LM741 OpAmp Voltage Indicator

Posted on Feb 5, 2014

A simple voltage status monitor built around the LM741 operational amplifier. These circuits are the best we have found in terms of accuracy, simplicity of operation, and cost. The LM741 is an operational amplifier. The wide range of uses of this simple and cheap integrated circuit do not need to be mentioned here - of importance is that an output voltage is turned

LM741 OpAmp Voltage Indicator
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on (or off) automatically when a measured voltage (e. g from a battery) reaches the same value as a reference voltage (manually set by the user). The circuit diagram shown above is configured to give a high voltage indication - LED turned on when the measured input voltage is above a certain level. The 100KOhm variable resistor is used to manually configure the voltage over and above which the LED will light. If the reference voltage arriving at pin 2 of the LM741 is lower than the battery voltage arriving at pin 3 via the voltage divider created by the variable resistor, then the output from pin 6 of the LM741 will be low. Therefore the LED will turn on as it has voltage across it. If on the other hand the reference voltage on pin 2 is higher than the battery voltage on pin 3, the output from pin 6 will be high, and so as there is no voltage across the LED, it will be off. The Zener diode should be chosen with a zener voltage of around half that of the target voltage - e. g. for a 12. 0 Volt indicator, a 5. 6 Volt Zener diode could be used. In order to obtain a low voltage indication - useful if you would like to stop using a battery before it is drained too deeply - the output from pin 6 is still directed through an LED (and current limiting resistor), but this time it is connected to ground (0V) rather than to the positive input voltage Vin as shown in the amended circuit diagram above. The output from the LM741 will...

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