Next step in large number of LEDS

Hey all,

I have ordered all may parts to get a large number of rgb leds going. i have the arduino going to shift registers then to transistors tip 122 then to the RGB Leds. The issue I have is when I unplug the additional 9v power supply. The RGB LEDS are still lit up like they are getting power from the arduino. Is there something I am missing. I can send a picture if that helps.

Yes, show us a schematic.

kdarius:
I have ordered all may parts to get a large number of rgb leds going.

Didn't ask us first...?

kdarius:
Is there something I am missing. I can send a picture if that helps.

We're definitely going to need something. Best would be a schematic. You can draw it by hand and photograph it if you want...

The issue I have is when I unplug the additional 9v power supply. The RGB LEDS are still lit up like they are getting power from the arduino.

Must be the case.

here is a rough schematic. let me know if this helps.

didn’t ask you guys first because wanted to try it all myself. All good though.

does the drawing help at all?

Hi,

You appear to be using the transistor as a high-side switch. The current lighting the led is flowing from the shift register outputs through the transistor base.

Paul

how would you suggest I fix this?

And can i fix this and still use the TIP 122 transistors or will I need to purchase something else.

Why do you want to fix it? The 9V supply will normally be there, right?

My advice would be buy a few components, ask advice, experiment. Only then buy large numbers of comonents for your project.

Tell us more about this project. What do you want to do?

well I already have ordered the components please take a look a the picture. I want to run allot of RGB leds from the Arduino.

and in order for me to run a large number of leds I have to use external power hence the 9v power supply.

You still have not explained why you feel it is a problem that the leds light when the 9V supply is not connected. Is it important that the Arduino can still run when the 9V supply is switched off?

Sorry for not explaining Paul. This is just a prototype and it will run 20 RGB LEDS. In the end project i would need 80 RGB leds. I believe I would not be able to run this from the board without burning up the board.

oh and i am sorry i uploaded the wrong picture.

kdarius:
Sorry for not explaining Paul.

You still haven't really. Are you concerned that if the 9V supply fails when running 80 leds, the 5V regulator will overload?

Using the transistors in the configuration you have used (not considered a very good idea, by the way) the current drawn from the shift registers will be far less when the 9V supply is connected compared to when it isn't connected. If you have a multimeter (and you really should have one) you can easily prove this for yourself.

kdarius:
In the end project i would need 80 RGB leds.

There is a much much easier, better and overall I bet cheaper way of running many rgb leds than using shift registers and transistors. Use ws2812B leds. Their nickname is "neopixels" You can get them on strips, matrixes or individual tiny pcbs. I have even seen some 5mm thru-hole ones.

Hey Paul,

So i have an adruino i want to run 80 always on RGB LEDS. I will look at the neopixels but want to try to get this to work. I thought with the transistor what current is coming from the arduino is only acting as how much current is passing through the transistor from the 9v power supply. I did not think they were really connected. Kind of like a dimmer switch my hand is the arduino and the the 9v is the power that goes through the dimmer. So my hand and the 9v are not really connect in any way.

Does that help at all. Sorry for my ignorance on the subject.

Rather than a dimmer switch, think of the situation of turning the steering wheel in a car. Turning the wheel is easy beause you have power steering. You turn the steering wheel gently and some power from the engine is used to steer the front wheels. But if you stop the engine, you can still turn the steering wheel but it is very hard and you must use much more of your own strength.

The type of transistor you are using is a bipolar junction transistor (bjt). It is a current amplifier. A small current flowing into the base and out of the emitter controls a large current flowing into the collector and out of the emitter. But if there is no current flowing into the collector, more current will be drawn from the base instead.

So hopefully that answers your question. Let us know if you need more advice on your prototype design or component choices.

Wow Paul that actually makes allot of sense. So that is why with the power supply disconnected I still get light.

With the current setup even though this is not a good circuit if I the power supply does not go bad the arduino should be ok running many leds? it will pull the 3.6 amps from the power supply instead of the arduino if the power supply is plugged in correct? Because this is really to test the software I have manipulated with this work for now? Final product I can change everything out.

Is there a chip you think I should replace the transistor with to act more like a the dimmer option i was talking about?

You mentioned using transistors like this is a bad idea. I do have a multimeter but not sure what to look for.

Now that you know what I am trying to do how would you do it without transistors and not using neopixels.

Thank you so much for your help it is starting to make sense.

kdarius:
if I the power supply does not go bad the arduino should be ok running many leds?

Yes, the Arduino will be OK if the power supply is OK. But if the 9V power supply fails, too much current may be drawn from the Arduino's 5V regulator and it could be damaged I think. Can anyone else confirm my theory?

However, if the Arduino is also running from the same 9V supply, then if the supply fails, the Arduino will stop will not be damaged.

kdarius:
Is there a chip you think I should replace the transistor with to act more like a the dimmer option i was talking about?
Now that you know what I am trying to do how would you do it without transistors and not using neopixels.

How many colours do you want? Would 7 (red, green, blue, cyan, magenta, yellow and white) be enough, or do you need more?

kdarius:
You mentioned using transistors like this is a bad idea.

Using that particular model of transistor in that particular configuration is a strange choice, although, as you have found out, it does work, but I think you have been lucky this time! There are many different types of transistors for different situations and you need to know which to choose depending on the use. The TIP122 you chose, for example, is a medium current NPN Darlington. Its not usual to use NPN as a high-side switch, although it can be done, but it is even less usual to use an NPN Darlington for this. NPN are normally used as a low-side switch. Also it has a current rating of 5,000mA but you are using it for only 20~30mA!

kdarius:
I do have a multimeter but not sure what to look for.

You would set the multimeter to, for example 200mA range, then put the multimeter between the shift regsiter output and your base resistor. Measure the current that flows when the 9V supply is connected and the LED lights normally. Then disconnect the 9V supply and see how the reading changes.