Here are my constraints:
- I need to build an am transmitter, but it has to fit in a 1 inch diameter and ~1 inch height box.
- Since the box is very small, I plan to use a CR2032 battery, which is also a considerable limit.
- Plus, I plan to use a carrier frequency of 1.52MHz (I can get a crystal to that frequency).
- The signal I want to transmit come from an electret microphone.
So far, I have tried to make this work with 555 and a 9V battery, here is what I came up with:
The aim is to mix the 555 output and the incoming sound together, so the antenna goes between R3 and R4. I had to change a bit the values of R2 and R1 to make the 555 oscillate at 304kHz, so that the 5th harmonic is at 1.52MHz.
Using an old alarm clock, I was able to get a very weak sound. I added a class A amplifier to get more power through the antenna but I am getting a weird effect. I can hear a sort of whistle added to the actual sound. The whistle frequency seems to change (it sweeps slowly) even if I'm not touching anything. Is it because of the square wave (and not sine) carrier ?
Despite this effect, I was planning to do the following:
- Use a flat CR20232 3V battery
- Use an ICM7555 instead of a regular 555
At least, it fits in the box... Any better idea ?
By the way, I'd like to use only DIP packages (max 8 pins) and discrete components.