Radio Transmitter

My first real project in electronics was an AM Radio transmitter that I made in the 9th grade.

One day at the end of 8th grade, while listening to the radio, I started wondering how radio transmission and reception works. I was intrigued by this wireless transmission of information and wanted to know how it works. I read a brief description of the phenomenon on how stuff works and was immediately hooked. I had an urge to make a radio transmitter myself, even though I had zero knowledge of electronics (did not even know about transistors!).  So over the summer, with the Internet and an 11th grade textbook as my teachers, I taught myself the basics of electronics, starting with semiconductors, diodes, transistors, capacitors, inductors and so on.

I researched the various kinds of modulation (AM, FM, phase modulation) and chose AM as it was easier to understand.

One of the most useful websites that I found was scitoys.com. In particular, http://sci-toys.com/scitoys/scitoys/radio/am_transmitter.html.

I spent a lot of time experimenting with various circuit diagrams available online. A lot of my learning came from experience. Studying what combination of components was common in circuits, I was able to identify the “functional groups” of circuits. I started improvising circuits, combining functional groups from different circuits to see how it affects gain and distortion. Although I could not design circuits, I knew what each component of the circuit was responsible for, through understanding and experience.

Circuit Diagram:

My final circuit was based on the following circuit diagram which I found on the website circuitstoday.com.

am-transmitter1

The circuit has two parts , an audio amplifier and a radio frequency oscillator. The oscillator includes Q2 and the tank circuit with inductance L1 and capacitance C3. The 100pF capacitance(C4) ensures that the oscillation is transferred from collector, to the emitter, back to the base again through the internal emitter-base resistance. The resistor R7 ensures that the oscillation will not be shunted to ground through the very low value internal emitter resistance, re of Q2. Q1 is wired as a common emitter audio amplifier. I modified the audio amplifier stage to facilitate audio input from an mp3 player rather than microphone.

I first constructed the circuit on a breadboard and later finalised it on a ‘zero pcb’/ vero board.

IMG_20151026_190527 IMG_20151026_201224

Making this project was extremely enjoyable and the start of a new hobby. I presented this project in the Science exhibition held in our school and won the 1st prize in the 9th grade category.