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Turn off delay circuit

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purpleanger

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Hi,

I'd like to design a circuit such that at first, the circuit is completely powered off. The user pushes a button, which injects power into the circuit. Then a mechanism maintains the power active, even if the switch has been released. At a later about 3 seconds, when the circuit does no longer need power,
it cuts its own power and sleeps until the user chooses to push the
button again.

How can i design this circuit?

Thanks.
 

grieblm

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Hi,

I'd like to design a circuit such that at first, the circuit is completely powered off. The user pushes a button, which injects power into the circuit. Then a mechanism maintains the power active, even if the switch has been released. At a later about 3 seconds, when the circuit does no longer need power,
it cuts its own power and sleeps until the user chooses to push the
button again.

How can i design this circuit?

Thanks.
Your requirements are too vague.

What type of load do you want to control (AC/DC) and how much power and what is the voltage? The type of circuit will depend on your answers.
 

Syncopator

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You could use a 555 timer i.c. in the monostable mode.

The output would switch a relay with more than one set of contacts. One set, in parallel with the button contacts, would close and maintain the power until the end of the 3 second pulse - at which time the whole thing is automatically switched off.

The other set of of contacts is free to switch whatever else you want.

Here's a suitable circuit.



When the button is pressed, power is applied to the circuit.

The capacitor on pin 2 initially holds the pin low so it is seen as a trigger.

The output, pin 3 goes high, energising the relay. The contacts close and maintain power to the circuit when the button is released.
Another set of contacts, not shown, switch whatever you want it to.

3 seconds after pin 3 goes high it goes low again, disconnecting the supply.
 
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grieblm

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You could also use a CMOS gate (very low power) to control a relay via a transistor and switch. See this link for details.
 

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