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.

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Hacking the MP3 player

I took apart the MP3 player and did some measurements on the switches.

State

Power button

Play button

off

1.5V/0V

0V/0V

on

3.2V/0V

2.8V/0V

playing

3.2V/0V

2.8V/0V

It's pretty convenient that I can detect power-on through the switch voltages. Whew. For playing I'll need to check that there's some signal on the output which could be a bit tricker. I think I'm going to compare to some arbitrarily low voltage and run the comparator output through a diode to a capacitor. The capacitor would discharge slowly with a resistor through the base-emitter junction of a transistor: the collector could be attached to a resistor to 5 volts — while playing, it would stay very close to 0 volts (logic 0) but if there was no ouput, it would slowly rise to 5 volts, eventually tripping whatever logic input.

I wired up little wires to the switches and battery case to hook it to the logic circuits and power. What a pain to solder to this surface-mount stuff:

Here's a super close-up of the play button.

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