Monitoring Circuit for Telephone Line Usage

Monitoring Circuit for Telephone Line Usage

Circuit Project Description

The circuit was designed to provide a monitoring scheme for a telephone line where several phones are installed, to indicate when one is being used.

Terminology 4N25 – a general purpose optocoupler that consists of a gallium arsenide infrared emitting diode driving a silicon phototransistor in a 6-pin dual in-line package used in microprocessor inputs, digital logic inputs, and power supply regulators due to its features such as high isolation voltage between input and output, operating temperature up to +110ºC, and available in wide-lead spacing4011 – a quad 2-input NAND gate integrated circuit, generally characterized by small fluctuation in voltage supply, very high impedance, outputs that can sink and source, one output can drive up to 50 inputs, high speed gate propagation time, high frequency, and low power consumption7812 – 3-terminal 1A positive voltage regulator with short circuit protection, thermal overload protection, output transistor safe operating area protection, output current up to 1A, and output voltages from 5V to 24V Circuit Explanation

The circuit is not only useful in indicating  a phone being used but it can also cut from a stereo amplifier some certain sounds that are too high, some sound of the television, or turn ON some light when a telephone rings that needs to be answered or unhooked. Included in the circuit is a pair of 12 V 2X2 relay contacts RL1 that are connected to a 2-pin connector 2.54 mm step J2. This can be used when connecting any appliance or equipment that is desired. The telephone line will be connected to J1 having the required polarity.

The unused or idle state of the telephone will possess a voltage of about 48 V to 50 V. the photodiode is activated by this voltage while IC1 isolates the circuit from the telephone line at the same time. The photodiode is capable of converting light into either voltage or current depending on the operation method while a phototransistor is more sensitive to light than a photodiode and produces more current output for a given light intensity. As the phototransistor turns ON in the IC1, the input of IC2A will become low while the output will become high since it is a NAND gate. This would then lead to a high input at IC2B through the delay circuit consisting of R4, R5, C1, and D6 and thus, having a low output. This would cause transistor Q1 to be OFF while relay RL1 is not active.

Upon raising the telephone, the telephone line voltage will drop to 6 V to 10 V. this would cause all the operation to work in reverse direction where RL1 will be turned ON. The duration that determines when the RL1 is closed or open depends on the type of telephone being used like the dial choice between disk or pulse system. The use of the delay circuit is to ensure the stability in the operation of RL1 as it is located between the gates of IC2A and IC2B and delays the changes on the input of IC2B. The relay RL1 will get activated when the telephone rings or the handset is raised, as long as the value of resistor R4 is 100K ohms. Changing the value of R4 to 1M ohms will cause the relay to be activated only when the handset has been raised. The circuit can be considered as a simple regulator due to its operation, while having a voltage of +12 V.

Part List

R4=100Kohm or 1Mohm
C1=100 or 220nF 100V MKT
C2=1000uF 25V
C3-4=100nF 100V
C5=4.7uF 16V
D5=1N5252 [24V 0.5W Zener]
D12=Red Led 3 or 5mm
IC1=4N25 optocupler
IC3=7812 [1A]
Q1=BD139 or BD679
T1=12Vac 500ma tranformer
RL1=12Vdc 2X2 relay
J1-4=2pin connector 2.54mm step
J2=6pin connector 5mm step
J3=2pin connector 5mm step
F1=Fuse 500mA [5x20mm]
PCB mount for Fuse
*All Resistors are 1/2W 5%


The usage of the circuit does not only focus on indicating if a telephone on the network is using the line to avoid interruptions where a LED is used as an indicator. It can also be a triggering point for other appliances in the house or office to perform certain operations and functions that is desired by the user.

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