Arduino Forum

Using Arduino => General Electronics => Topic started by: Sminkly on Oct 19, 2018, 04:54 pm

Title: Wiring a 4 pin Potentiometer to arduino
Post by: Sminkly on Oct 19, 2018, 04:54 pm
Hi,

I'm trying to use an older device with an arduino.

It's an old camera zoom control, you push it one way to zoom in and the other to zoom out, it is proportional, ie the further you press in either direction the faster it goes.

I've taken it apart and the zoom control is a 10k pot spring loaded to sit in the centre, then as you press either direction it adjust resistance, and if you let go it springs back to center.

I've measured the resistance with a mutlimeter.

At rest

pin1 - pin2 is 4k
pin 1 - pin 3 is 8k
pin 1 - pin 4 is 4k
pin 2 - pin 3 is 4k
pin 2 - pin4 is 0k
pin 3 - pin4 is 4k


When the control is all the way to the right

pin1 - pin2 is 5k
pin 1 - pin 3 is 8k
pin 1 - pin 4 is 4k
pin 2 - pin 3 is 3.3k
pin 2 - pin4 is 1k
pin 3 - pin4 is 4k

When the control is all the way to the left

pin1 - pin2 is 3.3k
pin 1 - pin 3 is 8k
pin 1 - pin 4 is 4k
pin 2 - pin 3 is 5k
pin 2 - pin4 is 1k
pin 3 - pin4 is 4k

So what this is telling me is

moving control to the right increases resistance pin1-2, decreases pin2-3, and pin2-4 increases(up to 1)
moving control to the left decreases resistance pin1-2, increases pin2-3, and pin2-4 increases(up to 1)

My question is to actually connect this to an arduino so I can work out which direction the control is going(and by how much.

I tried simply connecting pins 1 and 3 to 3.3v(the level the arduino I'm using uses) pin2 to ground, and pin 4 to A0. Using analog read it read something but the value was the same for either direction.

How exactly should this type of control be wired ?

Thanks.
Title: Re: Wiring a 4 pin Potentiometer to arduino
Post by: hammy on Oct 19, 2018, 05:58 pm
I've never seen a potentiometer with 4 connections except via google for those with wiper connections doubled up ( in which case I'd guess 2-4 would always be 0k)  do you have a picture ?
Is your multimeter ok? Maybe the pot is worn out ?

1-3 should be 10k always ( outer pins), 2&4 to 1 should go 0-10k whilst 2&4 to 3 should go 10k-0??

Those reading you have seem most odd - is it still in a circuit ?
Title: Re: Wiring a 4 pin Potentiometer to arduino
Post by: Sminkly on Oct 19, 2018, 07:13 pm
Hi,

thanks for the reply.

The zoom controller is probably from the 80s which might explain the lack of info I've been able to find about how to wire it. I'd disconnected the original wires and resoldered my own.

What I did manage to google was it might be a centre tap pot, a bit like the one mentioned here

https://electronics.stackexchange.com/questions/247922/4-pin-potentiometer-identification

one of the answers talks about model 271 centre tap pot.

If I could work out the wiring so I could read resistance of pins1-2 and pin 2-3 I via 2 analog reads that would tell me what direction and the amount, I just don't know how to get grnd and vcc into it and still get the resistance.

EDIT:

one thing that has just occurred to me is that it might work with some kind of signal(it's definitely from a pre digital age) ie the signal(for the sake of argument lets say it's a tone) and then the pot does something to the tone making it higher or lower to control the zoom.

Doesn't help me with wiring though.
Title: Re: Wiring a 4 pin Potentiometer to arduino
Post by: Sminkly on Oct 19, 2018, 07:24 pm
Just to follow up for reference this is the pot

https://imgur.com/a/Uf9r2JR

It's a sfernice 78CS103W00063 A (not that a google search gives anything about it)
Title: Re: Wiring a 4 pin Potentiometer to arduino
Post by: Bringamosa on Oct 19, 2018, 07:52 pm


Image  fixed
Title: Re: Wiring a 4 pin Potentiometer to arduino
Post by: MorganS on Oct 20, 2018, 05:40 am
Even in the 1980's this may be a hall-effect sensor. Is the rest of the electronics still functional? You may find that it is supplied with a constant 5V. Put 5V on those pins yourself and then measure the output voltage.
Title: Re: Wiring a 4 pin Potentiometer to arduino
Post by: jackrae on Oct 20, 2018, 10:26 am
It may be a 'sine' pot with three inter-connected windings.  The centre connection point may be the common or wiper connection.  The wiper then tracks around a circular resistance which has three fixed connection points around its periphery. The resistance measured between the common point and the other three connections then defines where the wiper is in respect to the X-Y plane.
Title: Re: Wiring a 4 pin Potentiometer to arduino
Post by: MarkT on Oct 20, 2018, 11:57 am
I've never seen a potentiometer with 4 connections except via google for those with wiper connections doubled up ( in which case I'd guess 2-4 would always be 0k)  do you have a picture ?

pin 4 is the centre tap - centre-tap pots allow a differential output without needing external voltage
divider - saves components and board area, or allows injecting a virtual ground into the pot so no
trimming needed.
Title: Re: Wiring a 4 pin Potentiometer to arduino
Post by: Sminkly on Oct 20, 2018, 04:37 pm
pin 4 is the centre tap - centre-tap pots allow a differential output without needing external voltage
divider - saves components and board area, or allows injecting a virtual ground into the pot so no
trimming needed.
Hi,

thanks for the info. Any idea how to actual wire it to an arduino though ?

Title: Re: Wiring a 4 pin Potentiometer to arduino
Post by: Paul__B on Oct 20, 2018, 04:59 pm
As Mark suggests, the fourth pin is the centre tap, so wire the end terminals (pins 1 and 3) to 5 V and ground, the wiper (pin 2) and centre tap (pin 4) to two Analog inputs.  You then successively read the wiper and centre tap voltages, and if they differ significantly, act accordingly.



It may be a 'sine' pot with three inter-connected windings.  The centre connection point may be the common or wiper connection.  The wiper then tracks around a circular resistance which has three fixed connection points around its periphery. The resistance measured between the common point and the other three connections then defines where the wiper is in respect to the X-Y plane.
No, totally different function - what you describe is a continuous rotation "synchro" pot used for remote indication.

It had five pins and two wipers 180 degrees apart.
Title: Re: Wiring a 4 pin Potentiometer to arduino
Post by: MarkT on Oct 20, 2018, 09:12 pm
I'm sure 3, 4 and 5 phase pots were all in use before encoders and digital control became ubiquitous - you
think of it, someone somewhere was making and using them...
Title: Re: Wiring a 4 pin Potentiometer to arduino
Post by: Paul__B on Oct 21, 2018, 05:15 am
The rotary encoder pot described was used for accurate indication of the fuel float gauge position in WW2 aircraft.



Title: Re: Wiring a 4 pin Potentiometer to arduino
Post by: Sminkly on Oct 21, 2018, 01:19 pm
As Mark suggests, the fourth pin is the centre tap, so wire the end terminals (pins 1 and 3) to 5 V and ground, the wiper (pin 2) and centre tap (pin 4) to two Analog inputs.  You then successively read the wiper and centre tap voltages, and if they differ significantly, act accordingly.

Hi,

brilliant, that's it. I can now tell which way the control is going and by how much.

Thank you very much.