Okay guys, I'm not happy with the price of this tool. I think a professional could justify paying $39 for it, but I would really like the price to be even lower. I'm planning on doing a relaunch of the kickstarter at a lower price, and so its time to do some work.
The circuit that the current design uses is a very old one that I came up with many years ago. Its a good circuit, sort of a mixed signal affair with discrete logic, an R2R (discrete!) DAC, and some opamps, which does the job, but its relatively expensive and uses alot of PCB real estate. Its by no means a kludge and I carefully engineered it long ago, but to really push the price of this device down I am going to have to go back to the drawing board. I'm proud of my old circuit but its not helping get this tool into the hands of trillions (yes trillions) of people so off with its head!!!!
In the past I tried a digital thermopile, Melexis makes one. I did not like how it performed. I didnt take good notes..but the prototype I made using it did not perform as well as the original design. I think it was a combination of the granularity of the temperature sensors digital output creating a stairstep in the audio tone which made it useless at detecting small temperature changes.
So my new attempt will be the next logical combo. An analog thermopile to an ADC on a microcontroller. This opens up all kinds of interesting possibilities, since you can do lots of stuff when that data is in a processor.
To kludge a proof of concept together I first used a microcontroller from a totally unrelated project with a very high quality thermocouple A/D connected to a PIC MCU. I connected the analog thermopile in place of the thermocouple and did some coding. It worked great! But that just proved it was possible with great (and expensive) hardware. So to get things real I kludged up another proof of concept. This time I connected the thermopile directly to the humble 10-bit A/D of a 16F616 PIC. Well, not directly, in between I put a precision AD8604 opamp with a gain of 470 and some RC filters. Some coding later and voila! It works very well, just as good as my ancient mixed signal design. And it should be much cheaper!
At very drastic and fast increases in temperature, like dropping a soldering iron in front of the sensor quickly, you can hear a stairstep, but that shouldnt matter in usage because scanning something that hot so quickly would not be a situation you would need to hear very subtle increases in temperature.
At very low changes in temperature, like a thumbprint on cardboard (see video), you can still hear the anomaly easily and smoothly.
Also, since the data is now in an MCU, that has multiple A/D channels, it opens up the possibility of making the much-requested absolute temperature measurements. But thats on the shelf for now. I'm going to properly redraw the schematic and get PCB layout going to see where we stand as far as real production costs.