Broken Charger Connection Alarm Circuit

Broken Charger Connection Alarm Circuit

Circuit Description

The above circuit can be useful to detect if the load of any battery
charger or plug-in adapter supply is not properly connected. The load
can be a set of batteries to be charged or any other type of battery or
low dc voltage operated device. The circuit can safely operate over a 3
to 15V range and 1A max. Current, provided the supply voltage is about
one volt higher than the voltage required by the load.

The circuit is inserted between the supply and the load;
therefore, until a trickle-charging current of at least 100µA is flowing
towards the load, D1 and D2 will conduct. The forward voltage drop
(about 1V) available across the Diodes drives Q2 into conduction and,
consequently, Q1 will be cut-off. If no appreciable load is connected
across the circuit’s output, Q2 will become cut-off, Q1 will conduct and
the Piezo-sounder will beep.

Circuit diagram:



  • R1 = 10K
  • R2 = 1K
  • R3 = 1K
  • Q1 = BC557
  • Q2 = BC557
  • D1 = 1N4007
  • D2 = 1N4007
  • D3 = Red LED
  • BZ1 = Piezo Sounder


An optional LED and its series limiting resistor can be wired in
parallel to BZ1, as shown in dotted lines in the circuit diagram.
In this case you may omit the Piezo-sounder in order to obtain a visual
alert only.

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