You can fix this circuit in the switch board that provides power to the loads such as Water heater, Electric Iron etc to remind you that the load is switched on. LED blinks indicating the load on status and the buzzer beeps every 30 minutes. This helps to prevent the wastage of energy usually happens when we forget to switch off these devices. This can be also used to remind, if remote lights such a terrace light is switched on unnoticed.
To make the circuit a compact one to enclose inside the switch board, a Resistor based transformerless power supply is used. The 1 watt 33 K resistor reduces the high volt AC to low volt AC which is rectified by the full wave rectifier comprising D1 through D4. Rectified DC is made ripple free by C1so that around 9 volts DC will be available to power the circuit. CD 4060 works safely between 5-18 volts DC and the rectified DC to IC1 is less than 9 volts in normal condition and will not destroy the device unless resistor R1 fails.
If loud beep is required, use a driver transistor for the buzzer.
The timer circuit is built around the popular binary counter IC CD4060 which resets through C2and R2 at power on and starts oscillating with the components C3 and R3. IC 4060 is an Oscillatorcum binary counter cum frequency divider. Its inbuilt oscillator is based on three inverters. The basic frequency of the internal oscillator is determined by the value of the capacitor connected to its pin 9 and that of the resistor in its pin 10. Each output goes high after the completion of the timing cycle.