This circuit will produce a visual as well as audible alert whenever the connection between the charger circuit and the battery being charged.What ever be the situation whether it be a loose contact or breakage the circuit will give you the alarm.Such a circuit can be very handy in the fail safe charging of the batteries in some applications , like as in burglar alarms etc.This circuit can be easily assebled from the components in your electronic junk box.
As long as there is a little current flows from the charger to battery the diodes D1 and D2 will conduct.This forward voltage drop of 1.4 V across the diodes make the Q2 ON.So the transistor Q1 will be OFF.When there is no at all any current flowing (when the wire is broken or there is a loose contact) there will not be any drop across D1&D2 and so Q2 will be OFF.This makes the Q1 ON,the buzzer beeps and the LED glows.
Circuit diagram with Parts list.
For this circuit to operate effectively the supply voltage must atleast 1V higher than the voltage required by the charged device.
The circuit can operate safely in a range of 3-16 V.
Assemble the circuit can be assembled on a good quality PCB or common board.
The transistors are not very value specific.Any PNP transistors will do the job.