In this project we are going to designed single transistor FM transmitter circuit that works effectively in 1-2 km range. In this circuit we have used electret condenser microphone as input source that gain signal in analog form. You can easily make this transmitter on PCB or without PCB because it consists on very few components. The coils and the antenna can be designed as a part of the PCB as antenna if you make it on PCB. If you want to increase the range of the transmitter then you must install the long high antenna with any wire. To understand the working of this transmitter we divide it into three main stage s as follows
- Input stage
- Modulation Stage
- Output Stage
1. Input Stage state of FM Transmitter
Input stage consists of capacitors C1 and C3, and resistor R1 and microphone.Input signals into the FM transmitter must not contain a DC component as it would adverse affect the modulation stage and therefore no FM signal would be generated therefore C3 capacitor is used to control the DC adverse.Resistor R1 (resistor) provides small amount of current to the base of transistor so in this way transistor remain on and 1st stage complete in this way.
The oscillator/modulation stage consists of C2(variable capacitor for adjusting frequency), C4,L1, and R2 with the frequency of oscillation being determined by C2 and L1 only.The modulation stage provides carrier signal that is modulated by the input signals keep in mind that input signals gained by the microfone or any input source.When we give power to circuit a rush of current flows through the L1 (inductor). This current pass through the capacitor C4 and due to this current flow through R2. R2 is connected with the emitter of transistor so the same voltage present on the emitter. As voltage increase on the emitter it reduces In VBe reduces the condition of transistor.When power is applied to the circuit, a rush of current flows through the inductor.
This rush of current will also pass through C4 via capacitive coupling which will result in current flowing through R2. Current flowing through R2 will result in a voltage drop across R2 and since R2 is connected to the emitter of Q1, the same voltage will be present on the emitter of Q1. As C4 begins to charge the small amount of current flow through C4 reduces and in this way less current pass through R2 due to this size of VBe increase and conduction increases.Due to increase in conduction c4 discharging and whole cycle start to repeat again.
3. Output/Emitting Stage
The signals that are generated by modulation stage are emitting through long wire (antenna).How to Receive Fm transmitter siglas The Fm Transmitter signals can be received through FM receiver such as simple FM radio and in the present age mostly smartphone has also the feature of FM receiver. The single transistor FM transmitter frequency can be searched through FM receiver between 88MH. This frequency can be changed with the variable capacitor or expands the rings of inductor coil L1. Fm transmitter frequency should be adjusted on empty area of searching otherwise it will disturb the frequency of near tuned radio which is not allowed by law.
Its very very Small FM transmitter you can make it with few components
- Transistor 2n2222/2655c
- Inductor coil 5 turns with 16 no copper wire
- pf 102(100nf)
- pf 47
- 100 ohm resistor
- 4.7k resistor
- 3 to 6v battery
Animated circuit diagram
Antenna should be connected with emitter point of transistor high antenna will increase the range of transmitter