The circuit was designed to accomplish a few things:
- Support on/off toggle via a single button
- Flash the taillights
- Allow battery charging in-circuit without damaging other components
Toggle is handled by a 3 transistor latch controlling an NFET. It is based on this EDN article
. Note I used the NFET since I had it lying around, you could just as easily used another BJT. Flashing is handled by a simple 2 transistor astable multivibrator. You can find other examples here
. Neither circuit is sensitive to in-circuit charging.
I simulated the circuit using LTspice to adjust the values for the various components. I would recommend adjusting the multivibrator values a bit to slow the flashing rate. The switch might also be more responsive with a smaller capacitor in the latch section.
The final design is in the LTspice ASC file. A slightly outdated version of the design (using different transistors) is included in the DCH file.