Introduction: Open Source DIY Automatic Bike Tail Light 2.0

About: After being laid off in 2009, I got rid of my car to save money. The difficult transition from a car to a bicycle led me to start Solarcycle and design USB chargers that keep smart phones alive.

An improved tail light that automatically turns on when it gets dark. Improvements include a custom PCB for easier assembly with better components, better component layout, and brighter LEDs.

Please click below to support our Kickstarter campaign to make this kit available for purchase online!

This circuit can be used for a night light, Arduino sensor, bike tail light, and much more! Save battery power outdoors 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.

PCB Gerber Files and Circuit BOM are available for Download Below

This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.


QTY Description

1 OPT101P Texas Instruments Photo Diode IC (This sensor detects visible and UV spectrum for great consistent performance)

1 EEC-S0HD334H Panasonic Super Capacitor

1 74HC86 NXP XOR Gate

1 SBH331AS-ND MPD (Memory Protection Devices) 3 X AA Battery Holder with On/Off switch

1 ERD-S2TJ101V 100 Ohm Panasonic Resistor

1 VS-19TQ015PBF Vishay Schottky Diode (I like these diodes because they have very small voltage drop)

4 WP9294SEC/J3 Kingbright Company LLC 5mm RED STRAW HAT LED

1 PCB (Gerber files provided)

1 2" X 2" Piece of Double Back Foam Tape

3 AA Batteries

Solder and Soldering Iron Required

Estimated Completion Time: 10 Minutes

Step 1: Assemble the Sensor Circuit

Solder the components to the PCB according to the labels on the PCB and the schematic. The notch on the Photo Diode IC and the XOR gate should be facing toward the top of the PCB as indicated on the board. Screw the leads of the battery holder to the Power screw terminals with the Red wire connected to the "+" terminal and the Black wire connected to the "-" terminal. Put the Double Back Tape on the battery holder above the switch then stick the PCB to the tape with the Photo Diode IC facing out. Insert the AA batteries and test the circuit by turning the switch on and putting the device in a dark space or covering the Photo Diode IC. When you put the device in the light, it should power down.

If you prefer a faster instant on and off response you can remove the super capacitor.

Congratulations you're done!

You can also encapsulate the circuit in clear epoxy for a water resistant coating, but I don't know what effect it will have on the sensitivity of the light sensor.

How the Circuit Works:

The key for this analog circuit to work is the CMOS XOR gate chip, which has a wide supply voltage range from 2 to 6 Volts. This allows the circuit to work off AA batteries and a 5V Arduino or USB output. Also, the large diode has a very small voltage drop, which allows the voltage to stay above the forward voltage of the LED's. The high quality photo diode IC has wide voltage supply range of 2.7 to 36 Volts, with a voltage output of Vsupply-1.15 and up to 15mA, which is just enough voltage to activate the XOR gate. If a TTL XOR gate was used, the voltage range would be 4.5 to 5.5 Volts and the photo diode output voltage would be too low to activate it with AA batteries. The super capacitor acts as a tiny battery to even out any power fluctuations that may turn the light on and off too fast.

Step 2: Many Other Uses! Night Light, Arduino Sensor, Bike Light, Etc.

Night Light

Set this light in a room to act as a night light that automatically turns off when daylight shines in to the room.

Arduino Sensor

This circuit has a sensor output, which can be wired to the Analog Input of an Arduino. This sensor can be powered by the AA battery pack or by the 5 Volt Arduino pin.

Bike Light

Cut an opening in a small bike bag to match the profile of the LEDs and the Photo Diode and cut a hole for the switch (I've found that Topeak Phone Packs work well). Insert the circuit in to the bag and close the bag. I found that wire cutters do a good job at cutting through this material. Attach the bag to the bike seat post and your done!

Arduino Contest

Participated in the
Arduino Contest

Sensors Contest

Participated in the
Sensors Contest

Full Spectrum Laser Contest

Participated in the
Full Spectrum Laser Contest