Do you work near a radio? Does that pathetic radio keep playing the same tunes 50 times a day in an endless loop? Do you get headaches from said looping? Then this is for you!
I
hate the radio at work. HATE IT! If I had the chance to smash it into little bits, torture it with intense UV light and throw the remaining bits into the acid bath, I'd do it. Alas, those ideas remain in my fantasy world. However, what I
can do is attach my own music to it. The radio doesn't support an input plug for music, so I decided to build a simple AM radio station to get the job done (and this way everyone will think it's a radio station).
Circuit schematic is attached.
It's a little more complex than most AM transmitters, but the frequency stays put and it should have excellent fidelity. It operates with "micro power" so there shouldn't be any legal issues. The range isn't that large, but you should be able to transmit about 5-10 metres if all goes well.
I'm making a prototype now and will upload the PCB data when I confirm it's functional.
Explanation of Circuit
Again, for those that are interested, here is a deeper explanation of the functionality.
Power Input
The circuit functions at around 15 V. Above you can see 2 input pins along with an electrolytic capacitor and a ceramic capacitor for additional smoothing and suppression of RF freedback.
Carrier Signal Generator
The carrier signal for this radio station is determined by the frequency of the quarz crystal, in this case 1 MHz. The capacitors C1 and C2 are made as large as possible in order that the frequency of oscillations can approximate to the series resonant mode of the crystal and is not dependant upon the values of these capacitors.
The transistor amplifies the signal a bit and is then decoupled by C22.
Audio Input
The signal is decoupled by C6 and fed into the base of the transistor. R10 and R17 place the current at the base at it's working point so the input signal can be amplified as linear as possible. The amplified signal is led to the differentiator.
Modulation of Carrier Signal
The two transistors act as a differentiator. The carrier signal is amplified more or less according to the audio input coming from the bottom.
Output Stage
The coil L1 and all of the capacitors in parallel must be tuned to the carrier signal defined by the quarz oscillator. The formula
fg = 1 / (2 * pi * sqrt(L * C)). C30 and the two coils L2 and L3 serve as the output tank for the signal. The shorter the antenna is, the more inductance the coil needs to have.
Debug Filter
This circuit is attached just before the output stage. What it does in layman's terms is it "collects" the out of phase signals and stores them in C33. So the more out of phase you tune the output stage, the higher the voltage will be in this debug filter. This makes it really easy to tune the transmitter.
I hope this was interesting for you all.
TheComet
