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AM Transmitter – Easy/Intermediate

Building the circuit:

 

 

To build this circuit you will need: A pack containing all these parts is available here

Ω      Two NPN Transistors

Ω      Two 1K Resistors

Ω      One 1M Resistor

Ω      One 4.7K Resistor

Ω      One 100K Resistor

Ω      Two 220pF Capacitors

Ω      One 100nF Capacitor

Ω      One 100pF Capacitor

Ω      One 100uH Inductor

Ω      One 9V Battery 

Ω      One 9V battery clip

Ω      A breadboard

Ω      Some hook-up wire

 

The Circuit:

AM Transmitter Circuit Diagram

Above you can see the schematic for the transmitter; C1 and C2 along with L1 provide the sine wave oscillator feeding into T1 to amplify it. The input goes into transistor T2 which modulates the oscillator. C3 is used to smooth the output. The antenna is made up of a single long piece of wire.

Construction:

AM Transmitter Construction

Start by placing the hook-up wires into the breadboard as follows. The yellow wire is a long piece of wire which will act as our antenna.

AM Transmitter Construction

Then place resistors in the breadboard as shown above.

 AM Transmitter Construction

After that place the ceramic capacitors I following the picture, the two capacitors close together are the 220pF capacitors. The one above is the 100pF capacitor, and the one on the right is the 100nF capacitor.

AM Transmitter Construction

Then place the two transistors in the breadboard as follows.

AM Transmitter Construction

Finally place the 100uH inductor across the left pin of the left 220pF capacitor, and the right pin of the right 220pF capacitor.

You’re done! Attach the input between the negative rail and the middle pin of the right transistor, then turn on an AM radio and try to tune in!

 

Overview/How it works:

This circuit takes an audio signal from an input and transmits it over short distances using an AM transmitter. AM transmitters work by creating a sine radio wave of a certain frequency (the carrier frequency) which can be picked up by a receiver tuned to the same frequency. However, just transmitting the frequency doesn’t transmit any information, so to transmit the information we also vary the amplitude, this is called amplitude modulation, or AM. The amplitude can be thought of as the loudness of the wave.

So how do we create the radio waves? Well, it’s very simple, simply applying a voltage to a wire creates a radio wave, so we can use this to transmit the signal. To create the sine wave we use a capacitor and an inductor, the inductor is a coil of wire which creates a magnetic field when you apply a voltage to it. When you stop applying the voltage, the magnetic field collapses back into voltage, this takes time, so the voltage slowly fades down. Combine this with a capacitor (or 2 in this case) and you get a sine wave.

AM Transmitter Modulation

 We take this sine wave and apply it to the base of a transistor to amplify it. The modulation is created by taking an audio input (in our case the stylophone we made earlier in the projects) and apply this to the base of a transistor which supplies the voltage to the oscillator, in this way you get an Amplitude Modulating transmission, like shown below.

This information is received by a receiver tuned to the same frequency (again using the capacitor and inductor oscillator), this is called a resonant circuit. To tune it, you use a variable capacitor in the oscillator circuit.

In our case, the receiver will be a standard AM radio, tuned to roughly 1300m Medium Wave.

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