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: