parts:
R1 10k ohm potentiometer
R2, R3 1k ohm resistor
D1 6 volt zener, 1 watt
Q1 2N3904 npn transistor
LED1 light emitting diode

all resistors are 5 or 10 percent tolerance, 1/4-watt
R1 controls the trip-point of the circuit. when the voltage falls below the trip-point, the led lights. adjust it to the desired level. to use this circuit to monitor a 6v battery, substitute D1 for a 3.3 volt zener diode and adjust R1 accordingly.

if tis circuit is a 12V battery monitor, where do we connect our 12V batt? Where is the input and where is the output? Is the +12V the Vcc?

I see this is a old topic but I would like to know how to make the monitor light the LED when the voltage drops to a certain point without the potentiometer. This circut would work great for what I need but I do not want to have to adjust each of the circut I will need to build.

I need to light the LED when the voltage drops to 10vdc.

Thanks

Hey Kevin,

It would be possible to substitute a regular old resistor in place of the variable resistor, but that would require some derivations, and it would depend on what type of transistor, resistor tolerances, etc.

If you calculated it you'd probably get close (a couple volts) but it is way easier to use a variable resistor and an adjustable power supply: set the power supply at around 9-10V and then adjust the potentiometer until you see it light up. Adjust the power supply over 10V and see if it cuts off.

Once you know it works, you can then use a multimeter to measure the resistance across the leads on the potentiometer then find out what value resistors you need to get it to work right. Then you can substitute two resistors for the potentiometer (you will need to measure the resistance from common to both leads of the potentiometer)

You can also use a 9v wall power supply or a couple batteries in series to get your test voltages.

Hope this helps, let me know if you try it. Variable resistors by the way are easy to get at radio shack (convenient)

As far as the last post is concerned, the battery positive is connected where it says +12v, and the battery negative is connected where is has those three horizontal lines (ground)

I built this circuit, and the LED stays on until the voltage drops below the trip point. I thought that the LED would light when the voltage dropped below the set point ? Am I missing something or did I construct the circuit wrong ?

Pls,i want 2 know d circuit diagram for 12volt battery monitor