Pump timer: relay with off delay

Oct 16, 2006 6 Replies
N

I have a 12V pump with an external push button switch. I want to use the existing switch to control a time-delay relay, so the pump runs for about 20-30 seconds when the switch is pressed. It would be nice, if a press during the countdown caused the count to restart.



It has been suggested that I use a 2222A transistor, a resistor, a capacitor and a small relay (with a diode shunt). Use the button to charge the capacitor up instantly, then it slowly discharges, while holding the relay on through the transistor.



This seems nice and simple, and avoids using a 555.



Can anyone confirm that this is a sensible solution, and suggest ball-park values for the C & R?


Nigel M
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G

You can probably skip the resistor and just let the capacitor discharge across the 2n2222, Put 100ufd in there and see what you get. It should be easy to extrapolate from there.

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N

Thanks for the info.

I'm assuming the (original) layout would be like this:

----------------------- +12 | | | / SW [RLA] - | | ^ | |----| -------(K) | | | = z | | | |

----------------------- GND

Where C is = R is z K is 2N2222

Nigel M
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H

Do you have a DC supply only? You can buy switches like this with a pneumatic off delay.

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N

I have to use the existing switch.

Nigel M
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H

There are relays you can add which have off delay.

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E

Put a 330 ohm resistor in series with the base for some protection against too much base current. Use a high value C and a variable R and you'll be able to adjust the timing.

Ed

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