/diy/ - Do It Yourself

>>2599393
A 5.5V supercap is like a dollar or two. If you remove the power LED (and any other LEDs that might light up during use) from the arduino, then operating it in power-save mode will draw a couple of µA. Say you can handle a ∆V = 2V as the cap discharges, we get:
>I = C*dV/dt
>I = C*2/30
>I = C*15
>C = I/15
If I=10µA, then:
>C < 1µF
Considering there's probably a 10µF MLCC on the board, you're probably fine without an extra cap if you design your circuit properly. Note that having a negative voltage across your linear regulator is likely out of spec, so you'll probably need a diode on the output. The 0.6V drop will be a bit of a pain, but the arduino should run just fine regardless. Assuming the I/O that activates the ignition is something like a MOSFET that doesn't have a significant drain current and will run just fine at 3V or whatever.

Also consider a CMOS 555 timer, it uses 250µA quiescent current but it's probably a lot simpler.