For the new lighting install in our camper van i wanted multi-way switching so that we can turn the interior lighting on and off from both the bed and the sliding door, so that we are able to turn the lights on when going into the camper at night and off when going to sleep at night without having to clamber about. I also wanted to be able to turn the lights on and off above the kids end of the camper van from the bed (the missus’ idea for when they wake in the night)
This would have required a decent amount of wiring and would have limited our switch selection to SPDT switches – which i cant find any i like. I thought about using latching relays but the majority of them draw power when they aren’t in use – not really suitable for a camper van with leisure battery, although it is only milliamps.
I came across an interesting schematic here which uses a latching relay and a simple but clever trigger circuit comprising of a capacitor, resistor and diode per coil (two coils per relay, one to set and one to reset). The 12v supply charges the capacitor through the resistor and when the capacitor is drained via the relay coil and a diode it toggles the relay and connects the opposite capacitor to the 12v supply ready to charge when the button is released. The resistor and capacitor arrangement is required as without it the relay would oscillate whilst the button is held.
I’ve prototyped the circuit using an HFD2/012-S-L2 relay (i’ve since found the HFD3/012-L2 at a third the cost, nevermind, next time…) and it works perfectly, toggling the relay on each button press with no oscillation and super low quiescent current at 10nA or less, my meter doesn’t measure any lower 🙂
I’ve designed a board to get manufacturerd and designed a enclosure which i’m going to print, I’ll post more info when they arrive / complete.
My version of the schematic included incase the original disappears, along with a board render and enclosure design:
Simon's tech blog 