Design of telephone remote control system module circuit based on single chip microcomputer

　　A telephone remote control system is designed. The system is based on the AT89C2051 single-chip microcomputer and the MT8870 dual-tone multi-frequency decoding integrated circuit. With the help of the public telephone network, the system can realize intelligent control of remote equipment through the telephone. The article introduces the composition, working principle and program design method of the system. The ring detection and analog off-hook control circuits are explained in detail. Users can use any dual-tone multi-frequency telephone (including mobile phones and telephone extensions) outdoors to remotely control various electrical appliances (such as rice cookers, microwave ovens, etc.) according to voice prompts. This device is suitable for homes, enterprises, institutions, stores and other places. It is simple and convenient to operate and has reliable system performance. It is a technological product with great development prospects in the future.

　　Ringing detection circuit

　　Working principle of the circuit : The ring detection circuit is composed of components such as optocoupler LE and gate circuit G4. When there is no ring current on the telephone line, the line voltage provided by the telephone exchange is a DC signal of 48V-60V. When the user calls, the telephone exchange sends a ring signal, and the 89C2051 single-chip microcomputer drives the on-hook and off-hook control switch circuit, DTMF signal decoding circuit, ring current detection circuit and voice prompt circuit and other circuit equipment. At this time, the light-emitting diode of the optocoupler LE is turned on, making the photosensitive transistor turned on, so the +5V power supply charges the 100uF capacitor through the 1K resistor and the diode. When the voltage on the capacitor is charged to the open gate level, the AND gate G4 outputs a high level and is detected by P3.5 of AT89C2051. For each ring, the gate G1 outputs a high level, that is, a positive pulse. The ring signal is a sine wave of 25±3V, and the effective value of the voltage is 90±15V. The ringing period is 5s, that is, 1s is sent and 4s is off. The positive pulse signal can be directly output to the interrupt counter input port of the single-chip microcomputer to complete the entire ringing tone detection and counting process. Circuit diagram design According to the characteristics of the ringing signal, the ringing detection circuit is designed as shown in the figure.

　　On/off control circuit

　　Working principle of the circuit: AT89C2051 first detects the output of AND gate G4 from P3.5. Each time G4 outputs a positive pulse, the phone rings. P3.5 must detect 8 positive pulse signals before sending a low level from P1.1 to turn on transistor T7. Then relay JK is energized to close the two pairs of normally open contacts JKa and JKb, and the 500-ohm resistor (in series with the winding of the small audio transformer) is connected to the telephone line, realizing the simulation of picking up the phone. Then P3.2 waits for the arrival of the positive pulse at the STD end of the DTMF decoder. Once the positive pulse at the STD end is identified, P3.0-P3.4 reads the binary code information output by the DTMF decoder. This information is the remote control command. AT89C2051 can judge whether it is a password or a command to control the opening and closing of a certain channel or a hang-up command.

　　The execution signal of the hang-up command is output from P1.1. When P1.1=1, T7 is turned off and the relay is released, which means that the simulated hang-up is realized. The signal for controlling the action of the controlled object is output from 5 channels, P1.3-P1.7. For example, if P1.3=1, T1 can be turned on and relay J1 is attracted; if P1.3=0, J1 is released. If P1.7=1, T5 can be turned on and relay J5 is attracted; if P1.7=0, J5 is released. However, it can be seen from the figure that P1.3 is not directly connected to T1. P1.7 is not directly connected to T5, but is separated by an integrated block 74LS273. 74LS273 is an 8D latch, which means that the chip contains 8 D flip-flops, with input terminals D0-D7 and output terminals Q0-Q7. If the clear terminal CLR is low, the device will be reset, and the outputs of Q0-Q7 will all be zero. If the clear terminal is high, every time the trigger terminal CLK has an upward jump, the state of the input terminal D0-D7 will be latched into the device and output from Q0-Q7. As long as the CLK terminal is no longer triggered, this state will be remembered forever. It can be seen that the signal output from P1.3-P1.7 of AT89C2051 is just memorized by 74LS273 before being sent out, and its control logic is the same as that of T1-T5 directly connected. The necessary condition for the input terminal D0-D7 of 74LS273 to accept the input signal is that there is a positive jump at the CLK terminal, which must meet two conditions at the same time: one is that the STD terminal of the DTMF decoder must be high, that is, the remote control transmitter has a DTMF signal sent; the second is that P1.2 of AT89C2051 must send a jump signal from 0 to 1. Only when these two conditions are met at the same time, the AND gate G5 will output a positive jump signal, and the 74LS273 can receive external information. This greatly improves the anti-interference ability of the circuit and prevents the AT89C2051 from malfunctioning of the controlled object due to unexpected interference.