How can I make this "automatic turn off" work using some GPIO pins on a MCU?

Question

I've made a simple measurement circuit and I wanted to make it work like that:

1. user pushes a simple tactile switch and device turns on
2. user can push the same button to measure again (and keep the turn off counter up)
3. device turns off automatically by switching the whole circuit down

The MCU's power consumption is not the problem. there are a lot of other stuff and I wanted to turn the whole circuit off at once so the sleep modes are not considered here. here's the schematic I've used which seems to be problematic:

^{simulate this circuit – Schematic created using CircuitLab}

The problem is, that the device turns on instantly, and stays on even when the output IO is pushed Hi. I read a value of about 4.3V on the gate of the mosfet. when I short the 47K resistor, the device turns off as expected so it seems the mosfet does not turn off completely. I've tried these values with no luck:

• Changing R4 to 10K
• Changing the diode to a 470R resistor. in this case device turns on and off rapidly
• lowering R1 to 10K.

What is the problem? is it possible to make it work without adding another component?

thanx for the time and knowledge.

Answer 1

When your device turns off, you still have a leakage path between the GPIOs and 5V IN. You need to block that somehow. Unfortunately it’s going to take some additional parts to do that.

Try this (simulate it here):

The additional nFETs isolate the CPU from leakage current when the power is off, while allowing the switch to be monitored when the power is on.

Here's how it works:

• To power the system on, the user presses the power key. Q2 turns on, turning on Q1 and thus the power
• user releases power key, gate pull-up Q4 gate latches the power on
• Power key state monitored by Q3
• After timeout, MCU powers the system down by pulling Q4 gate low

Maybe with some cleverness this could be simplified. Bear in mind that FETs are cheap, and this setup draws practically zero standby current.

MORE: Here's a version that uses bipolar transistors instead of nFETS (simulate it here):

The BJT's leak less than the nFETs so the off-state draw is reduced. Otherwise the principle of operation is the same.