LM741 OpAmp Voltage Indicator

Plans for a simple voltage high/low indicator built around an LM741 operational amplifier

We have previously published articles* describing electric circuits (1) which can be used to indicate when the voltage of a power source (for example, a battery bank (2)) either reaches a certain point, or falls below a certain point.
* Read Make a Simple Battery Status Monitor (3), TL431 Battery Voltage Monitor (4).

This article will introduce plans for a 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.

LM741 Operational Amplifier

LM741 Operation Amplifier integrated circuit

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 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).

Therefore, if we want an LED (5) to light up when a solar battery charger (6) reaches a certain voltage for example, we can achieve this with an LM741.
You can buy the LM741 (7) in the REUK Shop (8).

The LM741 specification sheet (9) can be downloaded here.

LM741 High Voltage Indicator

LM741 Op Amp High Voltage Indicator Circuit Diagram

The circuit diagram shown above is configured to give a high voltage indication. The 100KOhm variable resistor (10) is used to manually configure the voltage over and above which the LED will light. If the voltage arriving at pin 2 of the LM741 is greater than the voltage arriving at pin 3, the LED lights thanks to the output from pin 6. At all other times the LED is off (as is the output from pin 6).

The Zener diode (11) 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.

LM741 Low Voltage Indicator

LM741 Op Amp Low Voltage Indicator Circuit Diagram

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 resitor), but this time it is connected to ground rather than to the input voltage Vin as shown in the amended circuit diagram above.

Note that this circuit will only operate with input voltages in excess of around 3.8 Volts and so it cannot be used with a 2 AA battery charger.

NEW For an LM741-based circuit plan for a low voltage indicator specifically for a typical 12 Volt battery or battery bank, click here to read our article 12 Volt Low Battery Indicator - LM741 (12).

Configuring the Voltage Indicator

In order to set up the voltage indicator a variable voltage source should ideally be used. Using a digital multimeter set the voltage to the desired light up LED value, connect the voltage source to the circuit, and then turn the 100K variable resistor until the LED is just turning on/off. It is now configured to light up (or turn off) the LED at that exact voltage.

If you do not have a variable voltage source - as is the case for most people - then you need to find a way to supply the desired reference voltage to the circuit for configuration. If for example, you want to light up the LED when a solar battery charger has fully charged some batteries, connect fully charged batteries to the solar panel and then connect the whole thing to the circuit while you adjust the 100K variable resistor.
Similarly if you want the LED to light up when a 12V lead acid battery is 25% discharged (i.e. you measure a terminal voltage of 12.00V on the battery), then wait until your battery has discharged approximately this far and then set up the indicator circuit.

Web Link References

(1) http://www.reuk.co.uk/electric-circuit.htm
(2) http://www.reuk.co.uk/Interconnecting-Batteries-for-Battery-Bank.htm
(3) http://www.reuk.co.uk/Make-a-Simple-Battery-Status-Monitor.htm
(4) http://www.reuk.co.uk/TL431-Battery-Voltage-Monitor.htm
(5) http://www.reuk.co.uk/buy-5MM-RED-LED.htm
(6) http://www.reuk.co.uk/Basic-4-AA-Solar-Battery-Charger-Plans.htm
(7) http://www.reuk.co.uk/buy-LM741.htm
(8) http://www.reuk.co.uk/REUK-Products-For-Sale.htm
(9) http://www.national.com/ds/LM/LM741.pdf
(10) http://www.reuk.co.uk/buy-VARIABLE-RESISTORS.htm
(11) http://www.reuk.co.uk/What-is-a-Zener-Diode.htm
(12) http://www.reuk.co.uk/12-Volt-Battery-Low-Indicator-LM741.htm

Article from REUK.co.uk:
http://www.reuk.co.uk/LM741-OpAmp-Voltage-Indicator.htm
Published: 8th October 2007
© REUK 2009