Cheap, Simple, 2-Cell Low Power Indicator circuit

[tonyb]

Here is a low power indicator circuit that utilizes only 7 easily avalible components. The transistors are 2N2222 or generic equiv. Potentiometer should be set for the led to come on when voltage input to circuit is 2.2 volts or higher like 2.3 volts if you want more of a warning of discharge condition on batteries.

Sorry for the poor picture quality.

 

[zmoz]

Would it be possible to change a part of that for use with 4 cells?

 

[tonyb]

Yeah you can use it on a 4-Cell input, just put a 33 ohm or a 22 ohm resitor in place of the 10 ohm resistor going to the led. You should set the voltage to 4.4 volts and adjust the potentiometer for the led to just come on.

 

[PaulW]

Re: Cheap, Simple, 2-Cell Low Power Indicator circ

Tony,

Did you intend for the Q2 base to be connected to the Q1 emitter or collector? I'm not a circuit expert, but, as drawn, it looks like Q2 is always off.

Paul

 

[tonyb]

Re: Cheap, Simple, 2-Cell Low Power Indicator circ

Paul your right there was something wrong Q2 base was to go to Q1 collector. A voltage divider is set up between the 10k ohm resistor and the 100k potentiometer. The output of the voltage divider goes to the base of Q1 if the battery voltage is above the set voltage Q1 is on and grounds the input of Q2 keeping it off and thus the led is off. When the battery voltage falls below the set voltage Q1 turns off letting current to flow through the 100k ohm resistor turning on Q2 and thus turning on the led.

 

[PaulW]

Re: Cheap, Simple, 2-Cell Low Power Indicator circ

Makes perfect sense when I think about it. Nice circuit. Thanks for sharing it.

Paul

 

[tonyb]

Re: Cheap, Simple, 2-Cell Low Power Indicator circ

I had it breadboarded and tested the circuit earlier and in tranfering the schematic off the breadboard where the mistake was made. I works nice I plan to use it on my bike light.

 

[Doug Owen]

Re: Cheap, Simple, 2-Cell Low Power Indicator circ

Close to 'bare bones' for sure. You might consider more base drive for Q2. As it is you have a max of what 150 micro amps? At this sort of Ib, 2222 type devices have betas of a few dozen. Call it 50. This is only 3/4 mA for the LED, *at best*. A bit more drive (at the expense of battery life) might be in order?

You can easily make it one part simpler (and cheaper), there's no need for the ten ohms as drawn. Or you could fiddle with hystersis if you leave the ten ohm in, return the 10K to it's lower end (rather than the upper end where it is now). This way, as the circuit gets to the trip point and the LED starts to conduct, the voltage drop across the LED resistor will cause the 'detected voltage' to drop still further, which will in turn turn on the LED harder.... Now the circuit 'snaps on' and doesn't reset until the voltage is a bit higher than it's new threshold.

Neat circuit, thanks.

Doug Owen

 

[tonyb]

Re: Cheap, Simple, 2-Cell Low Power Indicator circ

Thats right it doesn't draw very much from the batteries I'm a minimalist just enough to see the led coming on. I guess you could change my design if you wanted too. I specified a low power red led in the schematic to aid in eff.(brighter). The way the circuit works is as battery voltage begins to drop led begins to dimly come on giving you prior indication of battery discharged condition until it comes on completely when discharged voltage is reached. If it had hysteresis the led would just snap right on as soon as batt. voltage dropped below thershold and would give no prior warning of discharge.