Telephone Line Monitor Circuit

Circuit Diagram 

Description

 If you feel that somebody is tampering with your telephone line you might find this little circuit useful. It detects if there is another telephone connected to the line, if there is a short or an open line. Sound and a flashing light will tell you which is the current situation. The speaker is practically cut out during a normal conversation thus preserving privacy, only the LED will flash occasionally. The circuit does not require any battery and takes the supply from the telephone line itself. The transistors used are wired in a reversed biased fashion thus behaving as oscillators. You might try the 2N2222A as an alternative (not tested). This monitor is, of course, suitable only for analogue lines. Watch the polarity of the input line: the circuit will not be damaged by a polarity reversal but it will not operate correctly. 

Author: smartkit
Source http://www.electronics-lab.com/

08:54 0 comments

Telephone Ringtone Generator Circuit

Description

This is a simple home telephone ringtone generator circuit which is built with applying only several electronic components / parts. It generates simulated telephone ringtone and requires only DC supply with 4.5V DC to 12V DC voltage.
One may possibly use this circuit in ordinary intercom or phone-type intercom. The sound is pretty loud when this circuit is operated on +12V DC power supply. Even so, the volume of ring sound can be adjusted.

Circuit Diagram

CD4060B is chosen to produce three kinds of pulses. Preset VR1 is fine-tuned to get 0.3125Hz pulses at pin 3 of IC1. At the same time, pulses obtainable from pin 1 will be of 1.25 Hz and 20 Hz at pin 14. The three output pins of IC1 are connected to base terminals of transistors T1, T2, and T3 through resistors R1, R2, and R3, respectively.

Working with a built-in oscillator-type piezobuzzer generates about 1kHz tone. In this particular circuit, the piezo-buzzer is turned ‘on’ and ‘off’ at 20 Hz for ring tone sound by transistor T3. 20Hz pulses are obtainable at the collector of transistor T3 for 0.4-second duration. Just after a time interval of 0.4 second, 20Hz pulses become again obtainable for another 0.4-second duration. This is followed by two seconds of nosound interval. Thereafter the pulse pattern repeats by itself.

Author K.UDHAYA KUMARAN, VU3GTH

07:17 0 comments

8 Line Intercommunication using 89c51 Circuit Diagram

Description

Private branch exchange
This project is aimed to make the low cost intercom system by which eight telephone lines can be interfaced with this hardware.

Main card:
In this card there is 89c51 micro controller is used. Micro controller is controller the DTMF encoder MT8870 and the Analog switch MT8816.

SLIC card:
In this card there is constant current source is used to activate the telephone. The lm339 is used to detect the hand off and hand on condition. There is same circuit are repeated because we have to interface the 8 telephone.

Summary:
When the telephone is hand on the output of the lm339 change, it is detected by micro controller that the telephone is hand on, then micro controller is make the analog channel by using the MT8816. the LM555 is used to generate the dial tone. 

 Circuit diagram

Click on Image And Zoom 

Author: Ashad mustufa
E-mail: mustufa66@hotmail.com
Source: http://www.electronics-lab.com

10:51 0 comments

Parallel Phone Tap Circuit Diagram

Circuit Diagram

Description

In a recent article, I discussed the difference between parallel and series telephone taps. In this article, I present a very simple and reliable parallel phone tap circuit useful for recording telephone conversations onto a tape recorder.

When installing a telephone tap, its advisable not to cut the telephone line in anyway. Cutting the telephone line for any time is detectable, and therefore series devices can be detected in this way. Parallel devices can be installed without cutting any telephone wires, and is thus harder to detect.

The diagram shows a simple circuit using just 6 very cheap and common components. To build the circuit, you will need some Veroboard or prototype board (whatever you favourite is), some wire, and a 3.5mm plug that will connect to your tape recorder’s microphone input socket. I have left it to the reader to work this bit out, as its very simple to do but quite awkward to explain!
 
Source:http://www.spyreview.co.uk/2006/10/08/parallel-phone-tap-circuit-diagram/

09:38 0 comments

Telephone amplifier

Circuit diagram

While talking to a distant subscriber on telephone, quite often we feel frustrated when the voice of the distant subscriber is so faint that it is barely intelligible. To overcome the problem, circuit of an inexpensive amplifier is presented here. It can be assembled and tested easily. There is no extra power source needed to power up the circuit, as it draws power from the telephone line itself. The amplifier will provide fairly good volume for the telephone conversation to be properly heard in a living room. A volume control is included to adjust the volume as desired. The circuit is built around IC LM386. Diodes D6 and D7 are used to limit the input signal strength. Transformer X1 is a transistor radio's output transformer used in reverse. As original secondary (output) winding is connected in series with the telephone lines, the speech signals passing through the lines cause change in the magnetic flux in the core of transformer and thereby induce signal voltage across the primary winding. This audio signal is used as input for IC LM386. Diodes D2 through D5 connected in bridge configuration constitute a polarity guard so that the amplifier is powered with correct polarity, irrespective of the line polarity, Zener diode D1 may have any breakdown voltage between 6 and 12 volts range. e. There is no need of a separate power switch as the circuit energises (via the normally open contacts of the cradle switch) when one lifts the handset. The circuit may be wired on a general-purpose PCB or by etching a PCB for this circuit. The circuit can be easily tested by connecting a 6 volts supply to line terminals 1 and 2. A hissing sound will be heard from the loudspeaker. Now connect 6V AC from a transformer to terminals 1 and 2 and observe hum in the loudspeaker. The volume of the hum can be changed through potentiometer VR1. Diodes D6 and D7 limit the input below ± 700 mV. The circuit is to be connected to the telephone lines in series with the telephone instrument, as shown in the figure

Source  http://www.electronics-lab.com/contact/index.html

08:14 0 comments