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I am building a project for school and am trying to find some schematics on a rf transmitter. I need it to be able to transmit on many different frequencies that I can chose somehow with a knob. Is this possible? Can a variable capacitor be used somehow for this? :shock:
There are a ton of low cost (<$10) kits out there that will do this. Keep in mind that there are limits to what you can do - power and antenna length. The kits are often described as "FM wireless mic." Keep in mind that any transmitter that you build has the potential to cause interference.
Most of the kits I am familiar with are such that you either adjust a trimmer capacitor or compress/spread the turns in a coil. With the trimmer capacitor version you might replace the trimmer with a slightly larger variable but it will take some experimenting because at these frequencies it takes very little to add capitance or inductance to a circuit.
If you end up with a version that requires compressing/expanding the turns of a coil then you likely find a fixed capacitor in parallel with it. You might remove the fixed capacitor and replace it with a variable.
In my experience none of the simple kits or circuits are particularly stable in terms of frequency. Change in temperature, vibration, proximity to metal or flesh can impact the frequency. You more or less set the transmitter frequency as close as you can then follow it with your receiver.
They do make and I am sure you can find plans for higher quality variations that are more stable and sound much better. As with many other things, it will cost more for the parts and take more time to build.
not necessarily looking for sound quality, but the option of being able to control different devices. Say for example, multiple different remotes for rf cars or switches. Instead of me having to build multiple remotes to control different frequency responding devices, I was trying to just have one that I could adjust dramatically. The output from the device doesnt have to have good sound quality, but just enough for a receiver to read the frequency to switch on...