Many times people forget to put the phone back on the hook properly due to which the telephone line goes busy and you can miss some important calls. To solve this issue in this tutorial we are demonstrating a telephone off-hook indicator.
The purpose of this circuit is to give an indication of when the telephone receiver is taken off the hook by activating the LED and a piezo buzzer. This circuit provides both visual and sound indications for your convenience, the LED will start flashing and the piezo buzzer will produce beeping sound signals.
Hardware Components
The following components are required to make Telephone Off-Hook Indicator Circuit
| S.no | Component | Value | Qty |
|---|---|---|---|
| 1. | Input Supply DC | 6-12V | 1 |
| 2. | Transistor | 2N4401, 2N4403 | 1 |
| 3. | Resistor | 3.3MΩ, 560KΩ, 4.7KΩ, 1KΩ, 470RΩ | 2, 1, 1, 1, 1 |
| 4. | IC | NE555 Timer | 1 |
| 5. | LED | – | 1 |
| 6. | Piezo Buzzer | – | 1 |
| 7. | Electrolytic Capacitor | 10µF | |
| 8. | Ceramic Capacitor | 100nF, 0.01µF | 1, 1 |
| 9. | Potentiometer | 100KΩ | 1 |
NE555 IC Pinout
For a detailed description of pinout, dimension features, and specifications download the datasheet of NE555 IC
2N4403 Pinout
For a detailed description of pinout, dimension features, and specifications download the datasheet of 2N4403
Telephone Off-Hook Indicator Circuit
Working Explanation
The circuit can be operated at any DC voltage ranging from 6 to 12 volts. It is using two transistors (Q1= 2N4401 & Q2= 2N4403), a 555 timer IC and a few other discrete components. The phone lines have a voltage of 48V DC when the phone is on-hook but it drops to 10 volts approx. When the phone is off-hook. Because of this voltage drop, the transistor Q1 starts conducting and the Q2 will conduct as well. This will activate the 555 timer IC and the LED will start flashing as well as the buzzer starts producing beeping sounds. the speed at which the flashing of the LED and beeps depends on the 100K variable resistor.
Applications and Uses
It is really convenient and can be used for many telephone-related tasks such as, it will indicate whenever you accidentally do not put the phone receiver back on the telephone correctly or when the phone that has many extensions is busy
