I have an AC voltage source which runs at about 6V to maximum 70V (usually around about 14V @ maximum 8W) which is then rectified in a diode bridge rectifier and later regulated to stable 5V. From this 5V I run a microcontroller.
Depending on the AC voltage I want to either include more or less capacitance (C1, C2, C3) before the bridge rectifier. To do this I use SPDT relays which are switched from the 5V rail using the microcontroller.
Now what I would like to do - mainly due to size (or rather height) constraints - is to get rid of the relays since they are just way too large. My idea was to use two back to back MOSFETs (with the sources connected) to replace every relay. I could drive them with my 5V rail through an optoisolator (like the VOM1271). But this way I'd have a normally open switch.
How would I get this to be normally closed?