Motor & headlight interlock

Since this vehicle is a motorcycle, the traction motor should not be operated without an interlock requiring the headlight to be illuminated either in a high-beam or low-beam state. So, the idea is to tap into the 12V going from the handle bar to the lamp from either the high or low beam light circuits to energize a small relay. This little relay will then interrupt the throttle signal, making the speed controller provide no power to the motor.

Sounds simple. Ah, but life isn't so simple. I can't just tap the high-beam wire & tap the low-beam wire, then tie them together so when either one is powered. Otherwise, both filaments stay on since one feeds the other where they meet which would be just before the relay coil. So, I need to put some diodes in to prevent this back-feeding. Also, when using the handlebar control to switch from high-beam to low-beam I noticed there is a tiny bit of time when neither circuit is powered. This can make the small relay drop it's contact, which is probably not a thing we want happening when underway. So, a capacitor has been added to power the relay coil for a brief amount of time during hi-low or low-hi switchover. The capacitor has a rating of more than twice what I needed so the relay will stay energized several seconds after the lighting is switched off.

Here's the schematic for that part of the circuit.  Part numbers are Radio Shack parts.  In the photo, I also used an optional relay socket in case the relay needed to be switched out quickly.  So far, the relay seems to be operating flawlessly.  So, a quick switch out socket probably isn't needed.