The proposed circuit was requested by one of the avid readers of my blog. It is a low battery warning indicator circuit using opamp IC 741 and can be used for monitoring a particular low battery voltage threshold.
Circuit Operation
The circuit may be understood with the following points:
1) The entire configuration is wired around the IC 741 and it becomes the heart of the circuit.
2) Basically it is configured as a comparator with one of its inputs clamped to a fixed reference level while the other input used as the sensing terminal.
3) Here as can be seen in the diagram, the non inverting input is provided with a fixed reference voltage through a resistor zener network.
4) This input is fixed to about 5 volts.
5) The other inverting input pin #2 is wired via a preset to sense the input supply voltage from the source.
6) The preset is adjusted such that the voltage level at this input becomes lower than the fixed reference voltage at the other pin of the IC as soon as the source voltage becomes lower than the desired threshold level.
7) When this happens the output of the IC immediately becomes high, illuminating the connected LED.
8) The illuminated LED immediately provides the indication of a low voltage situation so that the required actions may be initiated.
9) Optionally, the output may be replaced by a piezo buzzer instead of the LED for getting an audible response of the above situation, eliminating the headache of monitoring the LED condition every now and then.
The above circuit can be modified by adding a relay stage for controlling a particular stage which may be relevant to the low battery cut of actions.
How to Setup this Low Battery Indicator Circuit
The above low battery indicator circuit can be even further improved in the following manner for controlling both lower and the upper charging thresholds:
Initially keep the 100K preset link disconnected.
Apply a 14.4V source from the "Battery" side and adjust the 10K preset such that the upper relay just activates, confirm the triggering by subsequently moving the preset to-and-fro.
Glue it once fixed.
The LED will respond by switching ON to the fixing of this preset.
Now reconnect the 100K preset feedback link, and reduce the input supply to about 11.2V. Make sure it has a high resistance initially.
Next, slowly adjust the 100K preset such that the relay just deactivates (at the 11.2V).
Confirm by flipping the preset as above. Ignore the lower relay as it will switch ON as soon as the input supply is switched ON, so its operation is obvious.
That's it, the low battery warning circuit is all set now and will accurately respond to the above settings or any different setting that may be preferred and implemented by the particular user.
Low Battery Indicator Circuit with Relay Cut-off
The following circuit shows how the above low battery indicator can be enhanced with relays for achieving an automatic low charge and full charge cut for the connected battery, and also a cut off for the load during a low battery situation.
The upper relay becomes responsible for cutting off the battery during over charge and low discharge level, while the lower relay cuts of the load as soon as the battery reaches the unsafe low discharge zone and as soon as the upper relay reverts to charging mode
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