I recently found myself needing a simple circuit which could detect a low battery condition of a sealed lead acid setup, but also with a hysteresis function i.e. don’t re-enable the output until the battery voltage rises to a certain threshold.
The internet is practically exploding with low voltage detection circuits but many are quite complicated with exotic ICs and other fussy details.
Geez man. All it takes is a single comparator and a two resistors (three for hysteresis).
Okay so my circuit has a little more, that is because making something that is actually useful requires a bit of extra stuff.
With the above component values it will cut out at 11.2V and re-activate at 12V, which is good for most sealed lead acid batteries. There is also second comparator – this is purely acting as a logic inverter, because I needed a negative logic output. If you don’t need it, leave it out. One of the cheapest and most available comparators – the LM393 has two units anyway, so this works out well.
The main guts of the circuit is R1, R2, R3 & U1A. R4 & R5 are a simple voltage divider to get the input voltage inside of the 5V operating range of the comparator. R6, R7 R8 & R9 should be left as is.
Because I’m using fixed resistors, I’ve worked backwards, from a ‘components first’ approach, simply working out the formula for the circuit then plugging a variety of E24 resistor values in until I got what I wanted. I find this easier than working from a ‘results first’ approach i.e. starting with the desired voltages, to then being told by your workings you need a whole bunch of resistor values that don’t exist!
- VCC (Constant – 5.0): The output of the 78L05
- VL (Constant – 0.1): The voltage the LM393’s output transistor can pull down to. Yours may vary. The expression containing this term can be omitted if you are happy to call it zero.
- VIl: The low battery input threshold voltage
- VIh: The high input threshold voltage i.e. re-enable output when voltage reaches this level
If you wanted to adjust my thresholds, assuming a 12V setup, focus on R1, R2 & R3. Leave R4/R5 as is. If changing to a different voltage / type of battery, then R4/R5 need to be adjusted to bring the voltage at pin 2 within a 2-3 volt range.