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Author Topic: Arduino one REv3+TLC5940+tip122  (Read 1512 times)
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Hi!!! i've been working on a stair lighting proyect for a few days and got it working with de PWM channels of my arduino one like a charm, the leds go fade on one after the other and then off after a delay.

I use a TIP31 transistor to use a strip of SMD 12volts single color LEDs with an external power supply.

now i need to have more channels so i got a TLC5940 to extend my PWM channels to 16 or more, when i use a regular LED directly on any of the TLC's channels the LED works perfectly but when i conect the transistor circuit insted, nothing happens. i used this link from the arduino TLC5940 library page

http://sonicrobots.com/2012/04/24/the-pwm-controller-tlc-5940-the-arduino-and-a-high-current-output-circuit/

before using the TLC my arduino PWM channel was conected at the base of the TIP31, the colector to my LED and the emiter to the ground

i use the same circuit on the TLC and doesn't work i tried also using TIP122 like on the link and same result

please help!!!!
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West Yorkshire, UK
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Can you post a schematic of your curcuit? Hand-drawn on paper and photographed will be fine.

I suspect the problem is that the tlc outputs will only sink current, whereas the arduino outputs will both source and sink. This affects the choice and use of transistors.

Are you sure you even need the transistors? The tlc can sink quite a lot of current, 120mA per channel, I think.

Paul
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Can you post a schematic of your circuit? Hand-drawn on paper and photographed will be fine.

Probably no need to.  That circuit diagram is bogus - it shows a NPN Darlington connected upside down.  The emitter should go to ground and the collector to the LEDs.
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on the case that the problem is that the tlc only sinks ground what transistor should i use????? i need transistors cause im using at least a meter of LED strip and that can go up to an amp per channel and also i want to be safe on the current
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Ah yes, that circuit is more bogus than ever.  Clearly you need PNP transistors as emitter followers, base to TLC5940, collector to ground, emitter to the LEDs.  Whatever transistor can drive a couple of amps and stay cool.
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TIP32 then.

But you are suggesting using them as low-side switches. I was thinking high-side. I'm no expert in transistor theory (or practice for that matter). Why low-side?

Paul
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But you are suggesting using them as low-side switches. I was thinking high-side. I'm no expert in transistor theory (or practice for that matter). Why low-side?

Because the chip in question is a current "sink" - essentially an open-collector driver.
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so if i get the idea right because de TLC sinks current i need a PNP like TIP32 and the TIP31 is a NPN so TIP31 needs 5 volts or a high signal right????
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so if i get the idea right because de TLC sinks current i need a PNP

Yes. And you have to connect it between power-supply and LEDs ("high side")

When the TLC is active it will suck current out through the transistor base, turning the LEDs on.


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I have been using TIP125 (PNP) on TLC5940 channels to control LEDs and it works just fine.
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Another way to solve it is by using Inverting Logical Gates (such as 74hc04).

http://fritzing.org/projects/pwm-with-tlc5940-via-logic-inverting-gates

I hope this can be of some help...
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