Charge Lamp Driver

Some of my motorcycles, for one reason or another, do not have working circuitry to drive the "GEN" lamp (which indicates that the charging system is not working).

Initially, I designed a circuit to light the charge lamp when the battery voltage was "low", which could be interpreted to mean the charging system wasn't doing its job.

I later heard of a product from Euro Moto Electric that is meant to not only light the "GEN" light if the battery is low, but also *flash* the GEN light if the battery is way high (>= 15.5v).

I thought that was an interesting challenge, so I designed that bit also. Below is the result, with both circuits on one sheet.

The low circuit works by feeding base current into Q1 if the voltage is high enough to get past D1, a 5.6v Zener diode (and the voltage divider ahead of that). If Q1 is on, then Q2 is held off, and the lamp does not light.

The high circuit comprises, first of all, a LM555 timer circuit configured as an astable multivibrator (oscillator) running at a Hertz or two. The output drives Q4, which is "wired-or" with Q2, so either transistor can light the "GEN" lamp. The 555 oscillator is normally prevented from running by R4, a 1K ohm resistor to ground from the reset pin (4).

When the voltage climbs to excessive levels (designed for 15.5 v), then Zener D2 conducts, which pulls base current from Q3. Q3 turns on, and pulls the Reset pin (4) high, enabling the oscillator.

There is one clever bit. I found that, as Q3 slowly turns on, then the 555 timer enters a "linear" region where the output pin (3) starts to rise from 0, but the oscillator doesn't start. This causes the lamp to be "full on", which is not what we want, since that indicates low voltage, and our problem is high-ish voltage. So diode D3 is there to help pull up the Reset pin (4) as the output pin (3) goes high. As a result, the oscillator starts right up as Q3 starts to conduct.

Coming "soon" - the PCB version, with link to PCBWay where you can purchase the bare board. (Boards are being produced now, and when I get them and test them, I'll make the project public.)

Get Gerber and Drill files here.
Get KiCad files here.

Bill Dudley
September, 2024