Circuit principle:
The device circuit is shown in the figure, it consists of six parts: infrared emission circuit, infrared receiving circuit, amplification and frequency decoding circuit, monostable delay circuit, alarm generation circuit and power conversion circuit.
Turn on the power and generate the 40KHZ oscillating signal from the audio decoder integrated circuit A2 and the external resistor R6 capacitor C3, one as its own resonance needs; on the other hand through the A5 pin 5 (amplitude about 6V), R5 added to the transistor VT1 The base, after being amplified by the VT1 current, drives the infrared light-emitting diode VD1 to emit infrared light pulses around the crucible. Normally, the infrared photodiode VD2 does not receive the infrared light pulse emitted by the VD1, the pin 8 of the A2 output terminal is at a high level, and the typical monostable circuit formed by the A3 and the peripheral RC element is in a reset state, and the VT2 is not biased. The alarm electric horn HA does not sound when it is powered off. When a person or object approaches VD1, the infrared rays emitted by VD1 are reflected back by the human body or objects, and are converted into electrical signals of the same frequency by VD2, amplified by op amp A1, input to pin 3 of A2, and passed through the internal of A2. Recognize and decode, make its 8th foot output low level. The low-level signal directly triggers the monostable circuit set by A3 to be set, so that the third pin of A3 outputs a high level, VT2 is biased to full flow, and the HA power sends a loud warning sound. After the person or object leaves the VD1 monitor area, although VD2 pushes the infrared light signal to make A2 pin 8 return to high level, HA will continue to sound for a period of time (<60 seconds) due to the monostable circuit's delay reset function. Automatically return to the state to be alarmed.
The most important feature of the circuit is to achieve synchronous automatic tracking of the infrared emission and reception operating frequency, that is, the infrared emission part does not have a dedicated pulse generation circuit, and directly from the detection circuit of the receiving part to induce pulses (actually AZ's phase-locked center frequency The signal) not only simplifies the line and commissioning work, but also prevents the inconsistency of the receiving and sending frequencies caused by changes in the surrounding environment and the change of component parameters, and greatly enhances the stability and anti-interference ability of the circuit.
Component selection
Select the... 41 or LM41, MC1741 universal blood type operational amplifier, and use LM567 or NE567, MM567 phase-locked loop audio decoder people choose NE555 or 5G1555, LM555 time-based integrated circuit people choose 78to9 type small plastic fixed three End regulator IC. VD, Wb adopts Pi03 infrared emitting tube and PH02 infrared receiving tube respectively. VTI uses 9014 or 9013 type silicon NPN triodes and requires production of >150; VT uses 8050 type silicon NPN power triodes, requiring R>100. QD uses the QLIA/50V type silicon full bridge.
R1-R9 all use RTX-1/4W carbon film resistors. C1 and C3 use CTI ceramic capacitors, C7 use CT4D monolithic capacitors, and C2 and C4 to C8 all use CD11-16V electrolytic capacitors. HA uses voice acoustic horns. F uses a 250V, 0.15A fuse with a header. XP is a common alternating current two-pole plug for household appliances.
The launch tube is covered with a reflector assembly removed from a used flashlight, which has a spotlight and a reflective effect. This can prevent the launch tube from disturbing the normal reception of the receiving tube, and it can increase the reflection distance.
As long as the welding is correct and the components are well selected, the circuit can work. Adjusting the R1 resistance can change the infrared effective reflection distance (alarm warning range). In addition, the resistance value of R4 affects the amplification factor of A1. The values ​​of R6 and C3 affect the decoding frequency and the infrared emission frequency. Usually, there is no need to adjust. Selecting the parameters according to the figure can achieve a satisfactory result. If it is suspected that the HA delay sounding time is too long (or too short), the value of R8 or C6 may be appropriately reduced (or increased) to adjust.
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