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Telephone Line Engaged Indication

2016-02-07 23:00  
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Figure 1 

The circuit was designed to indicate that the telephone line is being used in an environment where parallel telephones are installed in a single telephone line.

BF256B – an N-channelJFETRF amplifiers designed forVHF/UHF amplifiers that is enclosed in TO-92 package
1N4001 – a 1.0 Amp Silicon rectifier with voltage range of 50 to 1000 Volts and possessing features such as guaranteed high temperature soldering, high current capability, diffused junction, low reverse leakage, utilizes void-free molded plastic technique for low cost construction, and carries Underwriters Laboratory Flammability Classification of 94V-0 by its plastic package
Zener Diode – a special kind of diode that permits the flow of current in just one or forward direction as a normal diode, but will also allow in the reverse direction if the voltage is above or larger than a certain value of the breakdown voltage
In an office, establishment or home, there are instances where a single telephone line is being used by several telephones connected in parallel to that line. This incident was normally referred to as party line or shared service line. The need for this circuit is to be able to indicate if one handset has been raised making the line engaged for conversation. The connection of the circuit will be positioned in parallel with the two cable lines of the telephone. The cables are typically made of twisted pair solid conductor or copper wire with 22 or 24 gauge.

The conversion of alternating current (AC) to direct current (DC) is done by the bridge diode composed of D1 to D4. The arrangement of the diodes represents a more efficient full wave rectification where the whole of input waveform is converted to one of constant polarity, positive or negative, at its output. By doing this diode structure, the circuit is ensured and protected from error polarity connection.

If the telephone line is not being used, the voltage in the telephone is approximately 50 V to 70 V. The gate ofFETQ1 is turned ON by this voltage through R1 & R2 so that the gate will not be inOPENstate. At the instance that the telephone handset is raised, the voltage will drop which will turn ON Q1 and simultaneously lighting D6. This will indicate the current status of the telephone line. The current ofLEDD6 is limited to 10 mA due to the current leakage that came from Q1 and R3. The voltage across Q1 is prevented to reach more than 10 V by the presence of the Zener diode while the parasitic pulses are filtered by the capacitor C1. The resistor R1 can be replaced with a higher value up to 220K ohms if some problems occur during the operation of the circuit. The circuit is being supplied by a 3 V to 9 V batteries since the power consumption is minimal.

R1=1Mohms
R2=100Kohms see text
R3=47 ohms
C1=1uF 63V
D1-2-3-4=1N4001
D5=10V 1W Zener
D6=LedRED
Q1=BF256B
B1=3V…..9V Battery
The main use of the telephone line engaged indicator is to alert an officemate or a house member that the telephone line is being used. This will prevent any abrupt interruptions during the conversation for courtesy purposes.

Source:users.otenet.gr/~athsam/clue_engaged_telephone_line.htm


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