We have published so many FM transmitter circuits at CircuitsGallery, so this time I am going to introduce a simple AM transmitter circuit based on 555 timer. Firstly let us have a look at what is Amplitude Modulation. AM is a modulation technique where the amplitude of carrier varies according to the input message signal. AM has been a widely used modulation technique due to its longer range but has the disadvantage of lesser noise immunity. It is commonly used in walkie talkie applications.Here 555 multivibrator generates a frequency of AM range so you can receive the transmitted audio signal in the AM radio receiver. This simple AM transmitter is suitable for school projects.
Circuit Diagram of Amplitude modulation transmitter
- 555 IC
- Transistor BF 194
- Resistor (10K, 1K, 2.2K)
- Pot 4.7K
- Capacitor (.001uF,.01 X2)
555 Transmitter Working
- For better understanding of the working, please read our article on 555 astable multivibrator which I had explained with animation.
- Here the carrier signal for AM modulation is generated by a 555 timer operating in astable multivibrator mode with the oscillation frequency set to AM range.
- Input i.e. the message signal is applied to the reset terminal (4th pin of multivibrator).
- If the amplitude of input message signal is increased, 555 starts to turn ON as it’s connected to the 4th pin. (Since 4th terminal connected to VCC causes it to turn ON because it is active low logic pin).
- And if the amplitude is decreased, it starts to turn OFF. (As 4th terminal connect to ground or 0V causes the 555 to turn OFF.)
- The output (3rd pin) is connected to the base of a BF194 transistor as it can operate at high frequencies.
- The emitter terminal of the transistor is connected to an antenna for better transmission. You can use a simple conducting wire as the antenna.
- BF 194 can drive high frequency, here astable multivibrator generates radio frequency of AM range, then you can receive the transmitted signal in the radio reciever.
Here is the screenshot of the simulation in Proteus.