Please help me with this potentiometer(?) - Page 1
Forum section: electronics / beginners

author: depletionmode
author_rank: Newbie
timestamp: September 02, 2016, 03:08:39 pm
content: So, I've been trying to modify an AM/FM radio and am having some issue with one of the potentiometers (or at least I've assumed it as such).It's 4-pin by the looks of things and whoever I wire it up I cannot get any response from it. I've only really used 3-pin ones in the past and I always assumed the 4th pin would either be either NC or a a duplicat eof one of the others.Please see attached pictures. The 'Tune' dial is attached to it (using some cord to turn it from the offseted dial.Any help on how I can go about wiring this guy up would be greatly be appreciated (assuming all traces to the board have been cut; I only want analogue values from it).

author: DmitryL
author_rank: Regular Contributor
timestamp: September 02, 2016, 03:12:40 pm
content: It is a variable capacitor...

author: edpalmer42
author_rank: Super Contributor
timestamp: September 02, 2016, 06:27:03 pm
content: The volume control is a potentiometer.

author: Assafl
author_rank: Frequent Contributor
timestamp: September 02, 2016, 11:47:42 pm
content: The 4 legged square with 4 adjustment screws is a tuning capacitor. It sets the frequency the radio receives (by changing the superheterodyne frequency to the IF mixer).I would not touch it if you don't have the equipment to recalibrate the dial.

author: albert22
author_rank: Regular Contributor
timestamp: September 03, 2016, 04:16:01 am
content: It is a miniature dual/quad variable capacitor. Also called Polyvaricon.The shaft varies two independent capacitors, one used for the "local oscillator" frequency and the other for "antenna tuning" if the radio is AM/FM then you will two sections, that is you have 4 variable capacitors. Each one has a small variable capacitor to allow a fine adjustment (trimmer), those are the screws that can be seen at your first picture.More info here:http://www.mitsumi.co.jp/latest/Catalog/pdf/varicon_misc_prodinfo_e.pdf

author: depletionmode
author_rank: Newbie
timestamp: September 03, 2016, 05:09:36 pm
content: Wow thanks guys!Makes perfect sense now; both in regards to what it was connected to (the AM/FM IC and 2 trimmers)!Yup the volume control is a pot; I have that working nicely...So I need to vary voltage with this... something like a capacitive voltage divider should do the trick right?

author: Assafl
author_rank: Frequent Contributor
timestamp: September 03, 2016, 06:45:43 pm
content: A voltage divider with a capacitor? You can for AC (for sure) - but why specifically use this component and not a real pot?

author: Zero999
author_rank: Super Contributor
timestamp: September 03, 2016, 06:51:51 pm
content: Quote from: depletionmode on September 03, 2016, 05:09:36 pmWhat are you trying to do exactly?

author: depletionmode
author_rank: Newbie
timestamp: September 03, 2016, 09:14:25 pm
content: I need to use the variable capacitor to the same effect as my potentiometer (modify voltage over range for my ADC). So I was thinking of connecting another cap in series with it, applying a voltage and thereby forming a voltage divider with which to feed my ADC. As the capacitance of the variable capacitor changes, so should the voltage at the point between the capacitors (the variable one and fixed value). Am I wrong?

author: depletionmode
author_rank: Newbie
timestamp: September 04, 2016, 04:05:53 am
content: Quote from: Assafl on September 03, 2016, 06:45:43 pmI think it'd work for DC too.I don't want to replace the component in the radio and just use it as it.

author: depletionmode
author_rank: Newbie
timestamp: September 04, 2016, 09:53:41 am
content: Quote from: depletionmode on September 04, 2016, 04:05:53 amIt seems I've been talking rubbish. Obv. won't work for DC .

author: Zero999
author_rank: Super Contributor
timestamp: September 04, 2016, 07:58:48 pm
content: Quote from: depletionmode on September 03, 2016, 09:14:25 pmAnd what to you intend to achieve by doing that?What's the desired outcome? Forget how you were originally intending to do it: that probably won't work. Why are you connecting this to a microcontroller?It seems like you either want to monitor the radio or control it using the MCU but so far nothing you've suggested will achieve this.

author: depletionmode
author_rank: Newbie
timestamp: September 05, 2016, 01:16:10 am
content: So the idea was to turn an old-style radio into a internet radio but with all the knobs still working.What i did in the end was connect all the caps in the varicap in parallel (to maximize capacitance) and used it in an astable multivibrator circuit which provided me with a nice square wave of around 1.8 - 7.2KHz.This I feed into an MCU (as I can sample frequency with that easily).Works a charm!

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: September 05, 2016, 02:31:18 am
content: Quote from: depletionmode on September 05, 2016, 01:16:10 amAs crazy as that sounds that is actually a pretty cool hack to do what you want.

author: depletionmode
author_rank: Newbie
timestamp: September 05, 2016, 07:46:24 am
content: Quote from: FlyingHacker on September 05, 2016, 02:31:18 amThanks!It gets sillier: The end target for this signal is actually a python script running on an embedded platform (NTC's C.H.I.P). It's linux based (hence not real time) and is too slow to sample the multi-Khz signal from a script naturally. So I used the MCU to do the sampling (as above) and simply convert it to a PWM signal (with the period proportional to the frequency swing). This I pass through a RC filter to convert it to a simple ADC-workable analogue voltage. This then goes through a MCP3008 ADC->SPI to the C.H.I.P.Lol, overkill? There was probably some simple way of getting the C.H.I.P to sample the signal directly...Perhaps I could have increased resistance on the astable multivibrator and hence reduce the frequency to something much slower but I didn't have 720M ohms of resistance lying about.In any event I'm currently sitting listening to the radio 'tuned' to 95.8 - Capital London so I'm happy it has worked out.

author: Zero999
author_rank: Super Contributor
timestamp: September 05, 2016, 11:15:49 am
content: An easy way to do this, is to use a counter as a frequency divider. The 74HC4060 is an astable multivibrator and 214counter on one IC. If you configured the astable for a frequency range of 18kHz to 72kHz, the 214counter would give 1.1Hz to 4.4Hz which should be easy to sample using your script.http://www.nxp.com/documents/data_sheet/74HC_HCT4060_Q100.pdf

author: depletionmode
author_rank: Newbie
timestamp: September 06, 2016, 04:42:27 pm
content: Quote from: Hero999 on September 05, 2016, 11:15:49 amMakes perfect sense. Thanks for the tip! I'll probably pick myself up some 74HC4060s for future projects; am sure I'll need something like this again.I unfortunately didn't have anything suitable lying around for this one so I just went with whatever I had on the bench as wanted the project done. As with these things I find it very hard to go back and redo stuff; usually I just let it lie once it works!