The battery monitor circuit in Figure 1 can be very helpful for any portable equipment powered by two 9-V batteries. When both batteries are in good health, an LED produces short bursts of light every second. If either or both batteries drop below 7.5 V, the LED stays dark. When blinking, the whole circuit draws 1 mA of current.
|Figure 1.||Four NOR gates and a gated oscillator tell you if your 9-V batteries are healthy or not.|
The circuit monitors battery voltages with two NOR-based comparators, R1-R2-G1 and R3-R4-G2. G4’s output is high only when both batteries are above 7.5 V. G4’s output enables the 7555 timer and it generates 1-second pulses with a 5% duty cycle. If one or both batteries fall below 7.5 V, G4’s output is low, the timer is disabled, thereby keeping the LED from blinking.
The values of the resistor dividers R1/R2 and R3/R4 set comparator circuit’s trip points at ±7.5 V in order to indicate the supply voltage is above the dropout voltage for most general-purpose voltage regulators, such as the 78L05 and 79L05. To extend battery life, you can use LDO regulators and adjust the resistor values to reduce the trip points accordingly.
In addition to a state-of-health indicator, the blinking LED acts as a reminder to switch off the power when your equipment is not needed. But, if you prefer the circuit to serve as a low-battery alarm, simply connect G1’s output directly to G4 and place G3 between G2 and G4. In this configuration, the LED will start blinking when a battery voltage drops below the comparators’ trip points.
The circuit in Reference 1 offers another way to achieve a 5% duty cycle with a 555 timer.
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