How do I make an electrical circuit that cuts off electricity below a certain level?

Question

I am trying to put together a circuit that will cut electricity off completely when demand falls below a preset or adjustable level and then turn the power back on full when demand for full power is required again.

For example: Cut the power completely when voltage draw drops below 9V but turn it on when draw is 9V or greater.

Answer 1

There is a hole in your logic.
If you cut power completely, the attached devices will have no power. If there is no power, they can't turn on and demand more power!!! (unless, they have a backup battery or secondary power source).

Answer 2

So many books, I can't even post one link because it would be too limiting. Use a search engine to look up 'OP AMP COOKBOOK'.

Op amps, a type of IC chip that is very cheap and easy to work with, are used exactly for that purpose. The supply voltage is connected to the op amp to measure its level. The op amp drives a transistor which switches on or off to power your device.

If the voltage falls below a threshhold level, the op amp turns off the transistor, interrupting power to your device. When the level rises above the threshhold, the op amp turns the transistor back on to full power and your device is going again. It's been a lot of years, but I think your basic 741 op amp is what you'd use.

#2 in the link shows a cut off circuit. You'd have to calculate the values for your particular voltage cutoff point.
http://www.electronics-lab.com/projects/motor_light/042/index.html

Answer 3

Look for Lm 555.It is a easy to use IC that has a two step comparator inside.Make the low step at 9V and high step at 9.2V(off at 9V and on at 9.2V).

Answer 4

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