Power On Hack

A misfeature of the iMX53 board is that it powers-on "Off", and you have to press a button to make it actually turn on. This is problematic with the board inside a case, so I've hacked together a timer circuit to effect a button press automatically.

The circuit uses a 555 timer IC (CMOS version). The R1/C1 network drives reset after power is applied. The circuit relies on the threshold voltage for RESET being lower than for TRIG, so the timer is triggered when it leaves reset. R2/C3 determine the length of the pulse, which must be long enough that TRIG is no longer active when it finishes.

The connection is made to the side of the power switch next to the reset switch. This must be grounded, which is done by Q1. I've added an LED for diagnostics.

I've assembled the circuit on a small piece of veroboard and fixed it above the SD connector. GND can come from the shell of that connector. 5V must come directly from the power connector.

This does seem to work reliably, as long as the system is turned off for at least 5 seconds. If the mains is pulsed off for less than that it doesn't work. Presumably an extra resistor somewhere would fix that.