Delayed shut-off PMIC?


Thread Starter

Dan Hurly

Hello all,
I've been working on a self powered device that uses a Raspberry Pi for its "brain".
because of the way the Pi is designed, I need a power management circuit that will interrupt the Pi's power supply after it has gone through an orderly shut-down. Normally, the Pi receives its power through a buck supply that's driven directly off a generator but a set of ultracapacitors takes over when the generator isn't active. When this is the case, the Pi runs some event dependent script for a timed shut-down after an elapsed time. Unfortunately, rebooting the Pi when it's in standby requires some operator interface unless the power has been completely interrupted so I've hacked up this circuit to disconnect the Pi from the capacitor bank at some interval following the shut-down sequence.
Can anyone see any potential problems with this approach?

I'm planning on using some LL FETs that I have laying around. The power requirements are minimal (< 15 watts).

I forgot to draw in the drain resistor for the first FET's gate (10kΩ to ground) and figured on tinkering with the unmarked capacitor and resistor values to get the delay I need: It will probably need to be around three to five seconds. The zener's just for the sake of my paranoia. Will this work?

Oh yeah, I almost forgot: The 5V output of the power converter is the power feed for the Pi.

It's easy to turn off the Raspi by this circuit with GPIO=low.
How are you going to power up the circuit when power returns? You will need GPIO=high but the Raspi has no power to set this signal. A dead end...

You will need to incorporate some kind of 'power good' signal to re-activate this circuit.
Hello Hurold,

I'm just assigning one of the GPIOs as always high upon boot-up (using some python code activated by the cron daemen). At shut-down, another python script executes the GPIO cleanup command and the drain resistor that I forgot to draw pulls the first MOSFET gate low. The intent is for start-up of the Pi to be brought about by the reintroduction of its primary power (i.e. the generator is active again) and that this circuit would allow enough time to complete a safe shut-down before disconnecting the secondary power supply (the caps). I figured the Pi is going to need a little more time (very little) after the GPIO cleanup is sent to safely finish locking its brain down.

Where in your circuit is the main power connected? If it is input through the top left wire (from the ultracaps), then there is no way the Pi will ever strat up because this circuit permanentgly shuts off power,
If the main power is connected elsewhere, please post an updated schematic to show where main power is connected. This will help getting a common understanding of this circuit.