This is a simple low voltage tester circuit. Even it can be a battery voltage monitor for others voltage sources that have problems. By showing on LED display and alarm sound. Which this circuit holds to checks a voltage of 9-volts battery.
Battery voltage monitor description
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 connects the probe input into the source, 9-volt battery. The current get in the circuit. Two pins are important to see right now.
- Positive supply pin is pin 3
- Negative supply pin is pin 12
At the same time, the battery current will flow through to a VR1 to the input pin 5 (non inverthing) of IC1.
Then, another way, R1 pass the current to limit in save level for the 6V Zener diode at pin 4 (inverting). It has the reference voltage. Both ways is compared the voltage inside IC1.
The LED1 and Buzzer is indicator of circuit. R2 controls the proper current through them.
When the input voltage is higher than 6-volts. It will have a high voltage state at output. But LED1 will still not works and not sound buzzer. Because each their legs are connected to positive suppy and output pin.
But when the input voltage is lower than 6-volts. Then, it makes LED1 glow brightly and the buzzer emit sound. Which we can set the level voltage input with adjust sensitivity 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
Application of battery voltage monitor
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.
I made a divide voltage as ladder level with normal diodes in series as Figure 2. A LED is used as load and R-resistors act as limmiter of LED.
We use volts meter to measure the voltage at the input probe. And take it to the 8.3V. Then, adjust VR until LED1 glow and buzzer emit.
Then apply the probe to lower level dropper voltage.
Please watch a video below for better understand.
‘Keep reading: Simple level voltage detector »
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