Practical doorbell circuit

Ding dong doorbell The picture below is the electrical schematic diagram of a doorbell that can make a "ding, dong" sound. It is composed of a time base circuit integrated block and peripheral components. Its sound quality is beautiful and lifelike, it is simple and easy to install and adjust, and its cost is low. One 6V laminated battery can last for more than three months and consumes less power. The IC in the picture is the time base circuit integrated block 555, which forms an astable multivibrator. Press button AN (mounted on the door), the oscillator oscillates with an oscillation frequency of about 700Hz, and the speaker makes a "ding" sound. At the same time, the power supply charges C1 through diode D1. When the button is released, C1 discharges through resistor R1 to maintain oscillation. However, due to the disconnection of AN, resistor R2 is connected in series to the circuit, causing the oscillation frequency to change, which is about 500Hz, and the speaker emits a "boom" sound. Until the voltage on C1 is no longer able to maintain 555 oscillation. The length of the reverberation of the "dong" sound can be changed by changing the value of C1. Battery-free dual-tone doorbell As telephones become more and more popular, more and more families have residential telephones. However, most residential telephones have very low usage rates. They use the 48V (60V) provided by the telephone home feeder. DC feed as the working energy of electronic doorbell is economical and practical. Now we introduce a two-tone doorbell circuit that does not require batteries. The circuit principle is shown in the figure. It is easy to see that the circuit in the figure is a variation of the conventional telephone ringing circuit. a and b are the positive and negative ends of the telephone line respectively. AN is a normally open doorbell button. When the phone is waiting, press AN and the 48V (or 60V) voltage provided by the program-controlled switch will charge the capacitor C1 through VD1 and R1. When the voltage Vc at the C1 terminal reaches the IC1 When the control voltage is turned on, IC1 vibrates and sends out a two-tone electronic ring current to make buzzer B sound, informing the owner that a guest is coming. When the phone is in use, the voltage between a and b in the figure is too low to reach the starting control voltage of IC1. At this time, even if the AN doorbell button is pressed, it will not work. This is because the value of R1 is relatively small. Large, much larger than the impedance of a telephone. Therefore, pressing AN has no effect on the normal conversation of the telephone. It also has no adverse effects on the program-controlled switchboard, and only makes incoming calls busy when using the doorbell. Analysis and improvement of an intercom doorbell. The circuit of an intercom doorbell is as shown in the figure. Its working principle is as follows: When hanging up the phone, contacts 1 and 2 of the hook switch HS are connected and powered by AC220V. V1 has a DC output. This The voltage not only charges the battery, but also is added to the ③ pin of the music IC. If you press S, the ② pin of the music IC is triggered, and the ④ pin has a music signal output. After being amplified by V2, it drives the speaker to produce sound, and at the same time, it drives Y2 and Y3 through R5. After picking up the phone, contacts 1 and 3 of the hook switch HS are connected, and the call circuit is powered on. At this time, intercom can be carried out. Due to the high gain of the audio amplifier IC2 (LM386) of this intercom doorbell, it is easy for Y2 and Y3 to produce howling sounds. The author has actually verified that as long as a small capacitor of a few pF is connected in parallel to both ends of Y2 and Y3, the whistling sound can be eliminated. Musical doorbell without buttons This article introduces a music doorbell without buttons. As long as the visitor stands in front of the doorbell, the doorbell will automatically sound. The circuit principle of the musical doorbell is shown in the attached figure. IC1 and other components form an infrared emission circuit. IC1, RP, R1, and C1 form a multivibrator frequency. According to the component data shown in the figure, the oscillation frequency is about 40kHz and the output current is 100--200mA. It can drive the infrared light-emitting diode D1 to emit 40kHz Modulated infrared pulses. IC2 is an infrared receiving chip with high sensitivity, high gain, good output waveform, and frequency identification function. After the infrared receiving tube D2 receives the infrared pulse with a frequency of 40kHz, it is converted into an electrical signal and sent to the ⑦ pin of IC2. After amplification, C5, L tuning, and IC2 internal circuit detection and shaping, the pulse signal is output from the ① pin. Normally, IC2 pin ① outputs low battery, D3 is cut off, music integrated circuit IC3 has no trigger pulse, does not generate music signal output, and speaker B does not produce sound. When someone stands in front of the door and blocks the infrared signal emitted by D1, the potential of IC2 pin ① instantly changes from low level to high level, and IC3 is triggered by D3 to output a music signal, which is amplified by V to drive the speaker to sound. IC1 uses NE555, IC2 uses μPC1373, and IC3 uses 9300 series music integrated circuits. D1 can use SE303A or LM66R 5mm round infrared light-emitting diode, and D2 can use PH302 square infrared receiving diode. V is 9013NPN tube, β≥100. B selects YD58--1 type, 8Ω/0.25W small diameter speaker. L uses 0.08mm high-strength enameled wire and tightly wraps 30 turns around the intermediate frequency transformer frame of the small transistor radio. Both buttonless music doorbells require push-button installation, so there are problems such as troublesome installation and easy loss and damage. The above disadvantages can be overcome by using a composite switch tube to replace the mechanical trigger switch with a musical doorbell. Figure 1 shows the vibration type. When someone knocks on the door with his hand, the piezoelectric ceramic piece YD installed on the inside of the door is vibrated and generates a corresponding audio voltage, which turns on the composite tube switches BG1 and BG2, and the music circuit CIC is triggered to play a piece of music. It is better to use piezoelectric ceramic sheets with larger diameters and use 502 glue.