Showing posts with label telephone. Show all posts
Showing posts with label telephone. Show all posts
Thursday, September 18, 2014
Telephone Tapping Indicator
This simple circuit can indicate a misuse or tapping of Telephone line through a loud alarm. The circuit 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 circuit.When the switch S1 is closed, circuit becomes active and the telephone enters into the armed state.
Telephone Tapping Indicator Circuit diagram
Setting
Connect the circuit 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.
Telephone Tapping Indicator Circuit diagram
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.
Connect the circuit 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.
Parallel Telephone with Secrecy and Call Prevention
This schema provides secrecy when two or more telephones are connected in parallel to a telephone line. The schema also prevents incoming calls to as well as outgoing calls from other telephones connected in parallel, except from the one lifted first.
When someone picks up the handset of the telephone connected in parallel to the original (master) phone for making an outgoing call, no dial tone is heard and the phone appears to be dead. But when a call comes, the ring signal switches the SCRs ‘on’ and conversation can be carried out. As soon as the handset is kept on the hook, the SCR goes off and the telephone can again only receive incoming calls.
When someone picks up the handset of the telephone connected in parallel to the original (master) phone for making an outgoing call, no dial tone is heard and the phone appears to be dead. But when a call comes, the ring signal switches the SCRs ‘on’ and conversation can be carried out. As soon as the handset is kept on the hook, the SCR goes off and the telephone can again only receive incoming calls.
Parallel Telephone with Secrecy and Call Prevention Circuit Diagram
When a call comes, conversation can be made only from the telephone which is lifted up first. To carry out conversation from the other telephone, the handset of the telephone that was lifted up first has to be placed on the hook and then the push-to-on switch of the associated schema of the other telephone has to be pressed after lifting up its handset. Thus the schema ensures privacy because both the telephones cannot be active at the same time.
Those who are don’t need parallel telephones can rig up the associated schema of a single telephone to work as an outgoing call preventer. An outgoing call can be made only when one lifts up the handset and presses the push-to-on switch of its associated schema.
The polarity of the telephone line can be determined by a multimeter. To avoid confusion, a bridge rectifier can be used at the input of the schema.
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