Applying the Quantum Touch QT1081 Touch Controller

March 17th, 2009

We had a customer come to us recently with a design proposal for a small portable device.  The product concept included a small LCD display, a couple LED’s, and a mylar keypad with 8 buttons.  I had been looking for a project which I could use as a vehicle to check out the Quantum QT1081 8 channel touch controller.  (Quantum is now owned by Atmel, see here.)  Because the product is lightweight, and may be either hand held or placed on a table when used, the force required to activate a typical mylar keypad seemed like it would interfere with the use of the product.  Plus there’s the cost of fabricating the keypad.  The button array of a capacitive touch keypad can simply be created on a standard single sided printed circuit board, and requires no force to activate.  Because a finger touch can be reliably sensed from several mm away, in this product the copper pads are on the back of the switch array, which is overlaid with a layer of plastic on which the key legends are printed.  Because of the dielectric characteristics of FR4 material used to make printed circuit boards, the board helps to “focus” the electric field coming off the pad.  As a result, the sensitivity to touch on the back side of the board is increased.

I found with the QT1081 chip that key presses were very reliable, with no issues with double hits, etc., that one might encounter with a mylar keypad.  The sensitivity of each key pad is easily adjusted to compensate for the differing amounts of stray capacitance associated with each key pad.  Best of all, the power consumption is very low.  A system using a PIC16F883 processor running at 4MHz with the QT1081 detecting a single key consumes only 2.4mA from a 3.0V supply.  An overview of the operation of digital capacitive touch technology can be found here.

Adjusting QT1081 key touch sensitivity

A datasheet for the QT1081 may be found here.  Please download this and turn to page 6, as figure 1.1 contains a reference schematic which will be used for the remainder of this discussion.  The values shown for the resistors and capacitors connected to the SNS** pins are good starting values.  The 1M resistors that are used to set the AT1081 operating modes will affect the sensitivity of their respective key inputs.  I discovered that SNS pins that were pulled to ground were less affected than SNS pins that were pulled off to VDD.  The values for CS0 through CS7 are MINIMUM values it seems.  I found that with the CS* capacitors set to 1000pF (1nF), keys with option resistors pulled off to VDD would not reliably detect a touch.  However, increasing CS slightly for those inputs to 2000pF resulted in working keys.  The datasheet claims that capacitors as large as 50nF (0.05μF) can be used for sensing capacitors.  Power consumption increases as larger values are used for CS.  Use no more capacitance for CS than necessary to achieve reliable operation.  Capacitor selection is discussed in section 3.3, page 11, of the datasheet.

Capacitive touch opens up many new design possibilites.  Other touch controllers in Quantum’s product line include slider bar and wheel sensors with up to 256 discrete positions that can be sensed.  Elimination of mechanical switches can be a huge win in terms of product styling and reliability, as well as cost reduction.  Because the Quantum components use a digital algorithm they are able to adapt to long term changes in the capacitance of the button array, such as water droplets on the sensing surface.  This reduces issues with “stuck” keys.

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When CD’s Were A Dream About To Come True

February 4th, 2009

I can remember the very first compact disk I purchased- I was in Japan in November of 1982 and CD’s were taking the county by storm (as things tend to do in Japan). It was a recording of Holst’s “The Planets”. Going through my archives doing some research recently I came across an entry from Len Feldman’s column “Sightings” in the August 1981 issue of db, The Sound Engineering Magazine:

“Those studios with vast libraries of analog master tapes may find little or no use for them when the world “goes digital” in a few years. But as for the fears about the obsolescence of the conventional vinyl disc, I think that such fears are over-exaggerated. The digital disk will make its way into homes slowly, I feel, and the analog LP discs which have been sold by the billions are not about to be discarded overnight, or even over a span of ten or more years. I’m not saying that they will never become collectors’ items— only that it’s a bit early to discard our collections of 12-inch LPs before we see which way the digital disk is really going to go— and how soon it’s going to get there.”

Hahahaha! This is one reason I love history, it’s so much fun looking back to see how things really turned out, and how often the experts get the future wrong. To be sure, it took a while for CD’s to make it to our shores (I didn’t purchase my first CD player until 1985, a Sony CDP-302, which is still in service by the way) but by 1986 vinyl disappeared virtually overnight from the stores. I think Len Feldman completely overlooked the convenience factor of CD’s. On good equipment vinyl sounds almost as good as a CD, and certainly a poorly recorded CD will sound worse than a good vinyl recording, but to be free of the hassle of handling LP’s and constant fiddling with turntables and styli, the cleaning, need to turn them over after 20 minutes or so, and the wear out issue, the same things that drove many to use cassettes to record their record collections when cassettes’ quality got good enough, was the big driver in the rapid embrace of CD’s. It’s hard to believe CD’s have been with us for almost 30 years. Which is why the current struggle of Blu-ray against ordinary DVD’s may not turn out the way futurists predict today. While the conventional DVD was a vast improvement over the VHS tape, many people can barely tell the difference between an upscaled DVD and a Blu-ray video image due to the long history in this country of people accustomed to watching sub-par video. Blu-ray is no more or less convenient than DVD, so from that perspective there is no reason at all to upgrade to Blu-ray. Even many folks with HD tv’s often can’t tell if what they are watching is in high definition or not! We’ve been down this road before, folks. When was the last time you purchased a superAudio CD? I thought so…

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Determining Rate of Heat Transfer in Hydronic Systems

January 2nd, 2009

A friend of mine recently asked me how I made all the heat transfer calculations on the new FOM heating system. He’s also in the HVAC business but does forced air and geothermal mostly, and has not dealt much with hydronic heating systems. We’ve both seen a number of “hack jobs” where outdoor wood boilers have been lashed up to existing forced air or new radiant systems. I asked one homeowner who was having some problems with his new radiant system to show me the heat loss and heat flow calculations for the job. The ensuing blank stare told me this was not going to be a fun visit. With all the computer programs out there these days that simply plug and chug and give you numbers close enough to work with, there’s no excuse for not doing things properly. But I digress, we are supposed to be talking about heat transfer!

Heat transfer calculations in hydronic systems are dead easy, so there’s no excuse for not running the numbers. In the USA we still use BTU for our units for heat. Recall that 1 BTU is equal to the amount of heat that causes a temperature rise of 1oF of 1 pound of water. Let’s say we have something like a geothermal system that uses a ground loop. A fluid, usually water or water mixed with glycol to prevent freezing, is circulated through the ground loop outside and a heat exchanger inside, either transferring heat from the refrigerant circuit (summer cooling) or transferring heat from the ground to the refrigerant circuit (winter heating).

  1. A geothermal system is operating at steady state in cooling mode. If the flow rate in the ground loop is equal to 10 gallons per minute, the temperature of the water entering the loop from the heat exchanger is 110oF and the water returning from the ground loop is 90oF, what is the amount of heat, in BTU per hour, that is being conducted into the ground?

Answer:

heat transfer(BTU) = ΔT * Flow rate(gpm) * 8.3 * 60, where ΔT is the temperature drop around the ground loop in degrees F from input to output.

We simply multiply ΔT times the flow rate times weight of 1 gallon of water times 60 minutes. We have to multiply by 60 to rationalize our units since flow is in gpm but we want to know the number of BTU’s per hour. Since 8.3 * 60 = 498 it’s common to write the heat transfer equation simply as ΔT times flow times 500, to make it easier to do ‘rule of thumb’ calculations in one’s head. So the answer to our question is then:

20 * 10 * 500 = 100,000 BTU per hour.

So remember, it’s simply delta T times flow times 500. Not so hard, is it?

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