A quick note about emergency lighting

I've been thinking about what would happen if the main battery went dead — the whole vehicle would go dark and silent. I had been thinking about adding separate markers on the steering tubes of the back wheels, but it would quite a bit of extra work. It dawned on me that I can set up emergency lighting with a relay and a couple batteries: as long as the power was on, the relay would stay on; if it shut off, the battery would connect to several of the lights on the string: i.e. far front, far rear, and the eyes … 6 lights at 20mA would be 120mA and would drain a 2200mAH battery in 18 hours, but if I got it down to say 10mA total, then it would last for 200 hours. If I went with green then it would be most visible. I dug up a very small relay with a 12-volt coil and it draws 15mA itself, but I found I could get it down to 7mA if I wired it in series with a capacitor (to let it latch) and a resistor (to keep it latched.)

I changed my mind and decided to make a pulse-width modulator with a low-power 555 timer and just hold the reset pin low via the control computer — if it failed, the 555 would start pulsing and drive the LED's.