timer mechanisme for solenoid valve

[teflon don]

hey guys
im planning on building a circuit that could regulate the time that a solinoid valve remains open
i want that each time i press micro switch that the valve remains open for regulated amount of time (preferably that i can dial in with a potentio meter

its important that each time the valve remains open for the exact amount of time regardless of how long i press the switch (for example if i press the switch for one second the valve remains open for 3 seconds if i press the siwth 10 seconds the valve still remains open for just 3 second)
but the systhem has to reset when i release and repush the switch (i.e. tapping the swich would cause the valve to be open continiously)

the valve works on 12 volt dc and uses about 60 amps (peak) and i want to keep the current trough the switch low to avoid arcing and contact erosion (a mosfet will solve that problem)
the circuit has to be as compact as possible and i can't programe pic
so can it be done

 

[crutschow]

You can do the one-shot function you need by using a retriggerable one-shot such as the 74HC123. You AC couple the trigger pulse through a capacitor so that holding the button down does nothing and the one-shot will time out after its designed period, but pushing the button again will retrigger the one-shot and start another complete delay cycle. As long as you keep pushing the button before the delay time-out, the one-shot will stay on.

 

[4pyros]

umm? use a 555

 

[teflon don]

umm? use a 555

that was my first thought as well
but i have nearly no expirience in designing my own circuits
so where could i find a scematic drawing or at least a basis to start from

 

[crutschow]

The problem with the standard 555 one-shot circuit is that it is non-retriggerable. This application (output stays active as long as you keep pushing the button) requires a retriggerable one-shot. To use the 555 requires a modified circuit where an additional transistor is added to provide the retriggerable operation.

Attached is a 555 circuit with the modification. As you can see from the Oscilloscope simulation, the solenoid on-time is nominally 3s at the 50% potentiometer setting. Continually pushing the button keeps the solenoid on and then it stays on for about 3s after the last push. Also if you hold the button down the circuit will time out normally. Is that what you need?

 

[teflon don]

thanx mate
that was exactly what i was looking for
and can i then turn down the time by increasing or decreasing the resistance?