Some electric bicycles come with LED tail-lights that are powered from their main (24V, 36V or 48V) battery. This is convenient as the tail-light comes on as soon as the main key switch is on and because there are no AAA tail-light batteries to manage. But because these electric bicycles are also marketed in Europe the tail-lights are non-flashing designs (certain EU countries have laws that prohibit flashing bicycle tail-lights). But here in the states, flashing tail-lights are the norm and most riders prefer them, so this instructable will show how to build a flashing LED tail-light that will work (with some component adjustments) with an input voltage range from 5V to 48V.
Step 1: Rev a Schematics
The Rev A design works for 24V, 36V, or 48V electric bicycles (it will not work for electric bicycles that have internal DC-DC converters to generate lower-voltages to run the tail-light .. see rev B for a solution to that problem). The design uses an LM7812 voltage regulator to generate a 12V supply for an LM555 timer circuit which handles the LED flashing. Since an LM7812 can only handle a maximum input voltage of 35V, the circuit must be configured using Zener diodes. A 24V system will need no zener diode and both D5 and D6 positions on the circuit are shorted using a jumper wire. A 36V system will require one 11V zener in the D5 position and a jumper wire in the D6 position. A 48V system will require an 11V zener diode in both D5 and D6 positions.
Step 2: Rev a Circuit Board Layout
Using a CAD program, I created this layout for the Rev A design.
Step 3: Rev a Circuit Board
Here is the fabricated circuit board. There are many circuit board manufacturers that can fabricate a circuit board from CAD designs (e.g. batchpcb.com)
Step 4: Connect and Verify the 12V Power Supply
Start the build by inserting the components for the 12V power-supply section (U2 and the correct configuration of D5 and D6).
Step 5: Add the Remaining Components
Once the 12V power-supply is verified then turn off the power and add the rest of the components. Once all the components are installed, turn on the power again and you should see the LEDs blink at about a 1.4Hz rate (85 flashes per minute). You can optionally adjust the flash rate by changing the R1/R2 values. There are online 555 timing calculators that can help you with this experimentation (try a Google search for "online 555 timing calculator").
Step 6: Rev B Schematics
Rev A cannot work on voltages less than 12V and since there are some electric bicycles that use DC-DC converters to generate a lower voltage (e.g. 6V) for the tail-light, Rev B was designed to handle this situation. Rev B is similar to Rev A, but it adds a few configuration options. For low voltages, U2, D5 and D6 are not installed. Also the following changes are required:
1) cut the trace between J3 and J5
2) jump (add a short wire) J3 to J4
3) jump J6 to J7
4) jump J8 to J9
Step 7: Rev B Layout
The Rev B layout is complete, but is has not been fabricated yet.
Step 8: Appendix A: Resistor Calculations
Here are a couple of spreadsheets to help with resistor value selection for R1, R2, and R3.
R1 and R2 values can be calculated using 555 Timer.xls
R3 values can be calculated using LED Current.xls