When select­ing a touch­screen there is actu­al­ly an inter­est­ing choice that I have to make. There are two com­mon touch tech­nolo­gies, resis­tive and capac­i­tive, many peo­ple prob­a­bly know it as old and new touch­screens. (Inter­est­ing aside, capac­i­tive screens actu­al­ly came first in 1965 but weren’t wide­ly adopt­ed, then resis­tive screens became com­mon before capac­i­tive screens resurged with phones and became the “new” style.)

Resistive screens

A resis­tive screen works by let­ting you push one side clos­er to the oth­er and detects this change.

Pro:

  • Elec­tri­cal­ly sim­ple, 4 wire sup­port built in to BananaPi A20 chip.
  • Cheap­er to man­u­fac­ture.
  • More accu­rate detec­tion than capac­i­tive screens.
  • Will work if wet.
  • Will work if wear­ing gloves, using elbow or poked with a stick.
  • Plas­tic is scratch resis­tant.
  • Can option­al­ly detect pres­sure.

Con:

  • Eas­i­er to dam­age screen.
  • Dam­aged screen is use­less, minor dam­age typ­i­cal­ly caus­es com­plete fail­ure.
  • Only sup­ports two simul­ta­ne­ous touch points.

Capacitive screens

A capac­i­tive screen detects pow­er loss to a near­by con­duc­tor such as a fin­ger. The mod­ern stan­dard is a pro­ject­ed capac­i­tive tech­nol­o­gy (PCT) pan­el which means a direct touch isn’t required, this allows a pro­tec­tive glass lay­er to be insert­ed on the top of the screen assem­bly.

Pro:

  • More famil­iar to peo­ple.
  • Eas­i­er to clean.
  • Sen­si­tive PCT devices can detect fin­gers through insu­lat­ing gloves.
  • Sup­ports com­plex mul­ti­touch pat­terns.

Con:

  • Fails if coat­ed in a con­duc­tive mate­r­i­al, such as sweat or food liq­uids.
  • Sen­si­tive to mechan­i­cal design, the met­al chas­sis is a con­duc­tor which can sig­nif­i­cant­ly impact on screen oper­a­tion and sen­si­tiv­i­ty.
  • Con­duc­tiv­i­ty varies on the per­son, the person’s hydra­tion, ambi­ent humid­i­ty etc.

Further pontificating

So for some­thing portable, like a phone, going capac­i­tive is obvi­ous­ly the bet­ter choice.

For a kitchen appli­ance it is far less clear. Wear­ing dish­wash­ing gloves or smear­ing toma­to sauce across the con­trols is going to be more com­mon.

The big fac­tor I keep com­ing back to is the famil­iar­i­ty. Peo­ple expect to have a sol­id capac­i­tive touch screen and attach a high­er qual­i­ty val­ue to it than a resis­tive screen. And that the lim­i­ta­tions of the capac­i­tive screen are well under­stood so peo­ple seem hap­py to work around them.

Basi­cal­ly the engi­neer in me leans towards resis­tive, the mar­ket­ing orphan I recent­ly adopt­ed says capac­i­tive.

So for now I’m get­ting both. I am already famil­iar with capac­i­tive touch pan­els and have one com­ing as part of one of my screens. So I just need to acquire a resis­tive pan­el to play with and under­stand.

Shopping

Buy­ing a resis­tive pan­el is far hard­er than it should be… most­ly due to lim­i­ta­tions with the won­drous site aliba­ba.

<rant>
Aliba­ba is fan­tas­tic for inter­act­ing with com­po­nent sup­pli­ers but the search engine is absolute rub­bish, pre-Google Yahoo rub­bish. (As Yahoo used to own a sub­stan­tial chunk of Aliba­ba maybe it is lit­er­al­ly the pre-Google Yahoo rub­bish.) Essen­tial­ly you enter some words, Yahoo search­es for those words in the title of each prod­uct, the prod­uct with the most words in com­mon is the top of the list. Which means that the retail­ers cre­ate new dupli­cate prod­uct entries with dif­fer­ent titles try­ing to hold the bin­go card that match­es your search words. One touch­screen man­u­fac­tur­er had a sin­gle 4 wire resis­tive touch prod­uct, avail­able in about 15 dif­fer­ent sizes. They had rough­ly 1800 bin­go cards/product entries try­ing to attract eye­balls.

(A nice trick with Aliba­ba direct is that they don’t keep these all updat­ed, so you can often get up to 20% off by find­ing a dif­fer­ent­ly priced entry of the same prod­uct from the same man­u­fac­tur­er.)

Once I find a man­u­fac­tur­er I try to fig­ure out if I can trust them. A warn­ing trig­ger for me is any­thing that they are obvi­ous­ly lying about in their mar­ket­ing, if there are obvi­ous lies I can see then there are prob­a­bly sub­tle lies that I can’t and will bite me lat­er. With resis­tive touch­screens how­ev­er EVERYBODYalmost every­body lies, pos­si­bly this is relat­ed to the above Aliba­ba is crap rant. And it doesn’t look like lies that help them, a 4 wire touch­screen has four ana­log pas­sive resis­tive wires, they state this, then in the inter­face sec­tion they state “RS232 or USB inter­face”… which I don’t under­stand. It clear­ly is a 4 wire FPC con­nec­tor, there is no RS232 or USB plug in the pic­ture, no men­tion in the descrip­tion. I sus­pect there is a series of check­box­es box for inter­face and they feel the need to tick some­thing… Anoth­er odd one is they list a res­o­lu­tion, num­bers vary but all are wrong. It is an ana­log device, any res­o­lu­tion lim­i­ta­tions come from the quan­ti­za­tion per­formed by the con­troller, which they don’t sell. Why pro­vide a fake num­ber and risk it being small­er than some­body else’s fake num­ber when the real answer is infi­nite and there­for bet­ter? *sigh*
</rant>

Buying

I have reached out to three sup­pli­ers for fur­ther infor­ma­tion:

I also found some inter­est­ing capac­i­tive touch man­u­fac­tur­ers in case I go that route: