Tuesday, October 16, 2012

An expensive replacement for a power switch

For a battery powered device you most often would want to be able to switch the device on and off for it not to consume precious power when not needed. Easiest way is to insert a switch that would connect and disconnect battery from the circuit.

But that's not geek. Our way is to spend some cents for a couple of transistors, spend some minutes for PCB layout and give away an MCU pin - but as a bonus get for your device a switch-on controlled by a push button and an automatic switch-off feature. Here's how to do it (design is taken 100% from a perfect manual for Bob Parker's ESR meter).

I guess it's plain and simple. When the device is in it's off-state, both transistors are closed and there's no current drown from the battery. If you press the push button S1, transistor Q2 will open, connecting +5V to VDD and powering your circuit. No your circuit must take over and pull MCU pin UP. Q3 opens and takes over S1 that can be now released. Your circuit will now remain being powered until MCU decides it had enough already and commits suicide by pulling MCU pin low. That would be it's last action as both Q3 and than Q2 close, leaving circuit non-powered until you next time press S1.

P.S. Yes, I'm building an ESR meter for me. Thanks to Bob Parker for this perfect guide and thanks to Dave Jones for a perfect guide on how to apply it.
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