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We are organizing a game for students where we are transmitting the daily objectives and hints over from an FM transmitter. Is there a way to limit basic radios to only be able to pick up a certain signal? For instance, 88.3 is not taken in our city, so would have the least amount of interference. We would like to limit it so that the student will dial through the stations (that will all be blocked) until they find the station that actually is broadcasting from a short distance away.

I hope that makes sense :)

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    \$\begingroup\$ Tune the radio then hot glue the dial. What's the big deal anyway if they pick up other stations when not tuned to yours? Tell them the frequency of your transmitter, and if they're not tuned right it's their problem. Better yet, join the 1990s and distribute the information on the web. \$\endgroup\$ – Olin Lathrop Mar 12 '12 at 21:11
  • \$\begingroup\$ Sorry for the lack of context. The quickest way to describe it is that it's a bit of a post-apocalyptic scenario. If possible, I was hoping to blank out the other stations to further the illusion of the game. I understand it's outdated, but that's kind of the point :) \$\endgroup\$ – JamieHoward Mar 12 '12 at 21:14
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    \$\begingroup\$ Olin's idea is probably the most simplistic. Go out and buy a good number of the same radio with an ANALOG tuner, crack them open, tune them in, and either glue the dial or just rip it out completely so it cant easily be changed. The key is they need to have an analog (dial) tuner, not a digital tuner. \$\endgroup\$ – Kris Bahnsen Mar 12 '12 at 21:50
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    \$\begingroup\$ It sounds like you want a "tuning" experience, where only one frequency actually "works." It might be easier to pre-tune the radio to the appropriate frequency, and then use a totally fake LCD display / button / microcontroller to display a frequency, and turn the speaker on with a transistor when it's tuned correctly. Also, you probably should check with FCC regulations that your transmissions are within allowable parameters. \$\endgroup\$ – Jon Watte Mar 13 '12 at 2:18
  • \$\begingroup\$ See this one: electronics.stackexchange.com/questions/306497/… \$\endgroup\$ – SDsolar May 21 '17 at 1:22
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The radio tuning dial will be some form of a variable component (potentionmeter, variable capacitor). The radio I built had a tuning capacitor attached to the tuning knob.

If you don't want to hot glue the dial in place like Olin suggests, you could turn the dial to 88.3, then remove the variable capacitor and measure its capacitance (probably using an LCR meter). Then replace it with a set capacitor of the read value. The process would be the same if your radio uses a different tuning component, just replace with a set value of that component.

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  • \$\begingroup\$ Thanks @JGord! More like the answer I was looking for. I'm not familiar with all of the details, but I'll get some of these radios together and see how it goes! \$\endgroup\$ – JamieHoward Mar 13 '12 at 1:27
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To meet your "must actually tune but find only one station scenario" you could try any of :

  • Use a microswitch or magnet and Hall cell to enable reception only when the tuning dial was in the correct place.

  • Provide a dial that was not really a tuning dial but which enabled audio at one position only (switch, cam, hall cell, ...) OR , better, which disabled the radio partially at incorrect settings so that signals were heard but were distorted or weak or whatever. A helpful local radio amateur could probably show you how to produce such partial disablement.

  • Connect a parallel tuned circuit from input to ground so that it "sucked out" signal at other than the desired frequency. Relatively cheap and easy.

  • Connect a series tuned circuit in the signal path so that it allowed only the desired signal through. The radio could be shielded with the series trap in the aerial lead.

  • "Radios" could be simple speech generator with a dial that triggered the desired output at one setting and other output or none at other settings. Using old-tech you could trigger a tape recorder mechanaism only at the correct dial setting. Compact tape decks probably available as surplus. Same using a CD player. Tape latency is probably superior.

Quantity?
Budget?

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