For higher voltage, the larger resistor is used. This lower voltage limit depends upon the type of the battery. If the voltage is lowwer than 9V, the zener must be replaced with smaller voltage zener and current-limiting resistor also replaced with smaller current-limiting resistor. Rechargeable batteries should not be discharged below a certain voltage level. The LED will light to indicate low-battery condition when the op amp’s output becomes low due to the voltage at the tap of the potentiometer drops below the voltage set by zener. Here is the schematic diagram of the circuit: The op-amp will compare the voltage between two inputs, one supplied with a voltage from a high resistance linear potentiometer which act as voltage divider, and the other is the voltage from zener reference. This circuit utilize zener diodes as a voltage reference and 741 op amp as voltage comparator to detect if the voltage falls below a preset value. Referring to the above diagram, the low battery indicator is formed by the two NPN transistors, while the additional BC557 and the relay are used for cutting OFF the battery from the load when it reaches the lower threshold, in this state the relay connects the battery to the available charging input. This battery indicator circuit will indicate the low battery condition by flashing the LED. A low-battery indicator circuit is shown in the schematic diagram. In the circuit diagram LEDs D1 toD10 displays the level of the battery in either dot or bargraph mode.
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