Making a simple telephone remote control device

Remote telephone control  can be achieved using telephone lines and this device . Not only does it have a simple structure and stable operation, its biggest feature is that it does not generate off-hook signals, which means that users will not worry about phone charges. This is more practical and convenient for remote control using mobile phones, public telephones or long-distance telephones. The device is completely isolated from the telephone lines and will not have any impact on the original telephone system and program-controlled lines. The following is a brief introduction to the working principle and production method of the device.

working principle:

The working principle is shown in Figure 1. The ringing current detection is composed of C1, R1, C2, D1~D4 and IC 1 (4N25). When a call comes in from the outside line, the ringing AC signal goes through C1, R1, step-down, rectification of D1~D4, and filtering by C2, so that the light-emitting diode in IC1 starts to work, and the phototransistor is turned on. Pin 4 of IC1 presents a high potential, which quickly charges C3 through D5, turning T2 and T3 on, and LED1 lights up to indicate an incoming call. At the same time, IC2 (CD4017) is powered, and C5 and R7 form a power-on reset circuit, which clears IC2 and Q0 outputs high level. At the same time, the high potential of pin 4 of IC1 turns on T1 through R2, relay J1 is attracted, J1-1 is closed, and a positive pulse is given to pin 14 of IC2. Since pin 13 of IC2 is grounded, IC2 is in a counting state. At this time, with the periodic ringing signal (about 1.5 seconds), IC2 starts counting. During the ringing gap time (about 5 seconds), because the high potential of pin 4 of IC1 disappears, in order to maintain the power supply of IC2, C3 discharges through R3 so that T2 and T3 continue to conduct. The maintenance time is designed to be 6 to 7 seconds, which is slightly larger than the ringing gap time. This ensures the normal operation of IC2. This time setting cannot be too long to avoid miscontrol due to successive calls coming in. As the ringing signals arrive one after another, C3 is charged again, and J1-1 also pulls in once, causing the output terminals (Q0~Q9) of IC2 to output high levels in sequence with the ringing signals, thus realizing the ringing signal. detection. C4 is to prevent interference when the J1-1 contact is working. Here, when processing the ring signal counting, pin 4 of IC1 is not directly connected to pin 14 of IC2.

Because it was found in the experiment that since the ringing signal is a periodic AC signal, if the above connection method is used, the IC2 count will often be disordered and the operation will be extremely unstable. Here, the free micro relays on hand are used to completely isolate this pulse interference, making the work very stable and counting accurate.

When the 8th ringing signal arrives, pin 6 of IC2 generates a high level, triggering the bistable flip-flop composed of IC3 (1/2 CD4013 ). The Q terminal outputs a high level, LED2 lights up, and T4 leads Pass, J2 pulls in, turning on the controlled load. Similarly, when the next pulse period arrives, J2 is disconnected and the load power is cut off. Thus realizing remote control on and off control. AN is the reset control. When pressed, Q outputs low level and J2 is disconnected. If the other party hangs up or the called phone picks up before the eighth ring, the ringing signal disappears, IC1 no longer works, and IC2 stops counting. After about 6 to 7 seconds, the discharge current of C3 cannot maintain T2. , T3 is turned on, and IC2 is powered off and enters standby mode. Here, the number of responses can be set according to your own needs, but it should not be too few to avoid miscontrol caused by other people calling.

The power supply part consists of K, B1, C8, C9, IC4, etc., providing 6V regulated power supply.                                   

From the above analysis, it can be seen that if the downstream execution loop is not considered, the device consumes almost no power when it is static, and only consumes a little current when there is a ringing signal. In this way, we can also consider using battery power and using wireless receiving and transmitting modules. Remote control can avoid practical applications sometimes being limited by telephone lines, making it more practical.

Production and debugging:

Please refer to Figure 1 for the selection of components and the pin distribution of each IC. J1 can use various types of micro relays with suitable voltages. The selection of J2 contact capacity is determined by the controlled load. Considering that the charging current provided by pins 4 and 5 of the IC is limited, in order to ensure the full conduction of T1 and T2, NPN tubes with slightly higher amplification (b>200) can be selected for T1 and T2. If the other half of IC3 has no other purpose, its input terminals (i.e. S, D, CP, R terminals) can all be grounded and the output terminals can be left floating. AN is the normal open button. No other licensing requirements are required.

When debugging, attention should be paid to adjusting the parameters of R2, R3, and C3. The principle is to ensure that J1 is reliably engaged and disconnected without jitter. At the same time, it is necessary to ensure that T3 can maintain the power supply to IC2 during the ringing interval. If you find that IC1 is not working, you can swap pins 1 and 2 of IC 1.