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Circuit to turn-on Edison from battery when USB power removed

Hello all,

For one of my projects I needed a circuit to turn on the Edison module from battery when 5V USB power is removed. Something like when Edison based device is unplugged from USB charger. I built and tested my circuit on a custom PCB which I am posting here.

1. If Edison is OFF and +5V USB is plugged in. That will turn ON Q1.1 and turn OFF Q2.1 resulting in logic high for PWRBTN# on Edison. Basically that will do nothing for Edison and it will stay OFF.

2. Note that since the Edison module is OFF, the gate of p-channel FET Q1.2 is 0 volts and the pin 2 of R4 is at VBAT while pin 1 of R4 is ground because Q1.1 is ON.

3. Now removing the +5V USB will make the gate of Q2.1 to VBAT thereby turning it ON. This will pull down PWRBTN# pin on Edison module.

4. Once Edison is turned on, net V3P3_Edison will become 3.3V turning OFF p-channel FET Q1.2. This will lead to the gate of Q2.1 pull down to GND by R5. This will bring back the PWRBTN# of Edison high.

5. Note that once Edison is turned on, if you insert +5V USB, nothing will happen.

I hope it helps someone.

Best,

- Aditya

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