A simple electromagnetic telephone ringer circuit is a device that helps you know when someone is calling without needing to connect directly to the phone line.
This is really useful in situations where you can not or do not want to tap into the phone lines.
The circuit works by picking up the electromagnetic field created by the ringing signal in the phone line which then turns on a buzzer to alert you about the call.
You can easily power this device with a 9V battery.
However, it is important to remember that using this kind of device or any other that is not officially allowed with phone lines might break the law in some places.
So, make sure to check if it is legal before trying to use it.
Circuit Working:
Parts List:
| Component | Specification | Quantity |
|---|---|---|
| Resistors (1/4 Watt) | 470k, 12k, 1k | 1 each |
| 2.2k | 2 | |
| Capacitors | Ceramic 0.1µF | 1 |
| Electrolytic 1µF 25V | 1 | |
| Semiconductors | Transistor BC557 | 1 |
| Transistor BC547 | 3 | |
| LED 5mm, 20mA | 1 | |
| Other Components | Air core coil make 50 close turns of 28 SWG enameled copper wire | 1 |
| SPST switch | 1 | |
| Piezo buzzer | 1 | |
| 9V Battery | 1 |
In this circuit several important parts are included like sensor coil, a group of transistors, capacitors, a bunch of resistors, a piezo buzzer, and a power source 9V battery.
When the phone rings the alternating current AC signal in the phone line creates an electromagnetic field around the coil L1.
This signal is picked up and boosted by the transistor Q1.
Then, Q2 processes this signal even more making it stronger.
The boosted signal then turns on transistors Q3 and Q4 acting like a switch to turn on the buzzer.
When a call comes in an LED1 lights up and the buzzer makes a sound to alert you.
The whole circuit runs on a 9V battery and switch S1 that lets you turn the circuit on or off whenever you want.
Formulas with Calculations:
The below mentioned formulas with calculations are used for Simple Electromagnetic Telephone Ringer Circuit:
Induced Voltage in Coil (L1):
V = -N (dΦ/dt)
where,
- V is the induced voltage
- N is the number of turns in the coil
- Φ is the magnetic flux
Example: If N is 50 turns and flux changes at 0.002 Wb/s then:
V = -50 × 0.002 = -0.1V
Transistor Amplification (Gain):
β = Ic / Ib
where,
- β is the current gain of the transistor
- Ic is the collector current
- Ib is the base current
Example: If Ic is 5mA and Ib is 50µA then:
β = 5mA / 50µA = 100
Capacitor Charging Time Constant:
τ = R × C
where,
Example: If R is 1kΩ and C is 1µF then:
τ = 1kΩ × 1µF = 1ms
How to Build:
To build a Simple Electromagnetic Telephone Ringer Circuit following steps are needed to be followed:
- Assemble all the components as mentioned in the above circuit diagram
- Connect the base of transistor Q1 to coil L1 and connect the coil L1 to any one wire of telephone line.
- Connect the collector of transistor Q1 to positive supply of the circuit through resistor R1.
- Connect the emitter of transistor Q1 to GND of the circuit.
- Connect base of transistor Q2 to collector of transistor Q1 through resistor R2.
- Connect capacitor C1 positive to positive supply for the circuit and negative of capacitor C1 between collector of Q1 transistor and resistor R2.
- Connect capacitor C2 one end to positive supply of the circuit and other end between resistor R2 and base of transistor Q2
- Connect emitter of transistor Q2 to positive supply of the circuit and collector of Q2 between the junction of resistor R3 and R4
- Connect base of transistor Q3 to the collector of transistor Q2 through resistor R3.
- Connect collector of transistor Q3 to positive supply of the circuit.
- Connect emitter of transistor Q3 to one end of piezo buzzer and other end of buzzer connect to GND.
- Connect collector of transistor Q4 to positive supply of the circuit through resistor R5
- Connect the base of transistor Q4 to collector of transistor Q2 through resistor R4
- Connect emitter of transistor Q4 to GND through LED1.
- Connect one end of SPST switch S1 from positive supply of the circuit and other end of S1 connect to +9V battery and (-) negative of 9V battery connect to GND.
Conclusion:
The Simple Electromagnetic Telephone Ringer Circuit is an easy and useful way to know when someone is calling without being hooked up to the phone line.
It keeps a safe distance from the line while still giving you sound and light alerts.
This circuit is great for homes, offices or places that are really loud where a regular phone ringer might not be heard.
You can change some parts to make it work better for different phone lines.