Electronics Touch Control Panel Design : New Schematic 05-02-2005 - NEED FEEDBACK

This is a design I'm working on for a forthcoming mod. It uses QT110 and QT118H touch control ics. I'm not posting the circuits for power on, cd eject, etc as they've been covered elsewhere.

The goal is to have the controls behind an unmarked smoked plexi faceplate so that everything is invisible. When you get close to a switch it will illuminate without triggering the switch. When you get closer or touch the faceplate the switch will activate. This will be accomplished by using two touch sensitive switches, one with higher sensitivity than the other. The higher sensitivity switch will trigger the LED illumination of the switch, the lower sensitivity switch will trigger whatever the function is.





The two circuits that haven't been covered in detail elsewhere:

This first circuit handles switch illumination on proximity:


This circuit will be used to switch on and off a bunch of LEDs wired in parallel used for illumination in the case:


I'm using the QT118H part instead of the QT110H because noplace seems to have the QT110H. I'm an electronics noob, so let me know what you think and if you see any problems with the circuits. My biggest question is if the multiple switches will interfere with each other. Thoughts?

 

be carful about wiring lots of LEDs in series as they take a surprising amount of power as the current adds up, so 4 20ma LEDs end up taking 80ma... not sure what the max output on a qt110 or equilivant is, but you may have to insert a transistor in there somewhere.

only way to be sure of interfearance is to test it.. i dont think that a washer is going to be enough to trigger the lighting though...

 

Good point. The other thing I could do is have the qt118h control a relay and have the lighting draw power from somewhere else as opposed to through the touch module... this is probably actually a really good idea.

I'm hoping the larger cap will do the trick... field sensitivity increases with cap size. If 20nH doesn't do it I can bump it to 30nH.

 

Updates

I've changed the lighting control switch to use a relay to isolate the LEDs from the QT118H.

I also wanted to say something about the scale of the parts. The washers will be 1" diameter with a 1/2" diameter hole for a 1/2" diameter plexi rod. The center metal rod will be 1/4" diameter. I'm hoping the metal components will generate a strong enough field. If not I can stack multiple washers and/or extend the length of the center metal rod in addition to using larger caps.

 

As a side note, make sure to provide an outside ground seperation between determined areas since the detection field will spread across the control panel. One example of this would be that one of the outer buttons will sense a finger and actuate on from a couple inches away from the predetermed area of sensitivity.

This is probably too late, but if you want to change active low to active high and vice versa, you can just utilize a 74xx04 Inverter IC from output to input. It also might be economical for you to purchase a QT140 instead of four individual QProx ICs.

 

I assume you mean a separate ground for each switch (ie Vss on each QTxx and ground for sensor metal).

I haven't ordered anything yet, I wanted to get some feedback first

I'm looking at the spec sheet for the QT140 now... It would seem to use 2 of these synced to prevent signal interference (even though the circuit diagram makes my head hurt)... though I definitely need more information on the 74xx04 inverter as the 140 is only low out, which would need flipped to hight to run the LEDs.

Like I said, I'm an electronics noob, so could you give a little more information on the 74xx04 Inverter idea? Thx.

 

Simple. The inverter IC does just one thing: it takes a TTL-logic signal (0v low, 5v high) and inverts it. You put a TTL signal into one of the inputs, and you get your inverted output on the corresponding output pin. All you'd have to do is hook up the outputs of the QT chips to the inputs of the 74xx04, then feed the outputs to whereever.

 

Cool, so basically I could use something like this part at digikey which has six inverters in the IC to flip from low to high for the LED circuits.

EDIT: Digikey part #296-14642-5-ND TI SN7404N since the Digikey URL apparently expired.

Doing the new diagram using QT140s and an inverter will be more of a challenge, but I'll post it when I have a chance to draw it up.

What software does everybody use to edit circuit diagrams? Having some decent software would definitely make things much easier/faster than doing it in Photoshop

 

Whypick1 always on the job.

Yes, you will probably want a grounding array surronding the sensor areas.

Something like this:


The Digikey part will do nicely.

I use MS-Paint myself.

