Free Electronic Circuits & 8085 projects

Electronic projects with circuit diagram and 8085 microprocessor projects.

Telephone line anti-theft alarm

Published on Mar 14 2011 // DIY Telephone Circuits

The alarm system to monitor and prevent the unlawful theft of your phone line. When someone outside of your phone line and machine Daoda when the alarm on the issue of interference signals, so that thieves can not dial-up, and with sound and light tips to the owner was stealing the phone. The figure is a schematic diagram of the anti-theft device, usually the post office sent out about the online program-controlled machine out of standby voltage of 50V, the D1-D4 above the positive polarity, D5 breakdown by R2 so that the base potential rise T1, T1 Guide Tong, T2 end, T3 end by T4, T5 does not work consisting of oscillator circuit. When the stolen phone to mention machine, outside the voltage down to 9V or so, can not make the breakdown D5, T1 was no bias due to the cut-off, T2 increased on-base potential. T2 to T3 the turn after turn and drive-level audio oscillator circuit, LED, DY audible alarm, and through the LED, DY will interfere with the signal plus out online, while outside the voltage down, so that stolen phones is not Dial-up and forced to hang up. Although machine owners put the voltage, increased the cut-off but T6 T1 or T7 (with T6, T7 is for two tubes, easy installation, regardless of polarity of the line can access, D6, D7 isolators) by telephone engine to obtain the base current and turns, so that T3 can not turn oscillator circuit does not work. Therefore, the normal dial-up phone owners can not be affected. Between the phone and outside due to the owner then there is only a small resistance R8, R9 zero vibration so the phone calls are not affected.

Telephone line anti-theft alarm circuit

Component Selection
  D1-D4, D6, D7, such as IN4007 rectifier can be used such as general, D5 optional 18V regulator about, DY is the 3V electromagnetic buzzer. Resistor 1/8W carbon film resistors use the other component model, as shown selected.