Hello,

How could I go about building a circuit that turns off after a certain delay? That is, after I hook up the battery, I want current to flow for a certain amount of time (some precise value that is somewhere between 10-30 seconds), and then shut down. This is the opposite of a time delay circuit, which seems to be a lot more common of a question.

I was thinking of maybe using a simple RC circuit, but the problem is that I need the current to be as constant as possible while the circuit is alive.

The circuit will run on 6V (4 AA batteries), and while it's powered on, it will be used to turn a small project motor.

Maybe it's possible to make the LM555 timer do what I want -- the problem is, I'm a beginner and I can't wrap my mind around even the most "simple and easy" tutorials on the web. Does this type of backward-time-delay circuit I'm trying to build have a name? What should I search for? I would like to see an example circuit, along with how I would go about calculating the time delay -- is there one on the web?

Thanks in advance for the help.

 

Yes, you can use a 555 timer for this:

To get the the effect you want with circuits 1&2, use the alternate contact on the relay. ie NC vs NO. To get the effect you want with circuit 3, swap the timing capacitor with the timing resistor (R1 & C1).
For more 555 circuits go to:
http://home.cogeco.ca/~rpaisley4/LM555.html

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