What transistor to use for this?

SarverSystems

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Background:

I've got an LED reel that uses common 12VDC input, and a different ground for each color.

I want to power these LED strips from multiple 12VDC inputs, but using a common ground.

I also want them to be triggered when a 6VDC source turns on.

Real World Example:

I want to use the LED strip to flash different colors when certain pinball playfield lights flash.

I know I'm going to need a transistor for the 6VDC source to tell the 12VDC LEDs to turn on and off. What transistor to use?

How do I convert the common anode LEDs to work with a common ground system?
 
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I drew up a quick circuit, it looks like I can use a transistor for each color to turn on and off the ground sides of the led strips.

I should be able to use this to trigger the 12VDC LEDs from the 6VDC source.

Still not sure which transistor to use, or which leg to connect to ground, 6VDC and LED grounds.
 
Better idea here -> work on this and evolve your idea.

(control signal on the base of the transistor)

led_string_2.png
 
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I'm no electrical engineer, but I can tell you your schematic is wrong. You've got your base and collector backwards.
If your using the RGB LED strips, each color, according to their datasheets, draws just under 1amp (assuming your using the whole 5 meter strip, but actual measurements the most one color uses is about 600ma). So, you'll need a transistor for each color that can handle about double that amount of current. And, your using a 12v source, so you'll want a transistor to handle about double that as well. A BC368 or a TIP110 should do the trick. Now, if your into pinball machines, you probably have a couple of TIP122's hanging around (bit overkill but if you've got em'). You don't need any resistors for the LEDs because, like you said, they are built into the lights themselves but, you will need a current limiting resistor from your signal source (5v from your pin lights) to the base of the transistor. That resistor value will be determined by the transistor you use and what its max base current (typically about 100 ohms would do). Also, you'll want to make sure that the pinball driver circuit can handle the added current draw to turn on the transistors. If it originally uses incandescent bulbs, then you should be ok. (Someone can correct me if I'm wrong)
 

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As an after thought, you'll want to add, at the very minimum, a blocking diode between your input signal (pin light) and the limiting resistor to the transistor. Should the transistor fail, you don't want your 12v LED source to back feed into your lamp matrix. Or, might want to consider using an opto-isolator to drive the high current LED switching transistor. Interesting project. Good luck.
 
Thank you CMNDR BRAIN.

I have many transistors to pick from, all of which are pinball related, such as the TIP102 you mentioned. I've got about 100 of them, so thats what I'd use.

Assuming TIP102's, what resistor do you suggest?

And just a standard ole 1N4003 for the 6V protection diode?
 
Thank you CMNDR BRAIN.

I have many transistors to pick from, all of which are pinball related, such as the TIP102 you mentioned. I've got about 100 of them, so thats what I'd use.

Assuming TIP102's, what resistor do you suggest?

And just a standard ole 1N4003 for the 6V protection diode?

Did you transpose a number? I believe I suggested a TIP122, but a TIP102 should work fine. Either way, you could use around 2.2k ohms for the base (not that critical, whatever you have 2.2k, 2k).

As for the protection diode you only need a 1n4001 which will handle up to 50v, but a 1n4003, which can handle up to 200v, will work just as well.

I built a test circuit with a TIP122, 2.2k ohm resistor, 1n4001 and a full strip of LEDs and was able to get it to work and handle the power requirements.

Now, the lights were ever so slightly dimmer through the circuit than if they were plugged in direct. As I stated, I'm not an electrical engineer and I don't fully understand the entire transistor Hfe gain and how it affects the load. Although, I am sure the circuit is safe. Maybe some one else could chime in.
 

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Your welcome. I would love to see some action shots when your done. Good luck.
 
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