What kind of circuit can turn off a power source (by "opening" a circuit) after a given amount of time?

Question

I have a motor powered by a battery (12V) and I was wondering if I could build a circuit to turn off the motor after a certain amount of time (for example, 2 minutes after the motor initiates). Is there anything that can help me do so?

Answer 1

Sure. Timers and relays will do it. You can substitute a power transistor for the relay.

Basically you build a 555 one-shot that gets triggered when the power goes on, and runs for 2 minutes. The back edge of the one-shot sets a latch which drives a power transistor that had been keeping power to the motor.

The latch has an RC on it that resets it on power up. In this mode it turns on power to the motor. After the 2 minutes, the back edge of the one shot causes the latch to set, which removes the power from the motor.

Timer info:

http://www.uoguelph.ca/~antoon/gadgets/5...

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Answer 2

go to a heating/air conditioning parts store and ask for a "delay on make" relay

Answer 3

You can build your own time delay relay,

Here's a link to a nice schematic...

http://www.aaroncake.net/circuits/relayt...

For the relay, be sure to use a "sensitive coil" type.

You can buy the parts from

www.digikey.com

Digikey also sell timers, but I was floored when I saw the over $100 price tag!!!  You should be able to buy these timers already made for around $30.  A common unit is from Altronix,  Check this one...

http://www.altronix.com/index.php?pid=2&...

I don't know where you can order it from but it should cost no more than $30 or so.  Contact them at the link provided to find a dealer near you.

Answer 4

A timer. Although you could make a device using electronics, timers are readily available. A simple clockwork timer (such as used in lower priced micro-waves) would do. Use the timer as the on/off switch.