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Analog dime delay circuit

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Nov 23, 2012
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My last lab in school (for circuits 1) is to design an analog time delay circuit. A user pushes a button and 10 seconds later a motor turns on. We are to assume a limit switch would turn off the motor. All I need is for the time delay and motor activation. All I understand for time delay is RC or RL circuits, but cannot think of a way to use these with something such as an op amp comparability to cause a 10 second delay, and to be able to be pushed a second time. I am truly stumped and do not even have a 3 second delay!

I am not asking you to do my homework, but does anyone have a friendly push in the correct direction?

If the user pushed a button, it could start the discharging of capacitor network. But when the user lets the switch go, the voltage will be applied to that network again and thus stop the discharging. Any ideas of how to keep this from happening?

I am not asking you to do my homework, but does anyone have a friendly push in the correct direction?
Have a look at the NE555 chip - it's ideal for this kind of thing, and very cheap. There's also a dual version (NE556 IIRC) if you need more than one.

Good luck!

Thanks for the reply. The thing is I cannot use that chip. I can only user RC or RL circuits and op amps. Not sure what to do...

If you can only use opamps and R's and C's:
You could use a switch and capacitor is series, with the switch connected to the + rail, and the capacitor connected to ground.
An op-amp could be connected as a comparator to see the voltage on the capacitor.
Thus, when the switch is pressed, the capacitor will be rapidly charged to the rail voltage (know as a sample and hold). (in reality
stick in a few ohms resistance to prevent damaging the switch).
The high input impedance of the comparator means that it won't discharge the capacitor in a long while, so the comparator
output will be high for ages.

Use the comparator output to slowly charge another capacitor, this time through a resistor. Use a second comparator connected to
this second capacitor, to detect when the voltage reaches a threshold (which can occur 10 seconds later).

Use a specific value resistor across the first capacitor, to slowly discharge it, so that it is ready for another switch press later.

Thanks for the response. That helped a lot!

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