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Battery Low Indicator
The purpose of the battery low indicator is to give a visual indication when a battery has been discharged below a certain level. This is especially crucial for re-chargeable batteries that should not be discharged below a certain voltage level. This lower voltage limit depends upon the type of the battery. The battery low indicator circuit can be typically used for 12V batteries to give an indication of the battery voltage falling below the preset value. The indication is in the form of a flickering LED.
Learning from the project
- Application of the LM319 dual comparator for level comparison.
- Application of the NE555 timer in astable mode of operation.
The heart of the battery low indicator circuit is the dual comparator IC LM319 (denoted as IC1) which compares the battery’s voltage to a reference voltage level. Only one of the two independent comparators that are a part of the LM319 is used in the circuit. The two inputs to this comparator are:
- The reference voltage, which is applied to the non-inverting input pin (4). It is held constant at approximately 1.2V by the zener diode (D1).
- Battery’s voltage, which is applied via a potential divider arrangement built around resistors R2, R3, and preset VR1 and fed to the inverting input pin (5).
The corresponding output is generated at pin 12 of IC1. Preset VR1 is very important as it sets the voltage level at which a visual indication of the low battery level is generated. For example: If the battery under test is of 12 V and an indication is required as soon as the battery voltage falls below 10.5 V, the voltage at the inverting input (pin 5 of IC1) should be adjusted to 1.2 V using preset VR1.
Initially, when the battery is fully charged, the voltage at the inverting input (pin 5) of IC1 is higher than the non-inverting input (pin 4- voltage level held constant at approximately 1.2 V by D1). This causes the output at pin 12 of IC1 to remain low. The reset pin (pin 4) of an NE555 timer (IC2) is connected to the output pin of IC1 and hence, a low gets applied to it correspondingly. Since the reset pin is “active low”, the NE555 is reset continuously and oscillations are not generated at its output (pin 3). As a result, LED1 does not blink.
Considering the example values, when the battery voltage falls below 10.5 V, the voltage at the inverting input (pin 4) of IC1 becomes lower than the non-inverting input (pin 5) and the output at pin 12 of IC1 goes high. This applies a “high” to the connected reset pin of IC2 (NE555 timer) and the astable multi-vibrator built around IC2 starts generating oscillations. LED1 is connected to the output pin (pin 3) of IC2 and hence blinks to indicate low battery voltage and the battery needs to be charged before further use.