Low-volts alarm

  
This inexpensive dc supply-voltage monitor sounds a warning when the voltage falls below a preset value. It is ideal for monitoring rechargeable batteries since it draws only a few microamperes when not sounding. The voltage at which the alarm sounds is determined by the zener diode. When the voltage falls below the zener voltage, the alarm sounds.
Low-volts alarm  - schematic

The alarm tone is determined by the RC time constant of the 39 k resistor and 0.01 mf capacitor.




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