This simple low voltage tester circuit can be used to as Battery voltage monitor for other voltage sources that have problems, by LED display and alarm sound. Which this circuit is used to check voltage of 9-volts battery.
We use a LM339 Quad Comparator 14pin IC is base of this circuit and a few parts (6 pcs.) include two resistors, LEDs, 10K-potentiometer without capacitors. When attack the probe input is connected to 9-volts battery. The battery current will flow through positive supply pin 3 of IC1 and Negative supply pin 12 as source of this circuit.
At the same time, the battery current will flow through to a VR1 to the input pin 5(non inverthing) of IC1,then other way the current will pass R1-1K to limit current to the 6-volts zener diode at pin 4 (inverting). as the reference voltage. Both ways is compared the voltage inside IC1
The LED1 and Buzzer is indicator of circuit by R2 control neat current flow them.
When the input voltage is higher than 6-volts will cause high at output but LED1 will still not works and not soud buzzer sine each leg them are connected to positive suppy and output pin. But when the input voltage is lower than 6-volts then will makes LED1 glow brightly and buzzer emit sound. Which we can set the level voltage input with adjust sentility of circuit by control VR1.
The component list
R1,R2_________1K 1/4W Resistors
IC1___________LM339 Voltage comparator IC
ZD1___________Zener Diode 6V 0.5W no. 1N5233
LED1__________Any color you like.
P1,P2_______Probes (See Notes)
Perforated board, wires, socket for IC
I experiment this circuit on breadboard and them will operate from 9V to 12V now we take it to check 9-volts battery. Which a good available battery will have voltage about 8V.
Here is video of lectronicsNmore that make this project worked very great. Thanks a lot.
We use volts meter to measure voltage at input probe. and take it to the 8.3V then adjust VR untill LED1 glow and buzzer emit.
Then apply the probe to lower level dropper voltage.
Please watch video below for better understand.