I'd like to power an Arduino using a supercapacitor. I already have that part of the circuit figured out. But I want to charge the supercapacitor using an external power source, without the supercapacitor discharging into the Arduino until its voltage gets to 5V. What can I use to do this? Sorry if it's a silly question - my knowledge of electronics is pretty basic. I've been playing with an SCR (datasheet here) but haven't been able to get it to "wait" until the supercap gets to 5V. The trigger current seems to be too low.
An SCR won't do what you want - it will naturally drop about 1 volt across it when connecting the supercap to the arduino so 5 volt in becomes 4 volts out to the arduino. I'd consider using a buck-boost regulator where the supercap (and charger) is on the input side and, the output side connects to the arduino: -
Vin can work from 2.5 volts to well over 5 volts in so you can choose, if you want to activate the arduino at voltage lower than 5 volts.
How is this activated?
The "RUN" pin is connected to VIN by a potential divider and only when VIN rises above a level determined by the potential divider ratio does the RUN pin activate the buck-boost regulator. It's called under-voltage-lock-out.