Phone Line Busy Indicator Circuit Diagram
A caution is issued regarding this circuit's potential for complications. Individuals should consider constructing an alternative design instead.
Have you previously experienced interruptions in communication due to another person answering a telephone call? This straightforward circuit is designed to resolve such issues. It indicates when the telephone is occupied by illuminating a red LED. When the phone is idle, a green LED is activated. It does not require an external power supply and can be incorporated virtually anywhere within the telephone, even internally.
Circuit Diagram
Components:
- R1 3.3K 1/4 W Resistor
- R2 33K 1/4 W Resistor
- R3 56K 1/4 W Resistor
- R4 22K 1/4 W Resistor
- R5 4.7K 1/4 W Resistor
- Q1, Q2 2N3392 NPN Transistor
- BR1 1.5 Amp 250 PIV Bridge Rectifier
- LED1 Red LED
- LED2 Green LED
- MISC Wire, Case, Phone Cord
Notes:
- This is a remarkably uncomplicated circuit and can readily be assembled on a prototype board and installed within the telephone.
- LED1 and LED2 cycle on and off during phone calls.
- It is unnecessary to confuse the Tip and Ring connections.
- The ring voltage present on a telephone line typically ranges from 90 to 130 volts. Precautions must be taken to ensure safety during line connections. :-)
- Depending on the specific location or jurisdiction, it may be necessary to obtain approval from the telephone provider before connecting this circuit to the line. An inspection might also be required.
- If the circuit introduces distortion to the telephone line, a 680 ohm resistor should be connected between one of the incoming line wires and the bridge rectifier.
Integrated Circuits: This circuit utilizes the 2N3392 NPN Transistors, which are commonly employed as general-purpose switching devices. These transistors act as switches controlled by the bridge rectifier's output. They allow current to flow when activated by the rectifier, enabling the LEDs to light up based on the phone line activity. The 2N3392 is a small-signal NPN transistor, frequently used in low-power switching applications. It's a reliable and widely available component for building circuits such as this one.