QT301 proximity sensor IC - Anyone tried it yet?

[NaBo]

In a bunch of recent threads about theremin/proximity controls, the QT301 has gotten a fair bit of attention as possibly the best way to get a reliable proximity control.  Here's the link to the site that's been posted a few times: http://www.qprox.com/products/qt300_301.php

Just wondering if anyone has any experiences to report, having used this chip.  Though 4 bucks each is a DEAL when you consider the ridiculous amount of awesomeness that can result, 4 bucks for any single part is kinda steep for it to either suck or be too complicated for the average hobbiest to ever integrate into a circuit.

Anyone get it and use it, with good results?  Also, any info you could give on how the callibration/programming part works would be very appreciated, and if anything else is needed for that (like the E3A under "Evaluation Boards" on that site).

I'm still a little confused as to the voltage range that this thing sweeps...  what "value(s)" pot does it compare to and does it function like a pot with a linear or log taper? Or is that programmable too?  I'd like to get a few of these when I order so the shipping doesn't kill me, but I feel uneasy about spending the cash when im not 100% sure how it works yet....  :oops:

Thanks guys

[NaBo]

... <_< ... >_> ... shameless bump

[slotbot]

the site you link too mentions it has a programmable eeprom on it. so you will likely need to program it before you can use it.

that may be more hassle then its worth for just 1 unit.

[NaBo]

i did summore research on this thing... it appears the evaluation board is over 100$!   :lol:!!!!!!!!!!!!  luckily, i think the thing isnt NECESSARY for programming.  its really just for precise programming (digital readouts of parameters) and testing how the thing works and what not.  kinda a demo of the product and tool for companies that will be buying tons of them and such.

from what i can tell, the "qt301 circuit" shown on the link i gave is all thats really needed, and it has to be integrated into each pedal...  the datasheet doesnt mention much else, so i guess you just callibrate the end points like how it says...  hmm...  maybe i'll just get one and if i 'splode it... then i'll know that i have no business screwing with programmable 4 dollar ICs  :wink:

[ErikMiller]

I'm still a little confused as to the voltage range that this thing sweeps...  what "value(s)" pot does it compare to and does it function like a pot with a linear or log taper?

It looks like the only output is PWM. You'd have to figure out how to convert that into a voltage or resistance if you wanted to control an existing circuit.

[NaBo]

oh, yes, thats right... i forgot to mention that.  the datasheet really cleared up some of those initial questions... just took a lot of time and brainpower to make a bit of sense of things.  everyday i wish i was taking EE instead of english.  every-frickin-day.  :x

you just feed the PWM output (whatever that means) to a simple little RC filter to convert it to voltage (right?), and the values you use in that circuit will kinda determine the "value"/"taper" of the control...  thats the impression i got from some quick web searching anyway.

what does it mean when they say "if an RC circuit is used, it is often best to put a voltage follower circuit on the output of the filter to buffer the output voltage"?  i can see that the "voltage follower circuit" must be the triangle looking thing in figure 2.7... but what does that part consist of?

hey, I feel like i might be learning stuff here!  err... but feel free to shatter my bourgeoning ego and PLEASE correct me if im wrong  

[The Tone God]

what does it mean when they say "if an RC circuit is used, it is often best to put a voltage follower circuit on the output of the filter to buffer the output voltage"?  i can see that the "voltage follower circuit" must be the triangle looking thing in figure 2.7... but what does that part consist of?

That "triangle looking thing" is an opamp. The buffer is being used to make sure the output is not loaded down by whatever it is attached to.

Your on the right track by looking through the datasheet but you have to look alittle closer at the small details. Thats what EEs get paid alot for. I'll give you some hints. Look at the PWM frequency, burst transfer properties,  the "taper" on the PWM output,  the voltage for the IC, electrode design, etc.

The devil is in the details. :twisted:

Andrew

[NaBo]

:?  oh... man...

in other words, i should leave this chip til the summer, when i can learn enough to use it properly.  ah well... i just used an LDR as a control for the first time, so i guess those will tide me over until then.

lol, geez, completing a specialist program in english, minoring in psychology, studying electronics in my spare time...  what am i doing to myself  

[Paul Perry (Frostwave)]

: lol, geez, completing a specialist program in english, minoring in psychology, studying electronics in my spare time...  what am i doing to myself  

Whatever you are doing to yourself, it has to be better than what those people watching TV are doing to themselves :wink:

The 100KHz these capacitive sensors operate at, is not that far from the optimium Theremin frequency. Can only be a matter of seconds (if not sooner!) before we see one..

[Paul Perry (Frostwave)]

Well it was sooner......
Toby Paddock from synth-diy has one already http://www.seanet.com/~tpaddock/c2cv.html

he doesn't give the schematic, but it is just the standard from the data sheet, plus a voltage controlled oscillator (in his case a 555 based one driving a piezo disc, so far as i can see).

[NaBo]

Hey, thanks Paul!  I saw that page linked on one of the other discussions about proximity controls but completely forgot about it!  that certainly makes using the chip seem a bit less intimidating...

[The Tone God]

In the application mentioned, the piezo buzzer driven thermin, I think the QT301 is a good solution but in the use of effects its not. The 100Khz PWM frequency (of which you cannot change the frequency of) is but one of the problems. There is going to be all kinds of noise leaking into the audio for different places.

I don't think the performance of the IC is going to be that good in this application. The IC is designed for messurement uses like fluid level detection. To dectect fluid levels you looking to sense a very small range of capcitance. Much smaller then what occurs with human contact. Add that you are looking at using this for foot control meaning the sensor will have to go through footwear, even if it just a sock, so you will have to set the calibration to be pretty high to sense through all that material but while you are able to now sense the foot it will only sense that original small capacitence range. When the foot passes by it will swing through the full 8-bit PWM range, which is actually a lousy range of 255 steps that would be diluted furthur with the RC filter, quickly ending up with a very small physical sensing range above the electrode which will I say will be about 2 cm. / 0.5 inch. Wingling your foot in that small sensing patch in the air will be real anoying but funny to watch for everyone else.

The IC may solve the problem of generating the thermin-like signal but with it you inheirt a new set of problems that your going to have to solve and they seem to be nastier then the original set of probelms. In the end your just moving the problems to a new location instead of solving them. Just my opinion.

I remember someone posting in a similar thread an opamp based circuit that they used for the same purpose. Give that a shot. You could continue with the LDR/foot control idea using a LDR that you cover with your foot to control the gain of an opamp driving an LED in an opto. You can put a gain control knob on to adjust the sensativity to deal with different lighting conditions. That would be pretty simple and have fewer noise issues.

Andrew

[NaBo]

aww crap... i knew there had to be a pitfall :x

I guess thats okay though, seemed like it would end up being too much work anyway...  think I'll let someone a little more advanced try it out before i even bother.  :wink:

Thanks for the LDR control tips!  I'm gonna test out that method in my idiot wah as soon as small bear opens up again and i can get some optoisolators.  I've had your Rock n Control article bookmarked for months, but since i wasn't planning on building a wah or expression pedal for a while, i completely forgot that it also deals with exactly the type of stuff i'm messing with now!