BJT voltage controlled switching for battery powered circuit

A circuit I did at work a while ago. The values of the transistors are likely different from what I actually used as the switching voltages are different values but all the same concepts still apply. A red/green LED combination for indicators on the right and a load being switched on the left. Intended for either AA or AAA batteries gives a realistically possible voltage range of between 1.6V-3.2V for the voltage source here. Decreasing the voltage will switch on the red low voltage indicator (here ~2.1V) and will also reduce current through the load on the left and switch it off completely at some critically low level (here ~75mA). These will not switch back to their regular states until a high enough voltage is reached (here ~2.5V-2.6V). When switching the circuit on and off the load will not be active unless the voltage is already above the high threshold voltage. This would be done by swapping out batteries, and we wanted to ensure the batteries were at least somewhat decently charged first to avoid the load potentially cutting off very shortly after turning it on with a set of old batteries. In particular the aim was on minimizing cost by using inexpensive components and reusing the same values as much as possible. In actuality the circuit we have turns the red LED on @2.0V and disconnects the load @1.76V then won’t turn on again until ~2.35V. I suppose there is a slight chance batteries might be stable in the small gap between the high threshold voltages of each side (maybe 10s of millivolts) where the red light is off but the load isn’t on. This should only be possible if I’m VERY unfortunate and have poor resistor tolerances which I shouldn’t with +/-1% resistors so I’d just rather not worry about it.

published 7 years ago