I replaced the incandescent turn-signal bulbs in my Kawasaki Concours motorcycle with LEDs because 1) they brighter 2) they last longer and 3) they require less current than incandescent bulbs. Because they require less current, the standard flasher unit (the little box that makes your turn signals blink) senses the low current and thinks there is a bad bulb and it therefore won't function properly. So the standard flasher unit needs to be replaced with one that is compatible with LEDs. You can buy these, but where is the fun in that? I designed a simple LED-compatible flasher circuit and retrofit it into the original flasher case.

Step 1: Locate and Remove the Flasher Unit

First locate and remove the flasher unit. This will be different for each vehicle. Consult your service manual for the location (or listen for the clicking noise when the flasher relay is operating).

Step 2: Remove the Old Circuit From the Flasher Unit

Some flasher units can be disassembled. If you can disassemble it, then by all means do so. In this case they had potted the timing circuit, so I used a hacksaw to cut open the flasher case.

Step 3: The Old Circuit

Here's the old circuit (encased in potting material).

Step 4: Re-use the Original Connector

We'll re-use the original connector and solder our new circuit to it.

Step 5: The Flasher Circuit Schematic

Here's the new flasher circuit schematic. It consists of a 555 timer circuit and a power transistor. The power transistor is probably way overkill, (it is rated at 10A) but it's what I had available. You can vary C1, R1 and R2 for to vary the flash rate and duty cycle. For the R and C values shown, then flash rate is about 1.4 Hz (which is about 85 flashes per minute). There is a ton of information about the 555 timer on the web, including programs that will calculate the R and C values for you.

Depending on the type of components that you choose, the R and C values can vary by 10% or more, so you may need to tweak some values. C2 should be a ceramic bypass cap, but it does not appear to be essential, my circuit worked fine either with or without C2. C1 should be a ceramic cap, but if you decide to use a tantalum or electrolytic cap for C1, then make sure that you install it in the correct polarity, or it could explode. If you have any doubts about your electronics capabilities, then you should not build this circuit.

Step 6: Parts List

Here's the parts list with DigiKey part numbers listed.

Step 7: The New Circuit Retrofitted

I point-to-point soldered the new circuit to the original connector. It's not pretty, but it works. An oscilloscope can be handy at this stage in case you need to debug the circuit. If everything is working correctly the D1 LED will flash at the desired rate.

Step 8: Button It Up

Once everything was tested and working properly, I used epoxy glue to enclose and seal the new circuit into the lower half of the original case. When the epoxy was cured, I re-installed the flasher in my Concours and it has been working well ever since.

Step 9: Voltage Filter Cap Fix for V12V Voltage Ripple Problem

An extra filter cap C3 was needed when this circuit was tried on a 1982 Yamaha XV750 Virago.
The problem was caused by excessive AC ripple voltage on the V12V voltage supply
when the motor was running at over 2000 RPM. The value of C3 is not super critical,
any value around 1000uF should be OK. C3 should be placed as close as possible to U1.

Thanks to Pofarm for testing and verifying this fix.