I'm looking for a simple circuit to light an LED when a SLA's voltage gets below ~11.5 or some preset value. I found this one online but am having trouble understanding it. Nor does it function as it should in a circuit simulator.
I'm trying to work it out mathematically and it shows (and simulator shows) the LED will never properly turn off. At 12.5V lets say, the zener will have ~10V across it, meaning the transistor base must see 2.5V. The BC557's max Vec is .65V, the diode would drop say ~3V at best, probably less, meaning Ve must be 3.65V. Even at 13.5V, Vb < Ve, how is the LED supposed to extinguish?
The description says the LED should illuminate below 11.6V only. Will this circuit work? And if so what am I missing?
Edit: This circuit below seems to make more sense, simulator shows LED lighting at 11.8V and extinguishing at 11.9V. The only problem is it "wastes" the LEDs current to ground while it is off. Can anybody suggest an improvement on this?