I built this circuit exactly according to the schematic on a breadboard except for changing the values for the tank circuit to match the presumed capacitance of the breadboard itself. It works really well with the electret and has quite good range.
If I were to remove the mic and replace it with a PWM pin or a mono audio input jack what would be required? If I just connect the base pin through the 4.7k resistor I get no RF carrier wave output (that I can see with a RTLSDR dongle).
I'm unsure if the capacitance of the electret or C1 (decoupling capacitor?) play an important role for getting the base signal right for the transistor to amplify the LC circuit into radio waves.
I'm wanting to try out different modes of transmission such as FSK or CW with the arduino in a cheap and dirty manner and try encode some digital signals.
My guess is R2 and R1 need changing to properly bias the transistor it for a normal audio jack to keep the transistor in 'linear amp' mode perhaps?
The electret I have works perfectly with the given R2/R1 value on the schem. It's getting it working with mono sound input (or just to transmit a plain carrier wave) that has me kind of stumped as swapping audio from a music player doesn't make it broadcast anything. Any ideas for R1/R2 to re-bias the transistor perhaps? It is amazing the electret works without a preamp and it's pretty damn sensitive too.
The transistor base bias is absolutely critical for proper operation, and that depends on the characteristics of the mic. The base is at RF ground because of C1.
You should be able to find a resistor (call it R3) to substitute for the mic and at least get a carrier signal. Then, you would feed in the PWM output via a capacitor (e.g. 1 uF) to the R1 R3 junction. Try measuring the base voltage when the circuit is operating normally and we can probably estimate a suitable value for R3, but R3=4.7K would be a place to start.
You will undoubtedly have to divide down the PWM output to get a suitably small AC signal.
Yes. Replace the microphone with an approximately 2.2k resistor. A 5k trim pot would be a good idea for it. Connect a 1.0uf capacitor between the transistor base and the input, and precede it with a resistive voltage divider as mentioned above. It will now be a terrible, but possibly working circuit. A big problem with that circuit is that the transmit frequency inevitably drifts all over the place. Also, it is hard to get sufficient carrier deviation without AM and huge distortion.
The transmit frequency stability depends on the quality of the tuned circuit mainly - use quality
components for that and it can be very stable - for instance a mechanically solid air-cored inductor
is good, ditto for tuning capacitor - then the component values depend only on geometry, not the
properties of the inductor core or capacitor dielectric. Alas you still have some temperature dependence
because of thermal expansion...
Thanks for the suggestions! I tried removing the microphone and placing a 4.7K resistor in place of the microphone (leaving everything else in place) and didn't have much luck finding a carrier signal. I then tried 2x 4.7K in parallel to create a near 2.3K resistor and no sign of RF signal.
During working operation with the mic the values I recorded around the transistor legs are:
(positive probe then comm)
Vbe 0.56v
Vbc -2.4v
After placing the 4K7 the voltages across the transistor legs were
Vbe 0.50
Vbc - 7.7v
I bought a 1K and a 10K trimpot so will continue trying combinations of those.
I made the inductor with some cat5e wire and dipped it in wax a few times so its a bit more robust and works okay. Its kind of crazy that the tuning circuit works better without a capacitor but I think its because the breadboard acts like a capacitor everywhere. I might transfer the build onto cardboard and solder or get some perf board.
Wax isn't a great choice as it isn't dimensionally stable. You need thick wire for VHF coils, to reduce
skin-effect losses. I've wound tuned circuit coils around glass tubes before (tightly) and epoxied them on.
Its standard practice to place coils inside a conducting can to prevent external fields affecting then, or
at least keep well away from other components. If you can find a copy of the ARRL VHF handbook it has
lots of practical details in VHF (through to microwave) construction.
Update: it works with carrier wave wooohoo! I wasn't looking hard enough and the frequency was wildly different at around 81MHz and had to add 22pF capacitor back in the LC part.
I added a 4K7 in place of the mic and reading values on the transistor of:
Vbe: 0.22
Vcb: -4.23
And with 2.3K (the voltages seem to switch polarity) stronger signal but perhaps more noise and harmonics:
Vbe: -0.4
Vcb: 5.06
As a practice exercise I was thinking of converting the circuit to work at 5V so the Arduino can power but I'll worry about that later.
