Homemade power outage notification circuit

This flood alarm does not require a backup power supply. There is no need for manual intervention when there is a power outage or a call. It will automatically stop working after a few seconds. The circuit is very simple and can be used as a reference for audiophiles.

The circuit is shown in the attached figure. When in use, plug the alarm into the mains socket. The circuit is powered by capacitor Cl step-down, diode D2 rectification, and 12V voltage regulator IN4742 voltage regulator. When the call just comes in, the voltage across capacitor C4 cannot change suddenly and is approximately 0. This makes transistor V cut off, the total reset terminal (4) of the time base circuit NE555 is at a high potential, and the integrated circuit NE555 connected to the oscillation circuit starts to oscillate, making the buzzer HA sound, notifying the line of the call. As C4 is continuously charged through R3, transistor V enters the saturation state, the (4) pin of NE555 becomes low, the oscillator stops, and HA stops sounding.

When the power goes out, capacitor C4 is discharged quickly through diode D4 and resistor R2. Since D4 is a germanium diode, the forward voltage drop is only 0.2V. Therefore, the voltage across C4 is quickly reduced to about 0.2V, so that transistor V is cut off again, and the total reset terminal (4) of NE555 becomes high potential. At this time, the large-capacity capacitor C5 supplies power to the oscillation circuit, causing HA to sound and inform the line of power outage. When C5 is discharged to a point where it is insufficient to maintain the circuit oscillation, the buzzer HA stops reporting.

Adjusting the resistance of resistor R3 can change the ringing time of HA when a call comes in; increasing or decreasing the capacity of capacitor C5 can change the ringing time of HA when a power outage occurs. Diode D3 can ensure that C5 only supplies power to the oscillation circuit when a power outage occurs.

Other components are selected as marked in the figure, without special requirements.

To make it right, find a used mobile phone charger that is slightly larger in size, dismantle its internal circuit, solder the circuit components shown in the attached figure onto a universal board with appropriate area and shape, and install it into the charger casing. Generally, it can work without debugging.