The working principle of the ringing detection circuit The working principle of the off-hook control circuit

The article introduces the composition, working principle and programming method of the system. The ringing detection and analog off-hook control circuits are described in detail. Users can remotely control various electrical appliances (such as rice cookers, microwave ovens and other electrical appliances) through any dual-tone multi-frequency telephone (including mobile phones and telephone extensions) outdoors according to voice prompts. This device Z is suitable for families, enterprises and institutions, shops and other places. It is simple and convenient to operate and has reliable system performance. It is a scientific and technological product with great development prospects in the future.

The article introduces the composition, working principle and programming method of the system. The ringing detection and analog off-hook control circuits are described in detail. Users can remotely control various electrical appliances (such as rice cookers, microwave ovens and other electrical appliances) through any dual-tone multi-frequency telephone (including mobile phones and telephone extensions) outdoors according to voice prompts. This device Z is suitable for families, enterprises and institutions, shops and other places. It is simple and convenient to operate and has reliable system performance. It is a scientific and technological product with great development prospects in the future.

Ringing Detection Circuit

The working principle of the circuit: The ringing detection circuit is composed of optocoupler LE and gate circuit G4 and other components. When the telephone line has no ringing current, the line voltage provided by the telephone exchange is a DC signal of 48V-60V. When the user calls, the telephone exchange sends a ringing signal, and the 89C2051 single-chip microcomputer drives the off-hook control switch circuit, the DTMF signal decoding circuit, the ring current detection circuit and the voice prompt circuit and other circuit equipment. At this time, the light-emitting diode of the optocoupler LE is turned on, so that the phototransistor is 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 door-opening level, the AND gate G4 outputs a high level and is detected by P3.5 of the AT89C2051. Every time it rings, the gate G1 outputs a high level, that is, a positive pulse. The ringing signal is a sine wave of 25±3V, the RMS voltage is 90±15V, and the ringing takes 5s as a cycle, that is, 1s sends 4s off. The positive pulse signal can be directly output to the interrupt counter input port of the microcontroller to complete the whole process of ringing detection and counting. Circuit Diagram Design According to the characteristics of the ringing signal, the ringing detection circuit is designed as shown in the figure.

Off-hook control circuit

The working principle of the circuit: AT89C2051 firstly detects the output of the AND gate G4 from P3.5, and the phone rings once every time G4 outputs a positive pulse; P3.5 must detect 8 positive pulse signals before sending low power from P1.1 The leveling makes the transistor T7 conduct, so the relay JK pulls in to close the two pairs of normally open contacts JKa and JKb, and the 500 ohm resistor (in series with the small audio transformer winding) is connected to the telephone line to realize the analog lift. 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 recognized, P3.0-P3.4 reads the binary code information output by the DTMF decoder. This information is the remote control command. AT89C2051 can It is judged whether it is a password or a command to control a certain way to open, close or an on-hook command.

The execution signal of the on-hook command is output from P1.1. When P1.1=1, T7 is turned off and the relay is released, that is, the simulated on-hook is realized. The signal controlling the action of the controlled object is output from P1.3-P1.7, a total of 5 channels. For example, if P1.3=1, T1 can be turned on, and the relay J1 is pulled in; if P1.3=0, then J1 freed. If P1.7=1, T5 can be turned on, and relay J5 is pulled in; 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, that is, the chip contains 8 D flip-flops, the input terminals are D0-D7, and the output terminals are Q0-Q7. If the clearing terminal CLR is set to a low level, the device is reset to zero, and the outputs of Q0-Q7 are all zero. The state of D0-D7 will be latched into the device and output from Q0-Q7, and this state will be remembered forever as long as the CLK terminal is no longer triggered. It can be seen that the signal output by AT89C2051 from P1.3-P1.7 is only memorized by 74LS273 and then sent out, and its control logic is the same as that of directly connected T1-T5.

The necessary condition for the input terminals D0-D7 of 74LS273 to accept the input signal is that the CLK terminal has a positive transition, which must meet two conditions at the same time: one is that the STD terminal of the DTMF decoder must be high level, that is, the remote control transmission There is DTMF signaling sent to the end; the second is that P1.2 of AT89C2051 must send a hopping signal that changes from “0 to 1.” Only when these two conditions are met at the same time will the AND gate G5 output a positive transition signal, and the 74LS273 can accept external information, which greatly improves the anti-interference ability of the circuit and prevents the AT89C2051 from being accidentally interfered. Malfunction.

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