If I built a coil like the one in this crystal set.

How could I use a micro to detect which point the alligator clip is on?

Basically I want to use the crystal set as a user interface for an audio playback system.

  • \$\begingroup\$ Thanks for the responses everyone. As pointed out i'm partly trying to gauge how easy it would be to do with the coil or wether to just fake it. Although feasible it sounds like it will be easier to fake it. \$\endgroup\$ – Justin Nov 17 '09 at 3:14

Resistance is a good solution, it also might be interesting to treat the coil like a resonator.

Try using a uC to give it a rising edge on the end clipped into and measure the rising edge on the other side. I could simulate this and see if it was feasible, but it would probably save hours to use a function generator and an Oscope, or just use a uC. A capture pin on a PIC and a PWM output would be my choice, pin goes high from PWM on a PIC at timer overflow, capture is relative to timers.

Just an Idea, if I think of any others I will let you know. If it could be done you could have two simple interrupts and just w check the variable they wrote to before doing any audio out. -Max

  • \$\begingroup\$ Thinking about it, a shmitt trigger is probably the piece I left out, by making sure that the trigger level is high enough I am sure you could accurately measure a delay. If the difference in delay would be enough, I am not sure. \$\endgroup\$ – Kortuk Nov 13 '09 at 10:39

If you used a very thin gauge wire you could measure the resistance from one end of the coil to the connection point. However, even with 28 gauge wire you're looking at a total of 6 ohms for the entire coil, so reading that with a micro is going to require additional components.

Alternately you can measure the inductance of the coil from one end to the tap, which would vary based on which tap was chosen.

However, you're just building an interface. Does it have to be a real coil, or does it just have to look like a coil?

Were I in your shoes, I'd cut the back side of the coil around the taps and use one of many methods for determining which tap was connected. This could be done with one input per tap, or with an analog input if you added resistors to the coil so the resistance significantly increased between each tap.

  • \$\begingroup\$ These are interesting solutions, but I'm not sure the differences will be enough (especially with resistance) to get an accurate measurement with common parts. Inductance may work better with an filter to slow down the discharge of the field. Longer delay = greater inductance, and you could adjust it to your measurement resolution. \$\endgroup\$ – Lou Nov 13 '09 at 13:36
  • \$\begingroup\$ Agreed, one would need to include additional components (opamp, constant current source, for one solution) to measure the small resistance changes. The inductance method also requires additional circuitry to measure the inductance change, and can be significantly affected by other nearby objects (if the user is wearing a watch it may affect the measurement enough to believe it's on a different tap) \$\endgroup\$ – Adam Davis Nov 13 '09 at 16:05
  • \$\begingroup\$ I would have a jumper wire for every possible location, and select from them using... a rotary switch. Since this isn't even a real radio (just an interface), you could use a multi-pole switch to select audio playback function. \$\endgroup\$ – Alan Campbell Nov 16 '14 at 2:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy