Production of infrared visitor notification device

How it works

This is a practical household electronic gadget. When someone comes to visit (standing in front of the door), it can inform the owner that there is a guest outside the door. It is equivalent to an active doorbell. Infrared is used as the detection signal. Infrared has strong anti-interference ability, high reliability and simple circuit.

Working principle of infrared visitor notification device

The circuit is shown in Figure 1, which consists of infrared transmitting and receiving parts. The transmitting part consists of an IC1 ( NE555 ) time base circuit and peripheral components to form a 5kHz oscillator. V1 drives VD1 to transmit infrared rays. The receiving part consists of a preamplifier circuit IC2 and a doorbell circuit. IC2, R6~R8, C4~C6 form a pre-selective amplifier with a resonant frequency of 5kHz. The converted photoelectric signal is preamplified. The signal is further amplified by V2 and doubled by VD3 and VD4. The rectified DC signal turns on V3 and triggers the doorbell circuit to work.

When a visitor walks into the range of the circuit, the 5kHz infrared rays emitted by VD1 are reflected by the human body to VD2, which then performs a series of processing. Since the range of action is not very large, there is no special requirement for the power of the circuit. The square wave oscillator connected to ICl has a relatively narrow pulse width, which can reduce the transmission power. The frequency-selective amplifier composed of IC2 and peripheral components has high efficiency and good circuit selectivity, which can enhance the circuit's anti-interference ability.

Component selection and production

IC1 is a 555 time base circuit. The author uses a CMOS type 555 ( NE7555 ). Because the driving ability of the CMOS time base circuit is not strong, V1 must be added as an amplifier (driver). This is very important. Many friends often encounter many problems in actual production because they do not understand whether the 555 is TTL type or CMOS type. In terms of working principle, the basic principle of all 555 is the same, but the voltage range of CMOS and TTL types is wider than that of the latter (CMOS is 3~18V, TTL is 4.5~16V). In terms of driving ability, the latter is much stronger than the former (CMOS is 1~5mA, TTL is about 200mA). Moreover, the static current of CMOS 555 is only about 0.1mA, while that of TTL 555 can reach 10mA. The op amp in the circuit is an I.LA741 single op amp, which can be directly replaced by LM741. Its pin function is shown in Figure 3. The component selection of the double-T feedback network composed of peripheral components is relatively important. The selected frequency is f0=1/(2πR6C4), and it must satisfy the balance formula: According to the component values ​​in the figure, the selected frequency f0 is about 5kHz.

The amplifier has a very high amplification factor for f0, but its amplification ability for other signals can be ignored. RP2 can be used for sensitivity adjustment. The doorbell circuit can use the KD series music integrated chip. Figure 2 is a connection example, and Figure 4 is an appearance diagram. R12 and LED in Figure 2 form a 2V simple voltage regulator (LED acts as a 2V voltage regulator tube) to power the KD chip.

C11 and R13 are connected to form a differential circuit as the trigger circuit of the chip. Y is a 2.5-inch small speaker. Of course, everyone can choose different music chips according to their preferences. If you don't need a loud sound, you can directly drive a piezoelectric buzzer.

Infrared emitting and receiving tubes are also the key to the circuit. VD1 and VD2 are emitting and receiving tubes respectively. VDI uses the low-power LN70 series, and VD2 uses the PN303 photosensitive diode with a wavelength close to its working wavelength. VD1 and VD2 should be installed in a black plastic block with two holes drilled. You can also use two small metal tubes to bond together in parallel, and then put a photosensitive element in each tube, as shown in Figure 5.

The debugging workload of the circuit itself is very small. After installation, generally only the sensitivity adjustment is required. Do not install the infrared transmitting and receiving tubes together at the beginning. After installing the circuit, move VD1 and VD2 about 2 meters apart and adjust RP2 to the minimum. Adjust RP1 to make the oscillation frequency of ICl consistent with the amplification center frequency of the frequency-selective amplifier (the frequency can be determined by measuring the voltage across Rll). As long as the circuit is installed correctly, V3 should be turned on at this time.

After adjusting according to the first step, VD1 and VD2 can be installed in the jacket shown in Figure 5. Turn on the circuit, let someone stand 0.5m away from the infrared component, facing the transmitting and receiving components, and adjust RP2 so that V3 is just critically conductive. In this way, the circuit debugging is basically completed and can be put into use.