Practical and convenient fax machine power supply control circuit

This fax machine power control circuit has a simple structure, is safe and reliable, practical and convenient, and has the following features:

① In standby mode. The fax machine is powered off;

②When there is a telephone ring or fax signal, the fax machine will automatically turn on and work until the reception is finished, and then automatically turn off the fax machine after a certain delay;

③If you want to send a fax or make a phone call, just touch the button and the fax machine will be powered on and enter working mode.

1. Circuit composition and working principle

The control circuit is shown in the figure below. The signal receiving circuit is composed of R1, C1, D1 and optocoupler G. In the standby state, the optocoupler G is cut off, the ② and ⑥ pins of the integrated circuit N1 are low level, the ③ and ⑦ pins are high level, the relay J is not attracted, and the fax machine power is turned off.

When there is a ringing signal, the optocoupler G is turned on, and the signal is amplified by vTl and VT2. The collector output of VT2 is high level, which makes D2 turn on. Pins ② and ⑥ of the integrated circuit N1 are clamped at a high level, and pins ③ and ⑦ are converted to a low level. Relay J is energized, the fax machine is powered on, and a ringing sound is emitted. After picking up the phone, the optocoupler continues to conduct. Start answering the phone or receiving text and pictures until the end. After hanging up, the optocoupler is cut off. At this time, the capacitor C2 is slowly charged through R6, and the potential of pins ② and ⑥ of N1 gradually decreases. After a delay of about 50s, when it drops to 1/3 of the power supply voltage, the output pins ③ and ⑦ of N1 turn to a high level, and relay J is released. The fax machine is powered off and enters the standby state.

When you need to send a fax, just touch the switch SW, and N1's ② and ⑥ enter the high-level reset state. Capacitor C2 discharges quickly, N1's ③ and ⑦ pins jump to low level, relay J is energized, and the fax machine power is turned on. The fax machine needs 3 to 5 seconds to enter the normal working state, and then pick up the phone. At this time, the "beep" sound in the telephone line turns on the optocoupler, and the signal is amplified by VT1 and VT2, and the light-emitting diode DL1 lights up. The ② and ⑥ pins of the integrated circuit N1 are clamped at a high level. During a call or sending a text fax, ensure that the ③ and ⑦ pins of the integrated circuit N1 are at a low level. Relay J continues to maintain the energized state until the sending is completed and the phone is hung up, and the optocoupler is cut off. At this time, C2 is charged through R6, and after a delay of about 50 seconds, the relay is released and the fax machine is disconnected from the power supply.

The secondary of the power transformer T is 12V, which is rectified by bridge, filtered by C4 and C5, and stabilized by the three-terminal voltage regulator integrated circuit N2, and outputs a stable voltage of +12V. C3 is a high-frequency filter capacitor. OL2 and R7 act as power indicators. K2 is a through switch.

2. Production and Use

This control circuit is relatively simple, with few components, and it is also very convenient to install with a universal circuit board. As long as the components are selected correctly and the circuits are connected correctly, it can be used after simple debugging. When powering on the test, first disconnect the diode D2. The left circuit is the receiving circuit, and the right circuit is the control circuit. Power on each. After both parts can work normally, connect D2 for debugging.

The test of the receiving circuit should connect the fax machine and the telephone line. The fax machine should be connected to the power supply. When it is in standby mode, the light-emitting diode DL1 (red) is not on. When the telephone rings, the light-emitting diode DL1 (red) flashes with the ringing sound. After picking up the phone, DL1 emits red light and no longer flashes. When making a call, as long as you pick up the phone, DL1 will light up and you can talk normally. This means that the receiving circuit is normal. Otherwise, you should carefully check the circuit connection. The parameters of the components and the quality.

What needs to be adjusted in the control circuit is the delay circuit and the length of the delay time. It can be set according to the use of the fax machine. Adjusting C2 and R6 can change the delay time. Just press the touch switch Sw, the relay J is energized, the power is turned on, and after a delay, the relay J is automatically released and the power is cut off. This shows that the control circuit is working normally. At this time, the diode D2 can be connected. When there is a ringing signal, the relay is energized and the power is turned on. When there is no signal, the power is turned off after a delay.

When you pick up the phone, the light-emitting diode DL1 keeps on lighting. As long as DL1 is on, the fax machine should remain powered on. If DL1 is not on, the control circuit will automatically cut off the power supply of the fax machine after a delay of several tens of seconds. At this time, the entire circuit works normally.

In use, some laser fax machines store the received text and image information first when receiving text and images, and start printing only after the signal is received. If automatic reception is adopted, when the signal is received, before all the information is printed out, the power of the fax machine is cut off, which affects normal reception. The delay time should be determined according to the fax machine used and the number of pages of text and images that are frequently received. We use a Panasonic fax machine, and the text and images that are frequently received are all within 5 pages, so the delay time is set to about 50s. If it exceeds 5 pages, a longer delay time can be set. If manual reception is adopted, you can wait until the reception is completed before hanging up the phone, or manually close the direct switch on the panel to complete the reception of multiple pages of text and images. If it is synchronous reception, that is, receiving and printing are carried out at the same time, the delay time can be shorter. As long as the fax signal exists, the control circuit will keep the fax machine in the power-on receiving state. In short, the delay time can be set according to actual needs.

The control circuit has been tested in severe convective weather for many times. Although the control circuit has been damaged by lightning for many times, the fax machine it protects is still safe and sound. Its "sacrifice pawns to save the king" effect is still worth a try.