DIY a practical audio-visual doorbell

The ordinary doorbells used at home have a single function and do not have the functions of monitoring and listening. The following introduces a visual (audio) doorbell that combines an ordinary doorbell with a CMOS camera, a small TV, etc. through an external circuit. It is easy to make and has low cost.

How it works

The circuit is shown in the attached figure. The doorbell still uses an ordinary music doorbell (mostly ICs are made of 9300 series). When the combination circuit is plugged into the AC220V power supply, the switch S is closed and the doorbell does not sound, only the electrolytic C1 and C2 are charged in the circuit (C1 is charged after passing through the J1-1 contact), and the doorbell trigger monitoring circuit is ready to work.

When someone visits outside and presses the doorbell button AN, the doorbell sounds. At this time, the audio voltage at both ends of the speaker in the doorbell drives the output side of the photocoupler IC1 to conduct after being processed by C3 and D2. At the same time, IC2 is also turned on to make the relay J1 close; the two sets of contacts J1-1 and J1-2 are activated, and the power sockets CZ1 and CZ2 are powered to supply the camera and small TV with working power. The machine is powered on and enters the monitoring state. At the same time, the closing of the normally open point of contact J1-1 causes the charge charged by C1 to discharge to the transistor b-e junction in IC1 through the contact and R2, so the output end of IC1 is locked in the delayed conduction state within the discharge time constant of C1 and R2. Even if the doorbell stops ringing, the circuit still supplies power to the camera and small TV for a certain period of time, so that the owner can identify the visitor through the working machine. When C1 is discharged, IC2 and J1 are turned off and released with the cutoff of IC1. After the circuit is restored, C1 is charged again through the J1-1 contact (normally closed point restoration), thus waiting for the next trigger delay process.

This circuit consumes very little power when static, and consumes almost no power when in standby mode, except when C1 and C2 are charged in the initial state.

Fabrication and component selection

The circuit device does not need major adjustments. The capacity of electrolytic C1 can be changed according to actual conditions to make the delayed working time appropriate (usually about 1 minute).

When selecting components, electrolytic capacitors C1 and C2 should be selected with sufficient withstand voltage and low leakage. Relay J1 should be a small AC 220V relay with two sets of normally open and closed contacts (such as JQX-13F, etc.). For the camera, buy an ordinary CMOS black and white miniature camera with infrared night vision and monitoring functions. A small TV can be 5.5 inches to 7 inches, and it should be switched to "AV" state after assembly.

The additional part can be independently placed in a box, and the power output sockets CZ1-CZ2 can be fixed on the outer wall of the box. The monitoring doorbell can work normally as long as the combination connection is correct.