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Capacitance Switch

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AstrumPreliator

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Alright, I've been working on a capacitance circuit with the Qt110-D IC (Datasheet found here) to light several LED's.
Here is the schematic (sorry for the photoshop schematic, clean format, don't have much installed yet):



The +v is 5.4v at 300mA. I'm not entirely sure what value of Zener diode to use. I was thinking a 3.6v Zener at 400mW with a 100 Ohm ballast resistor (R1). I don't know too much about Zener's though, so I'm not sure if that will work.
The capacitor is rated at 10nF. Resistors 2-7 are 22 ohm resistors. I'm not sure on what value to get for the transistor either. Without the Zener, the input voltage is 5.4v and the output is 3.9v.
Oh yeah, and the LED's are all 30mA with a votlage drop of 5v.

So if anyone can help me figure out the values of the transistor, zener, and ballast resistor it would be much appreciated. Thanks.
 

Phasor

Member
First of all, you will need to insert a base resistor between the pin 2 output, and the base of the transistor. About 2k - 5k should do.

The transistor can be any small npn, with a gain of 200 or more, and capable of carrying 200mA or so. For example, BC547/548/549. If you choose a transistor with lower gain, you will need to use 2 of them in a darlington configuration, as the chip is only able to source 1mA.

For the zener, 3.6V is fine. 100 ohms for R1 is ok, though you can probably use anything up to 500 ohms. No need to go below 100 ohms.
 

AstrumPreliator

New Member
Thank you very much for the help. I know the very basics of electronics, and thats pretty much it.

One more question though. Is the circuit design good? I'm not sure if I could make it better or whatnot. Thanks.
 

Phasor

Member
What exactly is it that you want to do with this? I'm not sure if you're aware- but the output from this chip is only a 75mS pulse, and as it states (on page 6 of the datasheet), "the pulse also has too low a duty cycle to visibly activate LEDs".

Don't be entirely put off - you can add something to the output to extend the pulse if you want... really, you can do anything with it. :lol:
 

AstrumPreliator

New Member
Your looking at the HeartBeat (tm) mode. I am actually using a toggle mode (by connecting pins 3 and 4 to ground). Look at 2.2.2 on page 6. Plus I know it can power an LED already. I have it hooked up (except with no zener, transistor, and only 1 LED).

It's actually for a touch sensitive picture frame. The main part is acrylic while the back part is aluminum (to extend the sensor over the entire width and height of the acrylic). I actually would have loved to use the LED's in series (making it loads easier to hide the wiring) but oh well. The board is going into a project box. The power comes from a wall adapter.

All I need to do is add the transistor, zener, and some resistors.
 

AstrumPreliator

New Member
The first time I looked at that I thought you couldn't do toggle mode. Then I re-read it and I found out how.
I really wish Radioshack carried some of this stuff. Radioshack is such a worthless store now. Have to order online if you want anything other than a -This sentence has been interupted to bring you the color red, in a shade of blue- resistor or two.

Thank you again.
 
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