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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.

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    \$\begingroup\$ Note that, in the schematic shown, the symbol for the battery is reversed, but the + and - signs are correct. The collector of the NPN transistor should be positive. Which way round do you have it connected? \$\endgroup\$ – John Honniball Nov 20 '14 at 9:47
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    \$\begingroup\$ One coil tapped at 6T. Not two coils unaligned. Must be wound in one direction. \$\endgroup\$ – Optionparty Nov 23 '14 at 0:46
  • \$\begingroup\$ Cool, I'll try that. I mistakenly thought that it would be equivalent. \$\endgroup\$ – Ravenstine Nov 23 '14 at 0:54
  • \$\begingroup\$ 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. Is this a good sign or am I fooling myself? \$\endgroup\$ – Ravenstine Nov 23 '14 at 2:35
  • \$\begingroup\$ FM receivers are not very sensitive to low power or long range. It sounds like you are on the right learning track. Experiment with other circuits until you find something that fits your needs. \$\endgroup\$ – Optionparty Nov 26 '14 at 2:35
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If you want 90 MHz, you're going to need a much smaller capacitor, I would think. The capacitor and the inductor are critical to determining what frequency this produces. The page you linked seems to have a 22 nF cap in that location, which is 0.022 uF. 10 uF seems much too large.

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  • \$\begingroup\$ I tried the 22nF cap, but still nothing. However, the 22nF cap is a polyester-film capacitor(223j100v). Should I be using a ceramic capacitor? I guess I will also try some other capacitors too. \$\endgroup\$ – Ravenstine Nov 20 '14 at 21:05
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    \$\begingroup\$ Well, you're going to have to find a way to either measure what frequency it is oscillating at or find a way to 'sweep' it into the band that your radio can receive. You won't receive anything on the radio if it is oscillating outside of the band. My guess is you need to experiment some more with the capacitor and the coil. \$\endgroup\$ – alex.forencich Nov 20 '14 at 22:10
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    \$\begingroup\$ Not sure if a SPICE sim will help at all with a hand-wound tapped inductor, especially if you don't have an LCR meter. The characteristics of that inductor are going to very significantly affect the operation of the circuit. \$\endgroup\$ – alex.forencich Nov 21 '14 at 19:43
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    \$\begingroup\$ @alex.forencich: Sure, it won't be perfect, but gets you reasonably close so that you can say 9 or 90 MHz. Those in the picture I would think amount to around 150 and 600 nH (I don't have my inductivity calculator at hand) so I would guess its all more 9 than 90 (also the 2n2222 bandwidth is not much higher than 90MHz anyways). Reducing the resistor to get a bit more output might help to detect if it works at all btw. \$\endgroup\$ – PlasmaHH Nov 21 '14 at 22:30
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    \$\begingroup\$ A SPICE simulation would have not caught OP's problem, as the issue was in the construction of the inductor. As I said, without a SPICE model of the inductor, the SPICE simulation isn't going to be of much help getting it working in the first place. \$\endgroup\$ – alex.forencich Nov 24 '14 at 4:36
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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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    \$\begingroup\$ Coils, especially hand wound coils, can be very "touchy".You may want to experiment with a thicker wire, or using a core (and glue). Try wrapping paper around a pencil, then winding a coil and hot-gluing it to the paper. Remove the pencil, and you have a coil less likely to move (and change tuning characteristics). Good luck! \$\endgroup\$ – Alan Campbell Nov 26 '14 at 12:52
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The coil in the schematic is a single coil with a tap, rather than two coils as shown in your first photograph.

You could put a very small capacitance in parallel with the coil to tune it more easily. You do get very small trimmer caps of a few picofarads, or use an inch of insulated wire connected to each side of the coil. Twist them together more or less to add more or less capacitance.

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    \$\begingroup\$ This is a good answer and I don't know why it was voted down. The bit about making a small capacitor with a pair of wires twisted together and then trimming the value by either cutting a bit off the top or changing the amount of twist is older than I am. I used to see things like that inside of the tuners of old-time TV sets. There is even a proper name for that twisted-wire capacitor: it's called a "Gimmick" by Amateur Radio enthusiests. \$\endgroup\$ – Dwayne Reid Feb 16 '15 at 16:01
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    \$\begingroup\$ A quick Google search for "twisted wire capacitor gimmick" brings up good descriptions of this technique as well as many photos of examples. \$\endgroup\$ – Dwayne Reid Feb 16 '15 at 16:05
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You'll have much better stability if you wind the inductor on a plastic dowel. After the winding is finished, apply some nail polish to keep it in place. That's why EEs steal nail polish from their SO's :)

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  • \$\begingroup\$ Actually, I did just that using a Bic pen tube and cyanoacrylate glue(with accelerator). It worked pretty well, though I still had to very gingerly pinch the coil into the right spot before I glued it; this did not exactly take a few tries! \$\endgroup\$ – Ravenstine Feb 17 '15 at 6:31

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