Traditional Chinese Practical Eight-way scanning responder: Circuit Design, Parts List & Workshop Prototype Guide

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Eight-way scanning answering machineWireless remote control answering machine, which consists of 8 transmitters and 1 receiver, can be used in intelligence competitions of 8 groups or less. Before the competition, the participating groups are numbered from 0 to 7, and each group is given a corresponding transmitter. Place the receiver in the center or front of each group. After the host presses the start button, the answering session begins. After that, whichever group presses the answering button on the transmitter first, the receiver will immediately display the group number and lock, and at the same time make three crisp "ding dong" sounds. From now on, there will be no response when pressing any of the answering keys. Only after the host presses the start button again can the next question be answered.Circuit working principle:(1)Transmitter circuitThe circuit is shown in Figure 143-1. An astable multivibrator is composed of a time base 555 circuit, and its oscillation frequency is given byf=1.443/〔(R1+2R2)·C〕decision. Press the answer key SB, the oscillator is powered on and vibrates, and the two transmitting tubes emit high-frequency infrared signals. The purpose of using two infrared emission tubes is to increase the emission power and the area of emitted light. This answering machine requires a total of 8 independent transmitters, the transmission frequencies are 10KHz, 20KHz...80KHz respectively. The low level time of pin ③ output and the total time are about 1:15. The values of R1 and R2 are different for each transmitter. The light-emitting diode is SE303A.(2)Receiver circuitThe circuit is shown in Figure 143-2. It consists of a frequency-selective preamplifier, an 8-channel phase-locked loop audio decoder, a scanning detection and display, and a "ding-dong" sounder.Figure 143-1IC1 (741) and VT1 (9013) form a non-frequency selective preamplifier. When the infrared receiving tube receives the high-frequency infrared signal from a certain transmitter, it converts it into a weak voltage signal and sends it to IC1 for amplification. Then, after being buffered by the emitter follower composed of VT1, it is divided into 8 outputs and sent to the input end of the 8-channel phase-locked loop audio decoder of the subsequent stage. The purpose of using four infrared receiving tubes is to enable the receiver to receive infrared rays emitted from all directions.LM567 is a phase locked loop audio decoder. The external resistors R6, RP and capacitor C4 connected to its ⑧ and ⑥ pins determine the center frequency of the voltage-controlled oscillator inside the integrated block. Its center frequency can be determined by the following formula:f=1/〔(R6+Rp)·C4〕When R6 and C4 have constant values, changing RP can change f0. This receiver requires an audio decoder composed of 8-channel LM567, whose center frequencies f0 are 10KHz, 20KHz...80KHz. Corresponds exactly to the transmitting frequency of each transmitter. Each audio decoder circuit only has a different value of RP. When the frequency of the signal sent by VT1 is consistent with the center frequency of a certain LM567, the ⑧ pin of the LM567 still remains high.IC3~IC7 and LED common cathode display screen form the scanning detection and display. IC4 is an RS flip-flop composed of 555. When the voltage of pin ② is lower than 1/3Vcc, pin ③ outputs high level; when the voltage of pin ⑥ is higher than 2/3Vcc, pin ③ outputs low level. IC3 is an 8-to-1 bidirectional analog switch. According to the different data on the address lines C, B, and A, its common pin ③ can be connected to pins 13, 14... ④ respectively. When the address code on C, B, and A is 000 (decimal number 0), pin ③ is connected to pin 13; when the address code is 001 (decimal number 1), pin ③ is connected to pin 14...; when the address code is 111 (decimal number 7), pin ③ is connected to pin ④. In this way, as long as the address code is changed sequentially in a loop, the scanning of each LM567 output terminal can be realized. The ⑥ pin of IC3 is the inhibit terminal. When it is high level, all analog switches are not turned on, and the ③ pin and 13, 14... ④ pins are in a high impedance state. IC6 is an astable multivibrator composed of 555, which provides counting pulses for IC5. IC5 is a presettable four-bit binary up/down counter. This circuit sets it to a non-preset up counter state. When its ⑤ pin is high level, counting is prohibited. Only when the ⑤ pin goes low does the counting start. The counter cycles once every 16 pulses. Here only Q1, Q2, and Q3 are used as the address codes of IC3, so IC3 can complete a scan every 8 pulses. IC7 is a BCD-latch/7-segment decoder. When its ④ pin is low level, it is in a blanking state. When ④ pin is high level, it decodes the binary address code into a decimal number and displays it on the LED screen.IC8, IC2, etc. constitute a "ding dong" sound circuit. …

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