LED / touch-switch question(s)...

[eggplant_reaper]

Hey everyone! I've been browsing through this forum for the past day or two, and it seems like a place with some very intelligent and helpful people, so I figured I'd drop my questions in here and see anyone had some input....

Here's the deal - I'm an art student (concentration on metals and glass) with limited background in engineering (I was an engineering major for a year, but I havn't learned anything past basic college level physics and calculus). Basically, I'm a good enough engineer to know what you can make, but not nearly good enough to know how to make it!

Anywho, for my next project, I was hoping to make a series of metal sculptures (basic shapes and forms) with LED's inside. What I hope to do, is to hook up a touch switch to the LED's so that when a person touches the sculptures, they light up for a minute or two and then go dark again.  For added effect it'd be ideal if the LED's could slowly light up when switched on (go from dim to bright in a few seconds or so) and then dim down after a few minutes. (as opposed to just turing on and off)

Here's the (tenative) specs for the project. The sculptures will be made from nickel silver and will be relatively small (metal cubes about 1.5 to 2 inches accross, cones about 4 or 5 inches high, etc). I was hoping for a very bright rich blue hue for my LED's. Anywho, here are my questiond....

1. Is it possible to make a circuit to do what I need?
2. If so, will it be easy enough for me (with no electronics experience) to make?
3. How big would said ciruit be?
4. What type of power would I need to power this circuit? (Battery, AC from wall...?)
5. How many LED's would I need to produce a very bright blue light from withitn the sculptures?

I found a link (from this forum actually) to small circuit produced that I think may be what I need to act as the switch. http://www.qprox.com/products/qtouch.php
...will this do what I need?

Anywho, I appoligize for rambling on, but hopefully you can help me out.
I very much appreciate any input!

[Rob]

Welcome to the boards, eggplant_reaper!

Straight to the questions:
1. Yes!
2. Yes!
3. Small!
4. 5VDC, possibly 3 AA batteries. Possibly even 3 button cells. The LEDs are a different story than the circuit.
5. The more the better

You have already found the correct solution. I would get a few QT113H-D chips (about 2 bucks each, get them from Digi-key. Get an assortment bag of ceramic disc capacitors in the nanofarad range. That should set you back a few bucks.

Next, work on building the basic circuit from that datasheet that lights one LED.

All it uses is one capacitor and one QTouch chips. The value of the cap sets the sensitivity, which is why I suggest you get a grab bag, to play around with it. With a breadboard, you should be making LEDs light within a few minutes, it's really simple.

Once you get that far, I can share some practical design tips based on my experience with this chip. But really, it will just take you a few bucks and some time to get off the ground, which is awesome.

[eggplant_reaper]

Wow!   Thanks for the very informative (and amazingly quick) responce!

I'll grab all those things and see what I can't put together.

Thanks again!

[Rob]

If the answer to number 3 needs to be "very small" it can be. For breadboarding the DIP package (QT113H-D) is very convenient, but if you were going to make say a batch of baords to install in a set of sculptures, using the surface mount QT113H-S would get your size down considerably, and get you to a one-sided circuit board with no drilling, which would be fantastic.

Ask all the questions you like about this, I actually have the chips on hand and have worked with them a good deal.

Also I should mention that the very informative datasheet can be found here.

[justDIY]

I'll just toss a wrench into the works for you both.

What I hope to do, is to hook up a touch switch to the LED's so that when a person touches the sculptures, they light up for a minute or two and then go dark again. For added effect it'd be ideal if the LED's could slowly light up when switched on (go from dim to bright in a few seconds or so) and then dim down after a few minutes. (as opposed to just turing on and off)

If size is an issue, minimal component count is a must.  To achieve a capacitive touch activation requires the q-touch, no way around that ... however, it comes in a huge dip package or a slightly better soic package ... darn no SSOP or TSSOP package ... so the qtouch is your biggest component

Now for the fade in, stay on for a few min, fade out ... you could probably achieve this using a large assortment of discrete logic ICs and timers ... one to time the fades, one to time the on-time duration... not sure how you achieve a ramp up/down fade using discrete logic, but I'm sure it involves more than a single chip.

The solution for minimalistic size?  enter the Microcontroller!

A simple and tiny pic 12f or 10f series microcontroller (only 6 or 8 pins) could offer a fade in, an on-time duration timer and a fade out.  it could be programmed to do more as well, like change color every time it was turned on/off, the sky is the limit.   The 10F202 offers 768 bytes of program space and 4 IO pins in an SOT23 or an uSOP8 package (both very tiny) or step up to the 12F509 in the uSOP8 package with 1.5kbytes of program space and 6 IO pins.   Each pin can drive a single blue/green led or a pair of red/yellow leds (assuming Vf of 2 to 3.5v and If of 20mA).  Throw in a few tiny surface mount transistors, and you could drive a large number of LEDs.

My estimates would place the entire circuitboard somewhere around the size of a quarter single sided, double sided, probably the size of a nickel.

[Rob]

He's right about size, but let's just put that monster into perspective:


That's a glamour shot of the QT113H-D.

