This is a clever Battery low power indicator circuit designed by G3AGX that provides a visual indication when equipment is drawing power, even at very low current levels. The circuit is particularly useful for monitoring standby power consumption or confirming that equipment is actually powered on.
Battery Low Power Indicator Circuit Analysis
The heart of the circuit is a 2N2646 unijunction transistor (UJT) configured as a relaxation oscillator. The UJT is an ideal choice for this application because it can trigger reliably at very low current levels while providing the oscillating action needed to flash the LED indicator. The oscillation frequency is determined by the RC time constant formed by the 150K resistor and 47μF capacitor.
Operating Principle
When current flows through the monitored circuit (as little as 320 microamps according to the specification), the UJT begins oscillating. During each oscillation cycle, the capacitor charges through the 150K resistor until it reaches the UJT’s firing voltage. At that point, the UJT conducts, discharging the capacitor through the LED and causing it to flash. The 100K resistor provides bias for the LED circuit and helps establish proper operating conditions.
Battery Low Power Indicator circuit provides a flashing pilot indicator for equipment where low battery drain is essential. The LED flashes approximately once per second and total drain current is only 320 microamps.
Ribless Programming cable for Motorola GM360/GM338 radio
About The Author
