This battery level indicator offers (2) incandescent lamps that light up progressively as the voltage increases. It is an adaptation of the 12V battery level indicator and is suited for motorcycle application. The reference voltage is from a 6.2V zener regulator and an LM2903 dual comparator switches the outputs. High current 2N4403 transistors drive the lamps.
This circuit was suggested by a reader. His requirements are as follows:
Two threshold voltages (12.5 & 13.5V)
Incandescent lamps (approx 70mA @ 12V)
D1 is the voltage reference zener. Tied to this is a string of divider resistors (R2-5) that set the two fixed voltage levels. R6 & 7 form a voltage divider to that reduces the battery voltage by a factor of 3. U1 is an LM2903 dual comparator that compares the voltages from the two dividers. To obtain the 80mA lamp drive capability, the comparator open collector output drives a PNP emitter follower transistor (actually a composite transistor connection, considering the open collector transistor of the IC).
Adjust R2 for 4.5V at U1-3, and then check for 4.17V at U1-5. U1-2 should run at Vbat /3.
What I learned
Something is learned from virtually every project and this one was no exception. The first was that incandescent lamps draw sufficient current to cause a ground loop voltage between the power supply and the indicator. This caused oscillation that resulted in a sloppy threshold –yes, comparators can oscillate! Adding C1 & C2 corrected this problem. Second, I learned that my ancient HP has significant output resistance when the meter is set to measure output current (actually the meter shunt resistance) –this too caused a sloppy threshold. Third, is that incandescent indicator lamps are all but obsolete –this is due the tremendous influence of LEDs. I really had to dig through my junk to locate two incandescent indictors.
Fuse protection (no fuse required)
I opted to make it short-circuit proof by adding a resistor in series with the positive rail of U1. As a result, even if the IC shorts (as they sometimes do), R8 supports all the voltage safely as it runs within its 250mW power rating. D2 provides reverse polarity protection in which the 70mA reverse current is blocked so it cannot destroy the IC. Everything else has series resistors that offer protection, and the incandescent lamps also have a finite series resistance.
For those who wish to use LEDs
LEDs are now the indicators of choice. To adapt the board for LEDs, remove R9, R10, Q1, Q2, PL1, PL2. Connect the LEDs between the comparator outputs and the cathode of D2 –use an appropriate LED series dropping resistor such as 1K or so. The actual resistor value may be selected for desired brightness.