On the hold-up time, that's programmable. Check the datasheet--you can get 10 seconds, 60 secnds, or infinite. A simple electrolytic cap in parallel with the LED would be my choice for dimming technology.

The real snag is load current--the QTouch chip only wants to source 1 mA. So you'd be best using it to turn on a transistor that drives the LED load.

[eggplant_reaper]

All that on a board the size of a quarter? That is ridiculously tiny! It think that should work perfectly.

Thanks again for all the input. It's very much appreciated! I'm going to order those chips and run over to the electronics store here at the university tomorrow and see if I can't pick up a breadboard, some capacitors and some LED's. If I can get that simple set-up running, then I'll be back here to discuss some programmable lighting effects. 

Thanks again!


one quick question.......do thay make an LED (or similar sized lighting device) that can cycle through different colors, or is it all one constant color output?

[Rob]

yes they do

[eggplant_reaper]

Got my circuits in today...a whole bakers dozen! I'm going t o head into the electronics labs tomorrow and see if I can't whip something up. I do have a quick question for you though.......How fragile are these chips? They came packed with an excessive amount of material and have all those static warning labels around and stuff.....just wondering how sensitive they actually are. I would expect I can' be throwing them around the room or try to plug one into the wall socket or anything, but am I going to harm them just by taking them out and looking at 'em here at my place?

[Rob]

you should observe ordinary static discharge precautions - they're actually microprocessor chips inside.

so they're not made of eggshells, but don't shuffle across the shag carpet holding them either.

[eggplant_reaper]

Alright.......so I stopped by the electronics lab today and threw the cicruit together......got it to work perfectly. I even found out how to toggle the momentary switch to and on/off by looking at the schmatic for the evaluation board that they sell for it (which I was pretty damn proud of myself for!)

Anywho, now I need some help getting the switch to turn the LED's on slowly (as described above). I know someone mentioned the programmable ciruit? I may go with something like that, but if anyonw has any other ideas, I'm open for suggestions.

[Rob]

Congrats on getting this far.

So tell us, how slowly is slowly? I think a microfarad-range electrolytic capacitor in parallel with the LED would be a simple way to achieve it. It would sure be quick to try out.

[justDIY]

all those static warning labels around and stuff

as I recall, the datasheet points out your 'switch' needs no electrical contact with the object doing the switching ... I think they claimed it would work behind "up to" an inch or so of glass or plastic.  they did warn that a substantial electrical discharge may exceed the breakdown voltage of the dielectric and tunnel through and zap the sensor... but the dielectric of plastic and glass is pretty darn high, I dont think 'normal' amounts of static would pose any problems.

a simple quick fix would be a layer or two of the clear packing tape over the sensor... a few mills of plastic dielectric should provide plenty of insulation... and if it still gets zapped, well, back to the drawing board!

as for handling the IC ... anything you buy from a supply house comes with excessive amounts of packaging and warnings on them .. just something the lawyers make them do.  don't be handling the IC while shuffling across a wool carpet in your silk slippers.

[eggplant_reaper]

alright....need some more guidance here.....

I'm wondering if there is a way to power three blue LED's (brightly) through the Q-Touch switch (5v max) from watch batteries (I have room for up to six batteries). I unfortunatly don't have space to use anything else to power the circuit. I'm wondering if there is a simple way to puch 5 volts through the switch and then kick it up afterwards to power the LED's (without adding much as far as circuitry). Also, I want to know whether it would be better to power all three LED's in series or to link them in parallel.....Also note that room is a big conideration here, so the less components the better.......

Thanks again!

(edit - I have no specs on the LED's unfortunatly, so I can't tell you what they need as far as voltage or current, but hopefully there is a general range at which they are all made in?!?!?!?)

[justDIY]

alright....need some more guidance here.....

I'm wondering if there is a way to power three blue LED's (brightly) through the Q-Touch switch (5v max) from watch batteries (I have room for up to six batteries). I unfortunatly don't have space to use anything else to power the circuit. I'm wondering if there is a simple way to puch 5 volts through the switch and then kick it up afterwards to power the LED's (without adding much as far as circuitry). Also, I want to know whether it would be better to power all three LED's in series or to link them in parallel.....Also note that room is a big conideration here, so the less components the better.......

you have a couple of choices here ... compactness is key I take it, so let try this with minimal parts.

your touch sw ic wants 5v, and most little batteries are 3v ... a pair of 3v button cells, in series, would yeild you 6volts ... throw in a standard switching diode to drop between 0.5 and 0.7 volts, and you'll have a 'safe' level to feed your IC - I'm sure it could survive at 6v, but just to be sure.

since you have 6 volts on the batteries, you could power 2 blue leds straight from the battery, no resistor required (batteries only, if you're on ac power, a resistor would be needed)

attached below is the simplified circuit

oh, I should add... T1 is a basic npn transistor - whatever you've got laying around, the 'shack sells a big multipack of them pretty cheap - its not real important what the part number is.  R1 is a current limiting resistor, and it is optional since the ic's internal gate resistance won't allow it to source much current - if you choose to use R1, make it around 1k, or just leave it off, no big deal.  D1 is any standard silicon switching (or rectifier) diode, ratings aren't really important, just something small