Pump timer: relay with off delay

[Nigel Molesworth]

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?

 

[[email protected]]

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?

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.

 

[Nigel Molesworth]

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.

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

 

[Homer J Simpson]

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?

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

 

[Nigel Molesworth]

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

I have to use the existing switch.

 

[Homer J Simpson]

I have to use the existing switch.

There are relays you can add which have off delay.

 

[ehsjr]

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.

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

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