Module circuit design of telephone remote control system based on single-chip microcomputer

A telephone remote control system is designed. The system uses AT89C2051 microcontroller and MT8870 dual-tone multi-frequency decoding integrated circuit as the core, and uses the public telephone network to realize intelligent control of remote equipment through the telephone. The article introduces the composition, working principle and programming method of the system. The ringing detection and simulated on-hook control circuits are explained in detail. Users can remotely control various electrical appliances (such as rice cookers, microwave ovens, etc.) through any dual-tone multi-frequency phone (including mobile phones and telephone extensions) outdoors according to voice prompts. This device is suitable for families, enterprises, institutions, shops 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

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

Off-hook control circuit

Circuit working principle: AT89C2051 first detects the output of AND gate G4 from P3.5. Every time G4 outputs a positive pulse, the phone rings once; P3.5 must detect 8 positive pulse signals before sending out low power from P1.1 The transistor T7 is turned on, so the relay JK is pulled in to close the two pairs of normally open contacts JKa and JKb, and the 500 ohm resistor (connected in series with the winding of the small audio transformer) is connected to the telephone line, realizing a simulated pick-up. Then P3.2 waits for the arrival of the positive pulse at the STD terminal of the DTMF decoder. Once the positive pulse at the STD terminal is recognized, P3.0-P3.4 reads the binary code information output by the DTMF decoder. This information is the remote control command. AT89C2051 can Determine whether it is a password, a command to open or close 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 cut off and the relay is released, which realizes the simulated hang-up. The signals that control the action of the controlled object are output from a total of 5 channels P1.3-P1.7. For example, if P1.3=1 can make T1 conductive, the relay J1 will close; if P1.3=0, then J1 freed. If P1.7=1, T5 can be turned on and relay J5 is closed; if P1.7=0, J5 can be released. But as can be seen from the picture, 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, that is, it contains 8 D flip-flops inside the chip. The input terminals are D0-D7 and the output terminals are Q0- Q7. If the clearing terminal CLR is driven to a low level, the device will be reset to zero, and the Q0-Q7 outputs will all be zero. If the clearing terminal is high, then whenever the triggering terminal CLK has an upward level transition, the input terminal The status of D0-D7 will be latched into the device and output from Q0-Q7. As long as the CLK terminal is no longer triggered, this status will be remembered forever. It can be seen that the signals output by AT89C2051 from P1.3-P1.7 are only stored by 74LS273 before being sent out. Its control logic is the same as that of T1-T5 directly connected. The necessary condition for the input terminals D0-D7 of 74LS273 to accept input signals is that there is a positive transition on the CLK terminal. This must meet two conditions at the same time: one is that the STD terminal of the DTMF decoder must be high level, that is, remote control transmission There is DTMF signaling sent to the end; secondly, P1.2 of AT89C2051 must send a transition signal from ‚0 to 1. Only when these two conditions are met at the same time, the AND gate G5 outputs a positive transition signal, and the 74LS273 can accept external information. This greatly improves the anti-interference ability of the circuit and prevents AT89C2051 from causing damage to the controlled object due to unexpected interference. Malfunction.