-operate.
I connected a test light across the solenoid coil and observed that the timer ceases but re-triggers before the solenoid releases effectively keeping it operated. But with the testlight connected I noticed a decrease in the number of "re-triggers".
In an effort to desensitize the trigger circuit I doubled the resister value to 20Kohms between +12dc and pin 2, the trigger pin.
With the 20Kohms and a 24v 3 watt bulb across the relay contacts in parallel with the solenoid coil the system has operated 100 plus times without a re-trigger.
I think the lightbulb is absorbing the back emf spike at the opening of the relay contacts effectively
1 letting the solenoid release, and
2 preventing "induction" re-triggering the 555 timer
I am happy now it works as I wanted it to, but would a capacitor do the same job as the bulb? If so what type of capacitor and size?

Alternatively is there another method?

Many Thanks
Denis Hibbs
Napier NZ