R1______________10K 1/4W Resistor R2_______________1K 1/4W Resistor R3_______________1K 1/4W Resistor (Optional, see Notes) D1,D2________1N4007 1000V 1A Diodes D3______________LED Red (Optional, see Notes) Q1,Q2_________BC557 45V 100mA PNP Transistors BZ1___________Piezo sounder (incorporating 3KHz oscillator)
The above circuit can be useful to detect if the load of any battery charger or plug-in adaptor 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.
- 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.