Introduction of the function principle of passive monitoring telephone alarm

This article introduces a self-developed passive monitoring telephone alarm. In addition to the main functions of the same type of alarm, its biggest feature is that it does not require power (provided by the telephone line) and does not change the line characteristics. Because of its unique passive design, it will be safer and more reliable when used in important places such as homes, warehouses and unmanned computer rooms.

Main functions 1. Two sets of alarm telephone (paging) numbers can be set, which can be changed at any time and stored for a long time. 2. When alarming, it will automatically dial out two groups of numbers alternately (the number of cycles is adjustable). 3. It can also monitor the scene of the alarm when answering the alarm. 4. Under the holding state, it will not affect the normal incoming and outgoing calls of the parallel phone.

Principle introduction: The alarm is mainly composed of the following parts.

1. The alarm trigger and the power supply form a circuit. Take the remote alarm transmission system as an example to briefly describe its working principle. When the communication equipment is faulty, the electrical contacts of the probe will be disconnected. At this time, the current of IC1 is changed to the charging current of C7 through R13, AN, and the current from the probe to ground, and Q6 and Q5 are turned on, and the current is limited by R12. After the external line voltage is added to C6 through D9 to form the working power supply VDD before off-hook. While Q5 is turned on, R16 sends a trigger signal to the S terminal of the alarm trigger IC3-2, so that IC3-2 is set, and the high level output from its Q terminal enables Q2 and Q1 to be automatically off-hook through R17. , And replace D9 by D8 to provide follow-up power VDD for the control circuit.

2. Analog telephone circuit. The function of this circuit is to complete the off-hook and dialing tasks and provide subsequent power for the control circuit. Mainly completed by IC1 and some parts. Under normal circumstances, the external line of the phone is added to Q1 after being oriented by D1 to D4, and R8 provides the maintenance voltage for the power terminal of IC1. When an alarm situation occurs, the off-hook signal sent by the alarm trigger will make Q1 and Q2 conduct, and realize automatic off-hook. The off-hook voltage provides power to IC1 via R4, R5, D5, and D6 on the one hand, and at the same time makes Q4 conductive through R7, grounding the {14} foot of the start end of IC1 to prepare for dialing. At this time, the dialing command sent by the cyclic dialing control circuit (equivalent to pressing the emergency dialing key M1 or M2), the number stored in the memory key of M1 or M2 is output by the {12} pin of IC1 as a dual-tone multi-frequency signal, and is sent through Send it after zooming in.

3. Circular dial control circuit. When the working power supply VDD is formed, the circular dial oscillator composed of IC4-2 and R19, R20, D12, and C11, one of the six Schmitt inverters, starts with 60 seconds high level and 2 seconds low level. Output high level, one of its functions is to provide a frequency-divided signal for the CP terminal of the "D" flip-flop IC3-1 of the special connection, and first make the Q terminal of IC3-1 output low level, which is sent as the strobe level. Enter the second input or NOR gate IC5-2. The second role is that after the delay of R21 and C12, the reverse from IC4-1 to low level is also sent to IC5 as the dial clock. When the input terminals of IC5-2 are all low level, the high level of its output will charge C14. This charging current will make Q8 connected in parallel with M2 instantaneously turn on and send a dialing command (equivalently press the M2 key), and send the number stored in the memory key M2. After dialing, the cyclic dial oscillator will provide about 60 seconds to answer the alarm and monitor the time on site. After 60 seconds, when IC4-2 outputs a low level, D11 will be turned on, forcing Q2 and Q1 to end to hang up and end the first call. After 2 seconds, when IC4-2 outputs high level again, D11 will be cut off, and the restraint of Q2 and Q1 will be released. Therefore, Q2 and Q1 are turned on again, realize the second off-hook and repeat the above process (the number of cycles is determined by the output end of the selected IC6) until the system is reset. The difference is that Q7 and Q8 are turned on alternately, and the two numbers are alternately dialed to improve the success rate of the alarm.

4. Alarm sound generation and monitoring circuit. When IC4-2 outputs a high level, D16 will be cut off, and the alarm sound control circuit composed of IC4-3, etc. will work. Its {8} pin will control the alarm tone oscillator composed of IC4-4 by outputting 5 seconds high level and 1 second low level, so that it will start to vibrate once every 4 seconds, and C17 will send the alarm tone Into the audio amplifier IC2 for amplification. The amplified signal is fed to the called party by the C4 feed line.