Connecting Non-10k Resistance-based Sensors

[audiocommander]

Audiocommander : I'm very interrested in the resistor values & how to connect them !!! If you could post that this would be very fine ! What kind of Spectra-stuff have you connected ?

I have some 8cm softpots and two 360° pots by spectrasymbol. Also one very nice FSR-Sensor to measure finger-pressure (Conrad has these).

The secret is, to get the load resistance value (impédance de charge, Lastwiderstand).

The formula for this is: RL = ? (Rmin * Rmax)

if(Rmin==0) the formula is: RL = ? Rmax

(if you don't see ?, it's "square root", "Wurzel aus")

Then you have to solder a load resistor (RL) between +5V and Signal (S); two of the pot pins have to be connected to Ground (GND) and Signal:

+5V   	----(RL)---+
                   |
                   |
S     	-----------+---(Sensor)----+
                                   |
                                   |
GND	---------------------------+

So here is an example from my FSR-sensor: - connect your multimeter to the middle pin and select either the left OR the right - measure the minimal available restistance value: Rmin = 1 k? - measure the maximal available resistance value: Rmax = 8 k? RL = ? ( 1*8 ) = 2.83 k? now, we don't have no 2.83 k?, so we simply choose the next lower 2.2 k? and connect this between +5V and Signal.

+5V   	----(2k2)---+
                    |
                    |
S     	------------+----(1..8k)----+
                                    |
                                    |
GND	----------------------------+

The AIN now should read values between 0.5V (min) to 4V (max).

To interpolate that value to 0..127 you can either connect an operation amplifier (that has to be described by someone else) or check out http://www.midibox.org/forum/index.php?topic=5270 where the interpolation by code is described. The idea is, to read the AIN value as 10bit value and calculate the rest. There's enough resolution to do this without loss of reading (although you loose processing units) ;)

Michael

Edit: small translation error

[audiocommander]

ahh one more thing:

I have some sensors that range from 0k? - whatever k?. I got some problems with too low resistance values (it's fun frying resistors ;D ), so I used to take 1 k? as min instead and round the RL down a bit. It may not be the finest mathmatical thing, but at least I can use it :)

Corrections welcome!

[stryd_one]

ahh one more thing:

I have some sensors that range from 0k? - whatever k?. I got some problems with too low resistance values (it's fun frying resistors ;D ), so I used to take 1 k? as min instead and round the RL down a bit. It may not be the finest mathmatical thing, but at least I can use it :)

Corrections welcome!

Firstly, you could test those resistors with a multimeter instead of frying them :) But hey, if you like the smell ;) heheheh

Thanks for the formula and method of doing this :) I have taken a long time to wrap my head around it but I think it's OK.

I think that maybe the radix/racine/wurzel aus is called something different, I can't find any reference to it in the context that you have used... I think maybe it is the same as what we call the Square Root... Does it look like a sort of 'tick'?

? square root

? ohm

Now we can cutnpaste those characters...maybe :\

[audiocommander]

yeah! I think root is the right word:

damn translation widgets! ::)

I'll exchange the signs :)

Thanks!

Thanks for the formula and method of doing this :) I have taken a long time to wrap my head around it but I think it's OK.

There's also one point about this method: I had real problems without these load resistors, even with sonsors ranging from 0-10k?, because somehow they impacted the AIN-values on neighboured lines!

I was about to become mad. But now everything works very smooth!

(sorry for the many edits :)

[audiocommander]

Don't miss this topic, where I got some helpful advises and background knowledge about potential problems this method may introduce  :-\

[illogik]

hi;

i've had also some fun with LDR's (i love these things) going to ain.

I didn't calculate load resistance but just took a small resistor (1k? i'll check), with a (10k?) pot in series, this having the advantage that you an "scale" the sensor, you can go from very little or no effect to full swing ;D used swith to go from sensor circuit to a normal pot

it's kinda experimental but useable..

wikipage on alternative AIN devices? i'll have some time next week for that

[stryd_one]

wikipage on alternative AIN devices? i'll have some time next week for that

That's a good idea man!

[audiocommander]

yeah, great!  :D

I'd post some entries too (not next week, but as soon as I got a bit more time, for sure)!

[dj_statikfire]

Also,

I have a ton of 500k pots laying around. To use them as 10k, I simply put a 10k across the contacts.

The result is having a non-linear 10k, where most of the action is in the first third or so of the turn range.

But it works quite well!

THX,

JD

[Wisefire]

ive used 50k pots without any problems with midibox

[audiocommander]

500k != 50k

500k = 50 * 10k

50k = 5 * 10k

50 / 5 = 10 => 10k  :o

  ;D

[Wisefire]

you are right.. however the topic suggestion is that you wouldnt be able to use any other pot than 10k and this is clearly not true.. i had some spare 50k linears lying around.. and they work great.. very accurate.. no fluttering of data..

also i think you can reduce non linearity by having a higher value resistor across the contacts.. but this is just my guess..

[stryd_one]

>10k = OK

<10k = You need AC's trick....

[audiocommander]

hey, I was just joking a bit...  :P ...and as always surprised by the result of my calculations ;D

also i think you can reduce non linearity by having a higher value resistor across the contacts..

no, on contrary. the higher the value gets, the less linear is the result.

As a 50k pot also takes the value of 5k somewhere (middle value of a 10k Lin-Pot = 512 as 10bit, 64 as 7-bit), it just reaches this point earlier or later (depending on the connection). With an unmodified 500k pot a lot earlier/later...

the most linear and best results come from high quality 10k linear pots.

[audiocommander]

So for the sake of flock, how to connect:

a] >10k sensors

b] <10k sensors

well, that's what I was trying to describe in this thread.

a WIKI entry about sensors would be great, esp. with a summary of different sensortypes:

http://www.midibox.org/dokuwiki/doku.php?id=sensors

(sorry, I'm a bit in a hurry right now...)

[Wisefire]

it must be that i am dumb.. but doesnt midibox just checks voltage.. as a pot is a voltage divider it doesnt really matter what kind of ohmage your your pots has.. as long as they arent 500k cuz those will probably start to flutter..