Thursday, 20 April 2017

oscillator - Does this simple RF transmitter circuit actually work?


I've come across this circuit a few times while searching for simple transmitter circuits, and decided to try it.


enter image description here


According to this site, it should be transmitting at around 90Mhz.


http://www.talkingelectronics.com/projects/Spy%20Circuits/SpyCircuits-1.html


I feel like I've followed the instructions to the letter, yet I have no results when chasing the dial on my radio.


UPDATE:


Here are some photos of the circuit I have constructed.



enter image description here


enter image description here


enter image description here


I've tried just about every area of the dial and fiddled with the coils. No effect or "quiet spot".


UPDATE 2:


I have changed the coil so that it is a single coil tapped at the 6th loop. I am now getting some effect, but it's not what I expected.


When I pinch the coil together, it seems to drown out whatever FM station I am on. I can't seem to narrow it down to one frequency. If I hold the coil together, it will add static or a faint whistling to any station. The range is also very poor; the phenomena occurs at a max of 3 feet away from my radio with the 12" antenna. If I use myself as an antenna(holding on to the antenna wire), I can create the effect from the other side of the room. Is this a good sign or am I fooling myself?


UPDATE 3:


See my answer.



Answer




The answer is yes.


My problem lied in the coil; not only was my initial coil diameter too small, but it needed to be a single tapped coil as opposed to two separate coils.


Indeed, it works and I get a clear silence on 90Mhz. But the coil I made is extremely hard to control(it's very springy), so the only way I can get it to the right frequency is by fiddling with it using my fingers until it resonates at 90Mhz, and this is very hard to do. I can usually keep it up for about 20 seconds before the frequency drifts. The website I referenced was also right in that the coil is very microphonic, so evidence of vibration from my hands shows up on my receiver.


While my circuit as it exists now is not practical in any sense(admittedly, I didn't have to make it that small), I believe it would work fairly well with a more easily tune-able coil. The range is also pretty good, though I haven't thoroughly tested it; it does transmit across my apartment. This turned out to be a cool project.


enter image description here enter image description here


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