Showing posts with label telephone. Show all posts
Showing posts with label telephone. Show all posts
Monday, September 8, 2014
Simple Telephone Tapping Indicator
This simple schema can indicate a misuse or tapping of Telephone line through a loud alarm. The schema is too simple and can be easily assembled on a common PCB. Line voltage of Telephone lines is around 48 volts DC in the On hook state. When the handset is lifted, this voltage reduces to 12 volt DC. This change in voltage level is used to activate the schema.When the switch S1 is closed, schema becomes active and the telephone enters into the armed state.
The high volt DC from the telephone line passes through R1 and VR1 and bias T1 into conduction. As a result, the collector of T1 goes to ground potential to inhibit T2 from conduction. Buzzer and LED thus remain off. When the handset is lifted, the DC voltage from the telephone lines drops to 12 volts. This turns off T1 and T2 conducts. Buzzer beeps and LED lights indicating that the telephone is using.
The high volt DC from the telephone line passes through R1 and VR1 and bias T1 into conduction. As a result, the collector of T1 goes to ground potential to inhibit T2 from conduction. Buzzer and LED thus remain off. When the handset is lifted, the DC voltage from the telephone lines drops to 12 volts. This turns off T1 and T2 conducts. Buzzer beeps and LED lights indicating that the telephone is using.
Setting
Connect the schema to Telephone lines using a telephone plug. The free socket of the telephone or Caller ID can be used. Close S1 and adjust VR1 till buzzer stops beeping. Lift the handset. Buzzer should sound. Otherwise, just adjust VR1 till buzzer beeps.
Connect the schema to Telephone lines using a telephone plug. The free socket of the telephone or Caller ID can be used. Close S1 and adjust VR1 till buzzer stops beeping. Lift the handset. Buzzer should sound. Otherwise, just adjust VR1 till buzzer beeps.
Thursday, September 4, 2014
Simple Telephone Security Device
This schema continuously checks the tel. lines and when it detects a misuse, it sounds an alarm. Additionally it transmits a loud disturbing tone through the telephone lines to prevent further misuse. The schema does not require any external power supply.
When the switch S1 is at the on position, the line voltage at the output of the rectifier diodes is approximately 48V which is enough to exceed the zeners (D5) break down voltage. So the T2 gets forward biassed. As a result the base of T1 is grounded and the remaining part of the schema doesnt get any supply. In this condition, schema draws so small current that is negligible and does not effect the normal operation of the telephone line.
When handset of any telephone connected to the line is lifted, the line voltage drops to 10V so T2 is switched off and T1 gets in forward biassed mode. The astable multivibrator schemary starts oscillating and the speaker starts sounding. The output of the astable multivibrator is also connected to the base of T1 so a loud sound is heard in the ear piece of the unauthorized telephone instrument.
Wednesday, August 20, 2014
Home Telephone FM Transmitter Circuit
Here the schematic diagram of home telephone FM transmitter. This schema connects in series with your home phone line and delivers the phone conversation through the FM band any time you pick up the telephone handset. Transmitted signal could be tuned by any FM receiver. The schema features an "On Air" LED indicator and also gives you a switch that can be utilized to turn off the transmitter. A special characteristic of the schema is the fact that no battery is required to operate the schema because electrical power is taken from your phone line.
The transmitter schema works by using only a short piece of wire aerial about 4" / 10 cm long to transmit the signal and a portion of the RF signal can also be radiated via the phone line itself. The schema may possibly be implemented to share or record conversations, but will not be meant for illegal use.
Source: Home Telephone FM Transmitter Circuit
Subscribe to:
Posts (Atom)
