Design Ideas November 21, 1996

 Simple circuit monitors battery voltage

Yongping Xia, Philips Lighting Electronics Co, Torrance, CA

  The circuit in Figure 1 monitors four NiCd rechargeable batteries and causes the LED to flash if the voltage of the batteries goes lower than 4V. The circuit consumes only 8 mA of current under normal operating conditions.

  IC₁ is an eight-pin IC that includes an op amp, a comparator, and a 1.2V zener diode. The zener diode connects to the negative input of the comparator in the IC as a reference. R₆ and R₇ divide the battery voltage, and the IC compares the result with the reference voltage. If the battery voltage is higher than 4V, the comparator output is high. Otherwise, the comparator's output is low.

  R₁ through R₄, C₁, and the IC's internal op amp form a 1-Hz oscillator that drives the LED through R₈. When the battery voltage is higher than 4V, the high output of the comparator stops the oscillator using D₁. When the battery voltage drops below 4V, D₁ becomes reverse-biased, and the LED flashes to indicate the low battery voltage. (DI #1949)

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