Our instructor in Electronics engineering gave us a project, that is, to make a wireless microphone that will transmit in AM bandwidth. Because this is our first design project, I don’t know where to start. I even search the web for schematic diagrams for an AM wireless mike, but I couldn’t find any, instead, only FM wireless mike.
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No. 1 — March 12th, 2010 at 5:18 pm
that because fm is far more superior than am transmittion.
No. 2 — March 12th, 2010 at 5:51 pm
An AM transmitter is just an oscillator whose output strength can be varied in response to some modulating signal (in this case, audio from a microphone).
The simplest one-IC transmitter is this: Wire five of the gates of a 74HC14 hex schmitt inverter in series, with a 1MHz crystal completing the loop. Connect the input of the “spare” gate to one of the intermediate outputs and connect a 220 ohm resistor from the output of this gate to 0V. Connect the antenna to the junction of the gate output and the resistor. Connect a carbon microphone (from an old telephone handset) in series with the supply (4 * 1. 2V AA cells in series).
Modulation comes from the fact that the mic varies the voltage available to the oscillator (but the crystal oscillator keeps running at a constant frequency). The 220 ohm sink resistor is there to ensure that there is a severe current gradient everytime the oscillator switches.
Note: you will not be able to pick this up on a set with a digital tuner, because it broadcasts on 1MHz and digital tuners only go up in multiples of 9kHz.