Our market is flooded with cheap and cheerful AC/DC Multi-Tune Musical doorbells. Most of these doorbells are built around customized microcontrollers from China, and usually run on two penlight (1.5V AA type) cells. We can safely trigger these doorbells using the existing AC 230V mains supply wiring of the doorbell switch.
Did you know? With a slight alteration, you can build your own security devices from these doorbells. For example, recently I made-up a compact Shadow Sensor Alarm with the help of my AC/DC Musical Doorbell for a school science fair project. One 5mm LDR is the one and only additional component required for this simple yet wonderful task. Now I wish to share the “hack” for the benefit of Electroschematics readers!
Shadow sensor alarms are usually used for protection against theft. A shadow alarm is a device that sounds an alarm when a shadow falls on it. Our shadow sensor alarm unit is capable of sensing a moving shadow in a restricted area, and can be easily installed on a wall, window or door to protect our valuables from theft. Note that constant lighting is required in the confined area to detect the moving shadow. This shadow sensor alarm raises an alert in the form of a distinct musical tone (for a finite duration) whenever a shadow is sensed by the photoresistor (LDR).
It’s of no concern about the microcontroller (melody generator chip) used inside the doorbell. Just look into the inside electronics to find the AC trigger circuitry and take a note. If the circuitry is similar/nearsimilar to he one shown here, you can hack your doorbell within minutes. Otherwise, be prepared to do a serious homework before the actual “hacking” process.
This is the internal circuit diagram of the doorbell used by me. This unit is built around a small microcontroller HL-06 (HLS5TA06), which is a low cost voice and melody synthesizer with 4-bits CPU. Here, this 14-pin DIL microcontroller is programmed as a 16-Tune English Melody Generator. A PNP transistor (BC557), ofcourse with some supporting components, is connected in the trigger input circuit to trigger the microcontroller HL-06.
From the circuit, I removed the 10K resistor, and soldered a 5mm LDR in that place. Next, I placed a short jumper wire between the points of the AC 230V Trigger Input (you can connect an spst on/off switch in lieu of this jumper to enable/disable the alarm unit). Wow! a compact, fool-proof, battery-operated Shadow Sensor Alarm is ready for use.
Testing of the alarm unit is simple. Point LDR towards a light source and power the circuit from 3V battery. Using your hand,block the light falling onto LDR. You should hear the alarm sound (musical notes) from the loudspeaker. When you remove your hand, the speaker should stop after a pre-determined delay. This completes the testing.
Any False Triggering?
Since this circuit is based on an ordinary light sensor (LDR) working in tune with ambient light, you might be in doubt about the false triggering possibility at night. Don’t worry, in a dark room,nothing haps in this shadow sensor alarm; it remains in idle state!
Have a deep look at the trigger circuit wired around BC557. Any idea spark? Is it possible to incorporate this trigger mechanism into any of your project? If so, revert with your update!