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I'm trying to find out what the device is called that is found on a car radio. The dial that you turn to change stations. I want to track one down to see how it works, but I have no idea what it's called.

Does anyone know?

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    \$\begingroup\$ depends on the radio I guess, are you talking about the older radios that have a needle and the stations behind it? (EG turn the knob and needle goes left or right to tune) \$\endgroup\$
    – jsolarski
    Jul 5, 2011 at 12:27
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    \$\begingroup\$ Nope, the new ones that just spin and have a bit of a 'click' or 'bump' as you spin it, Wouter got it below. \$\endgroup\$
    – TomCDona
    Jul 5, 2011 at 12:31

3 Answers 3

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In older radio's this was the variable capacitor that (in the common super type radio's) determined the frequency of the first stage and the oscillator, effectively selecting the single radio station you wanted to hear by analog means.

In modern radio's I guess it is a rotary encoder. You can tell by the subtle clicks you feel when you turn the dial. This rotary encoder is in effect two switches, that inform a microcontroller how the dial is turned. (This answer illustrates how a rotary encoder works.)

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  • \$\begingroup\$ that's exactly what I'm looking for, thanks! Off to digikey I go :) \$\endgroup\$
    – TomCDona
    Jul 5, 2011 at 12:30
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    \$\begingroup\$ @Tom - This question may help you make your selection on Digikey. \$\endgroup\$ Jul 5, 2011 at 13:38
  • \$\begingroup\$ Home radios used variable capacitors, but car radios used variable inductors for the same purpose. (I took apart a lot of radios in my youth.) \$\endgroup\$
    – markrages
    Jul 6, 2011 at 19:42
  • \$\begingroup\$ ... could also be a resistor adjusting the voltage for a cap-diode.... You can also tell it's an encoder when, after some time, you turn in one direction and the radio changes the station in the opposite direction... No, I don't hate anything modern, but I wish for old stuff whenever I find modern things don't even keep up with what they aim at replacing... \$\endgroup\$
    – zebonaut
    Jul 27, 2012 at 22:31
  • \$\begingroup\$ Not every rotary encoder has detents ('clicks') — they can be with or without. Generally any free-turning (no end stops) knob is likely to be a rotary encoder. \$\endgroup\$
    – Kevin Reid
    Aug 16, 2012 at 22:44
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The last dial-type auto radio I saw used a variable inductor, basically a slug that was pulled along the axis of a hollow form with a coil wound on the outside. It'd be hard to find one of those for sale today (it isn't a kind of tuner that can easily be mated to digital controls and readouts). The vibration in an automobile was not conducive to the use of air-variable capacitors (another tuning knob component of yesteryear). There were variants of the tuning capacitor with plastic-sheet insulation that were possibly used, the floppy plates wouldn't matter if the plastic maintained a constant thickness. Variable capacitors are expensive, but probably can be found.

Nowadays, it's likely your radio uses a no-moving-parts tuner based on the capacitance variation of a "varicap" diode with voltage. The knob would be a position encoder, reporting clockwise or CCW rotation to the control computer.

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To tune a radio it is usual to vary the capacitance in one or more "tuned circuits"*. Occasionally inductance is altered but this is less common.

In radios that are fully electronic the control dial or buttons interface with a microcontroller (small computer) that alters the capacitance of an electrically variable capacitor. An electrically variable capacitor is known as a varicap (not surprisingly) or a varactor or a varactor diode.

In older radios the capacitance was varied using a device called a "variable capacitor" (again no surprise :-) ). This is what you are most liable to find useful for initial investigations.

Added: In some cases when systems are tuned by varying inductance a brass slug is inserted into the coil to progressively REDUCE the inductance. This is not usual and the effect is usually proportionally much lower than increasing inductance by inserting a "ferromagnetic" core such as ferrite or powdered iron.

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  • \$\begingroup\$ I see I'll have to be quicker to be ahead of Wouter ! :-) \$\endgroup\$
    – Russell McMahon
    Jul 5, 2011 at 12:37
  • \$\begingroup\$ I see I'll have to be quicker to be ahead of both of you. Usually its stevenvh that seems to answer questions instantly. \$\endgroup\$ Jul 5, 2011 at 14:07
  • \$\begingroup\$ @Olin - It's OK, I'm still alive! :-) The weather was just too fine, and I went for a long ride on my bicycle. \$\endgroup\$
    – stevenvh
    Jul 5, 2011 at 17:10
  • \$\begingroup\$ It's not a race, guys! The best answer should win. Being the first sometimes gets you an edge, but quality is more important. That said, you all post high quality answers. Keep it up! \$\endgroup\$ Jul 6, 2011 at 15:40
  • \$\begingroup\$ I must be missing something - I didn't realise "winning" was the object :-). I thought I was just answering people's questions. :-). I've got a vague idea of the "system" but see I need to read up on the details. Fun to see the numbers rolling up for no (apparently) completely logical reasons. (Yeah. I know it's all 'rule based' but still interesting when unused to it). \$\endgroup\$
    – Russell McMahon
    Jul 6, 2011 at 17:38

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