This project is for a bike tail light that automatically turns on when it gets dark. This can save a lot of battery power during the day where there are lots of shadows like tunnels, buildings, or trees where you want to be visible without having to get off your bike and turn the tail light on. It has a super capacitor that allows the light to stay on when passing briefly in to the light, like under branches of trees or by fences, which would otherwise make the tail light blink randomly. Turn the switch on and let the light detector do the work for you while you ride. The light only draws about 3mA so it should last a long time.
NEW Brighter Version 2.0 of this project is available now! https://www.instructables.com/id/Open-Source-Light-Sensor-Kit-20/
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1 OPT101P Texas Instruments Photo Diode IC http://www.digikey.com/product-detail/en/OPT101P/2...
1 EEC-S0HD104 Panasonic Super Capacitor http://www.digikey.com/product-detail/en/EEC-S0HD1... (I got the idea to use this super capacitor from http://universalelectronix.blogspot.jp/2010/10/sup...)
1 74HC86 NXP XOR Gate http://www.digikey.com/product-detail/en/74HC86N,6...
1 SBH331AS-ND 3 X AA Battery Holder with On/Off switch http://www.digikey.com/product-search/en?WT.z_head...
1 330QBK-ND 330 Ohm Resistor http://www.digikey.com/product-detail/en/CFR-25JB-...
1 30BQ015PBFCT-ND Vishay Schottky Diode http://www.digikey.com/product-detail/en/VS-30BQ01... (I like these diodes because they have very small voltage drop.)
4 5mm RED STRAW HAT LEDs (they seem to be very bright and very affordable) www.ebay.com
1 2" X 2" Thru Hole PCB (I had to hacksaw a piece of PCB to get the right size.)
1 2" X 2" Piece of Double Back Tape
1 Small Bicycle Seat Post Bag with Seat Post Strap
3 AA Batteries