Low Resistance READADC

[joely87]

Hi All,

I am wanting to create a touch game as a student project. I would like the students to come up with a range of games using different code in the 08M picaxe chip. One idea that I am trying to prototype is measuring how far along the wire they get before they touch the maze.

My idea is that if I use some sort of resistive material I can do an READADC to a word variable giving a value between 0 and 1024. If you get half way I would like to return around 500, and if you get all the way somewhere around 10...

The only resistive wire I have found so far is Nichrome Wire which according to wiki 1.2 E-6 Ohms/m. I will only be using about half this so therefore only have about 0.6E-6 Ohms to play with.

Does anyone have any suggestions with voltage dividers to cope with such small values.... alternatively does anyone have any ideas on more resistant materials that could be bent to use in a maze / touch game / skill tester.

Thanks again in anticipation,

Joel.

 

[obroni]

You might want to try using an opamp to see if you can boost the voltage drop into a more measurable range.

 

[srnet]

The only resistive wire I have found so far is Nichrome Wire which according to wiki 1.2 E-6 Ohms/m. I will only be using about half this so therefore only have about 0.6E-6 Ohms to play with.

Ummm, thats Ohms per square ?

I use Nichrome wire for cutting foam, 22swg (0.7mm) is around 3ohms per metre by length.

 

[Jaguarjoe]

31 gauge alloy 675 nichrome wire is 8.5 ohms/ft or about 28 ohms/m. You should not have a hard time working with this if you use an op-amp to help get to 0-5 volts. Current sense amplifiers would be pretty good for this.

 

[inglewoodpete]

I use 30 or 32 gauge nichrome wire: 2.3 metres in length gives about 30 ohms, from memory. The wire is OK if the current remains under about 180mA. I use 5v across the wire, operating it as a potentiometer into one of the PICAXE 28x1's analogue inputs.

It depends on the range of numbers you want to return. If you put a 27ohm 5watt resistor in series to limit the overall current with the shorter wire and used ADC10, you should get a reasonable range of numbers.

Experiment!

 

[BeanieBots]

I don't want to rain on your parade but the resistance of the wire and making a potential divider is the least of your problems.
The act of making an intermittent 'touch' on the wire is in itself resistive and will give all manner of varying ADC voltages.

Try it !

Make a simple test circuit using 3 or 4 known resistances (say 1k) and briefly 'touch' the junction and see if you can get a sensible reading.

 

[premelec]

There are resistive paints and conductive paints used for shielding - might work out... plain old graphite works [# 1 pencil try measure resistance on heavy line on paper]. Contact resistance won't be a problem with low resistance track [like wire] as the ADC input impedance is high. I don't fully understand what you are trying to do...

If you only need a few spatial points put a bunch of resistors [20?] in series and access the taps...

 

[MartinM57]

One of these I presume - with a "distance" readout ?

http://www.toyday.co.uk/shop/images/uploads/buzzwire2.jpg

 

[premelec]

OK Martin... if that's the form then definitely an op amp is in order as the wire has to be pretty fat... I recall the old strip chart recorders that had a single wire and DC motor to drive the carriage along to null the input voltage - in those days chopper amps were used - now we have cheap and good high gain op amps. So nichrome or even iron wire for a start could do - as long as you can shine it up for good contact and use a sample hold [perhaps just capacitor] to capture any touch to the 'maze' wire. There's probably stainless steel wire or tubing that could work... but only millivolts of DC so stray AC etc will have to be rejected.

No simple answer... yet...

 

[Dippy]

I think you should heed BB's points.
It strikes me that it's similar to a noisy old pot on your 30 year old radio volume control.

I would do some simple experiments before spending too much time and money on it.

It's all very well to thinking of instruments with wires (or wire-wound pots) but their contacts will be nicely made constant pressurre multi-doo-dah fingers/brushes and probably some low pass filtering/integration to smooth out the crackles.

Experiment with the 'front end' techniques before applying all your op-amp mx+c.
i can see perfectly what you want to do in principle , it's just the execution side of things where it all falls down.

 

[joely87]

Thanks all, I must have forgot to check the email allerts box as I haven't had any emails (which I thought was strange for such an active formum). Thank you for your suggestions / concerns. I will have a bit of a play... It is not the be all or the project, it was just a thought for some of the more advance students to pursue and problem solve.

I have been successful with a nintendo ds touch screen using a range of adc reads. I thought it should be the same principle just using a smaller resistance. I shall try getting an series array or 10 10ohm resistors to read different values and will update from their.

Thanks for your support

 

[hippy]

There may be some merit in, rather than having the wire and contact as a potential divider, having the contact to 0V and forming two dividers from the rest of the wire ...

-.-    ___                                         ___    -.-
 `----|___|---.---=========================---.---|___|----'
              |                 /|\           |
              A                  |            B
                                _|_

 

[premelec]

AC excitation

Note that you can run an AC signal in the wire and then use an AC amplifier which is much easier than a very high gain DC amplifier - also the AC amplifier can be very selective to only amplify the excitation frequency - you can use a synchronous rectifier switched at the excitation frequency and then feed that to READADC... this all takes some analog design which is not so available to newcomers to the electronic arts...