 

You're thinking about parallel, not series. He wouldn't be able to run 4 leds in series off of a QProx chip. But you shouldn't even run one for that matter. As I recall, when I was working with a QT140 I feel like the max current was something like 5-10 ma. I could be off - but I remember that it was low enough that I buffered the outputs.

 

Check the QT118H data sheet, section 2.2.6. The output can only source 1mA, which is not enough for an opto coupler or relay. However, it can sink 5mA in the "off state", which is enough for an opto coupler (I wonder what the designers logic is behind this).

You can work around that problem by connecting a resistor and opto-coupler in series from +5V to GND. The resistor should be selected for about 4 mA and it must be at the +5V end of the connection. Connect the output of your chip through a diode so it can only sink current ground. Now in the OFF state the output will short out the opto coupler, which gives you the inversion.

If you choose to go the (proper) inverter route look for the 74ACT series. The chips are cheaper, more available (current technology) and can sink/source 24mA at their outputs. Since you can connect your LED to either +5V or GND you can use about any buffer or gate there is, whether it inverts or not. For your 4 buttons 74ACT00 works just as well as 74ACT241 or any other (about $0.50 at www.digikey.com)

Since you don't need speed, for the opto coupler choose one that has MOSFET outputs (sometimes called photo-relay). I just checked www.Digikey.com and they have TLP222G in stock. It has 350V standoff @120mA, so you can connect directly from +12V to -12V on your PSU. That's 6 white diodes in series @3.5V each (with room for a resistor). You can drive at least 3 strings in parallel for a total of 18 diodes.

 

so uh... whats an opto-coupler? : )

other than that, i think i understand most of what you guys are sayin

 

Optoisolator

 

Should have known better than to think this would be simple

On the bright side, compared to building this the rest of the system should be a cake walk

Anyways, I was thinking about this last night. It's looking like I'll be using two QT140s for the switching controls. One will control the 4 proximity LED 'button indicators' the other will control the actual button functions. They will not be synced as it would prevent the button function from activating when the LED activation is tripped.

nick01: I need to read through the spec sheet for the QT140 a bit more thoroughly to check what the outputs are and see what will be needed. I'm hoping the outs are enough to trip a relay or optoisolator so I can power LEDs etc from a separate source with a minumum of components. BTW I REALLY appreciate your listing actual part numbers for components. It makes my life 1000x easier in figuring this out

 

You are welcome; if you use the QT140 it gets even easier (according to the datasheet, I didn't test it). Section 2.2.3. specifies that the outputs become open drain sinks with OC=1. That means you don't even need the diode in the short-out scheme any more. 1kOhm resistor at +5V, optocoupler LED in series to ground and connect the output of the sensor where the two meet. I think I'll get some of those chips too.

 

I don't think that washer will be sensitive enough. perhaps another type of sensor like IR or light sensitive would be possible. But i'm eager to see when you build and test it out.

 

Hehe it may be a bit. I work full time as a software engineer and am finishing up a degree (on the fifteen year plan) and next week is my last finals week. So it may be a week or so before I even have time to draw up the new schematic using the QT140s. After that my evenings and weekends are my own again, so things should move rather quickly

For those that are curious where this is going to end up, the project is going to be called Evolution: NeXT and is a conversion of a NeXT cube with a dead motherboard over to an Athlon64 based PC. I should be starting a project log once I know if this faceplate is going to work

 

If the QT118 can only source 1mA (as nick01 said), then that will be not enough to energize a relay (depending on the size and type of relay).
http://zone.ni.com/devzone/conceptd.nsf/webmain/7C83114818EAA85786256DD400569EB7

-special [k]

 

I'll be using QT140s for the touch circuits

 

Crap. I'm reading the QT140 datasheet. It looks like I'll be using two QT140s and a QT110. You can't set the output mode on individual switches on the QT140, so using 3 of the sensors on SC Out mode for power, reset, cd eject, and the fourth in toggle mode to turn lights on and off isn't possible (unless I build a flipflop, which increases component count still more, and also means I need to find a schematic and figure it out)

BTW does anyone know what the conductive properties of zinc are like? The washers at Home Depot that are near the size I need are almost all zinc plated.