Convenient and practical telephone automatic redial controller

This device can be used to dial some difficult-to-reach telephones, such as TV and radio station hotlines, some popular consultation telephones, etc. It automatically redials the telephone number to be dialed at a certain interval until it is dialed or canceled. The circuit is shown in the figure below.

Working principle: This device is designed based on the principle that pressing the redial key of the phone can send the most recently dialed phone number again. The circuit is mainly composed of power supply, clock signal generator, program distribution simulation off-hook, redial, on-hook and ringback tone detection after dialing.

IC2 is a six-inverter CMOS digital integrated circuit CD4069, and F1-F6 are its six NOT gates. Among them, the square wave oscillator composed of F1, F2, RP1, and C3 provides the clock signal for the circuit. F3, F4, C6, R3. F5, F6. and C8, R4 respectively form two monostable triggers triggered by positive pulses. IC3 is a decimal counting divider, which is used for program control of automatic redialing. Optocouplers IC4 and IC5 are used as executive elements isolated from the telephone circuit. IC6 is a phase-locked loop audio decoder LM567, which together with the two-input four-NAND gate CMOS digital integrated circuit IC7 (CD4011) form a ringback tone detection circuit after the phone is connected.

When the button AN is pressed, the 220V mains electricity is stepped down by the power transformer B, rectified by VD1-VD4, and filtered by C1 to obtain a DC voltage of about 12V, which is then stabilized by the three-terminal voltage regulator IC1 to obtain a 9V DC voltage to supply the circuit. At this time, the ⑧ foot of IC6 is high, the output of the NAND gate YF1 is low, the output of YF3 is high, the transistor V is saturated, the relay J is attracted, and after releasing AN, the circuit is self-locked and powered by J1. At the same time, the F1 and F2 oscillators work, and the square wave with an output period of about 2.5 seconds enters the CP input terminal of IC3. When IC3 is just powered on, the C4 and R1 differential circuit resets it, Q0 in the output terminal is high, and the rest are low. With the input of the rising edge of the CP terminal pulse, Q1~Q9 output high levels in turn. When it is Q7's turn to output a high level, the high level triggers the F3 and F4 monostable circuits, F4 outputs a high f level pulse, the LED in IC4 lights up, the photosensitive tube is turned on, the "hands-free" key of the phone is short-circuited, and the phone enters the off-hook state. (Note: This circuit uses an electronic bistable hands-free push-button phone. If a hands-free phone with a mechanical button switch is used, the circuit should be modified accordingly). When the next pulse rising edge of F2 output enters IC3, Q8 becomes a high level, the F5 and F6 monostable circuits are triggered, IC5 works, the "redial" key of the phone is short-circuited, and the phone sends the last dialed number.

After the redial is completed, if the call is connected, a ringback tone is sent on the line, which is a 450Hz signal with 1 second on and 4 seconds off. Since the central oscillation frequency of the voltage-controlled oscillator in IC6 is also set to 450Hz by RP2 and C4 connected to pins ⑤ and ⑥, when the ringback tone enters pin ③ of IC6 and is demodulated by the internal circuit, it outputs a square wave with a width of 4s and a duty cycle of 4:1, and then after being inverted by YF1, it becomes a square wave with a width of 18 and a duty cycle of 1:4. While the high level of this wave is added to pin ⑧ of YF3, capacitor C14 is charged through RP3, and YF2 outputs a low level. When C14 is charged to about 0.5 seconds, the potential of the input terminal of YF2 is as low as the gate conversion voltage of CMOS, and the output of YF2 flips to a high level and is added to pin ⑨ of YF3. YF3 outputs a low level, V is cut off, J is released immediately, and the entire circuit loses power. At this time, normal calls can be made. After the call, press the "hands-free" button to hang up.

If the call is still not connected after the redial, a 450Hz blind tone signal with a period of 0.7s and 50% pulse modulation is transmitted on the line. The signal is demodulated by IC6, and YF1 is inverted to become a square wave with a period of 0.7s and a duty cycle of 1:1. The high level is added to the YF3⑧ foot and charges C14 at the same time. Since the positive pulse width is only 0.35s, when the YF2 input terminal has not yet reached the CMOS gate conversion voltage, the YF1 output has become a low level, and the same is true when the next waveform arrives. Therefore, YF3 can never flip, and the circuit is powered on normally. When the IC3 CP terminal gets another rising edge, Q9 becomes a high level, and the high level triggers the F3 and F4 monostable circuits again through VD7, IC4 works, and the "hands-free" key is short-circuited again, and the phone simulates hanging up. After a period of time, Q0~Q6 take turns to be high level, and the phone is on-hook until it is the turn of Q7, Q8, and Q9 to output high level. The circuit repeats the above process again... and so on, automatically dialing every 25 seconds or so until the call is connected.

Component selection and production: The component models and parameters used can refer to the data in the figure, and there are no special requirements. Among them, AN can use a small electric bell button, and B uses a power transformer with a secondary 10-12V AC output, and the power is enough to have SVA. During production, all components are welded on a circuit board and placed in a plastic box. The tube end of the power indicator LED is exposed by drilling a hole on the box cover, and AN can also be installed on the box cover. When wiring, pay attention to connecting the c pole of the photosensitive tube in IC4 and IC5 to the corresponding key high potential end, and the e end to the low potential end.

Debugging: (1) Adjusting the resistance value of RP1 can change the oscillation frequency of Fl and F2, thereby changing the automatic redialing time interval; (2) When a dial tone is received on the line, adjust RP2 to make the center frequency of IC6 450Hz. At this time, pin ⑧ outputs a low level; (3) Adjust RP3 so that the time from the time C4 starts charging to the time the YF2 output terminal flips to a high level is between 0.4 and 0.8 seconds. In this way, the ringback tone and the blind tone can be accurately distinguished.