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| Simple AM Transmitter Circuit |
AM transmitter circuit using only one transistor is described in this project.This is a low power AM transmitter.
How to build an am radio transmitter
We use a single transistor to achieve this circuit and a very few components less than only six parts is used for making this am transmitter circuit.circuityep.com.The power of this transmitter is around 100 milliwatts.
The range of the transmitter under idle conditions is around 100 meters.But the power can be incredibly increased using an Rf power amplifier which can be easily designed using transistor / mosfet.
Components used
» Transistor - BD 139
» Crystal - 500 KHz
» Torroid / Inductor - 100 Micro Henry
» Capacitor - 1mfd / 25 v (electrolytic), 224 pf (ceramic)
» Resistor - 10 ohms, 4.7 kilo ohms, 10 kilo ohms, 1 kilo ohms
» Power supply - 12 Volt DC
» LED - Red
Working
The heart of the circuit is the medium powered transistor which is BD 139.In olden era we use valves to achieve this.Even now valves are available in retro stores/ some amplifiers still use that old technology.
Those electronic valves consumes a lot of electricity, also the price is very high.The durability of some valves is less.So I use this cheaply available transistor.It cost for me around 1 dollars only.
So the whole project can be completed under 2 dollars which is very much exciting to beginners in electronics as they don't worry about cost.The main advantage of this circuit is that the troubleshooting/ servicing and understanding the circuit is so simple.
The transistor base is biased to positive of the 12 volt DC supply using a 10 ohms resistor which is clearly labelled in the figure.
As this transistor(BD139) is an NPN transistor in nature we must give the base terminal of this transistor a positive voltage.This is done to turn on a transistor, as a result current flows from positive terminal of power supply and enters the collector finally leaves to ground through emitter of transistor which results in a current flow through the circuit.
The frequency is generated using a crystal oscillator of 500 Kilo Hertz frequency.This frequency is in AM band.So that using a radio receiver we can easily tune the frequency in 500 KHZ AM band.
The signal to be transmitted is given into base terminal of the 139 transistor using a stereo pin.A mono pin can also be used.
We can transmit music using a music player/computer/mobile phone etc.The sound itself can be transmitted using microphone.A preamplifier is used while we are going to use microphone as input.
The music signal we are feeding is mixed with the 500 KHz Am carrier frequency wave.This carrier wave is then transmitted to the open space.The antenna propagates the signal.As we know this is a simple electronic project for beginners we don't use much longer antennas as the power output here is only a few milli amps.
Here we can use an antenna length of 1 meter.
But using an RF power amplifier the output power can be increased to a greater level.At that point we can use long antennas or dipole antennas which can transmit the signal to a greater
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