This circuit will raise an audible alarm as soon as the battery voltage goes above a predetermined over voltage limit.
The astable section of a high voltage alarm is similar to the circuit of a low voltage alarm. However, digging out the voltage from pin 4 of 555 is quite simpler in this circuit.
At a voltage of around 10.5 V, D1 would not be biased and would not get conducted. There is also a possibility of D1 getting biased due to which a low voltage of 0.5 V is developed across R4. So, the alarm sound is inhibited and no sound is generated.
How the Circuit Works
To get the alarm ringing, a Voltage of 10.5 Volts must be provided across D1. This leads D1 to become highly biased so that the current passing through R3, R4, and D1 produces a minimum needed potential of 0.5 V to appear across R4. This makes the alarm bell to ring again.
Like the Low Voltage alarm, D1 can be provided with different threshold voltages to maintain a 0.5 V difference with the zener diode fitted in the circuit.
Also, as with the low voltage alarm, the high voltage one should not be exposed to very high voltages, especially at the IC1 point. Also, having a voltage lower than 5.1 V at IC1, the circuit will not operate systematically.
The circuit of High voltage alarm circuit must be placed in a desired equipment as the common boards will have no space to contain the circuit completely. Therefore, the components are kept in their own boxes and connected with a 3.5mm jack plug. This jack plug is that connected with a jack socket to feed the current from the mains.