Really simple project, but I really don't know how to do it

[ghouck]

What I'm looking to do it switch on/off an LED. What I have is a 3v+ signal that I need to trigger a transistor, which in turn will illuminate an LED. The 3v+ signal won't support the current for an LED, so it has to trigger the transistor. My plan was to use a NPN transistor, and feed an LED 12 volt through a 470k resistor. What I need to know is if there is anything more to it than that, and what transistor to use. The LED is 3v 20ma. I see schematics on the 'net for this, but they're all different, some having several resistors. What I 'think' I need is a transistor, an LED, and a 470k resistor. Do I need another resistor for the base if the signal is 3 volts? Anything else I need? Thanks, Greg.

 

[MikeMl]

How much current do you want to send through the LED? Are you trying to save power, or do you want the LED to be as bright as allowed by the LED specs? Can you power the LED off the 3.3V supply instead of a 12V supply?

The current limiting resistor in series with the LED might be close to 470Ω, but not 470KΩ.

Yes, you likely need a resistor between the (CMOS?) logic output and the base of the transistor. How much current can the logic output source?

 

[ghouck]

How much current do you want to send through the LED? Are you trying to save power, or do you want the LED to be as bright as allowed by the LED specs? Can you power the LED off the 3.3V supply instead of a 12V supply?

The current limiting resistor in series with the LED might be close to 470Ω, but not 470KΩ.

Yes, you likely need a resistor between the (CMOS?) logic output and the base of the transistor. How much current can the logic output source?

Thanks, and yes, I meant 470 ohm. As far as current I can get from the output source, I do not know. The manufacturer will not say exactly, only that it is "signal only, and can not be used to drive an LED". What this is is a RAID card in my server. The manufacturer has some assembly that attached to the outputs, and as they say, "switches on and off LEDs that are powered by another source". The problem is that the 'assembly' is some $600 drive cage with the lights built-in, obviously this isn't feasible. Thanks, Greg.

 

[MikeMl]

Ok, I didn't mean to power the LED from the Device Logic signal. Can you tap into the 3.3V POWER SUPPLY?

 

[ghouck]

Possibly. It'd be easier using 12 or 5 volts because I can get that right off one of the molex connectors. The only reason I was leaning toward 12v is because I already have resistors for that. I could use the 3.3v, but it wouldn't be as neat of an install. Thanks.

 

[MikeMl]

Ok, here is how to do it: The current drawn from the logic is only 100uA. Current through the LED is ~20mA.

 

[ghouck]

Ok, here is how to do it: The current drawn from the logic is only 100uA. Current through the LED is ~20mA.

Thanks Mike, I do appreciate the help. Hopefully I will have the time to learn a bunch of this stuff on my own.

 

[ghouck]

I couldn't get my hands on any 2n4124 transistors, but got a few of the following:

2N3904
2n2222
2n4401

I was messing around with me breadboard with them and they seemed to work, but I can't make heads or tails out of their specs, so don't know what will really happen if I build with them. Thanks, Greg.

 

[MikeMl]

I'd try 4401, 3904, 2222 in that order.

 

[ghouck]

Thanks, what about 2n3906's?

 

[MikeMl]

Thanks, what about 2n3906's?

They are PNPs; you need a NPN.

 

[ghouck]

They are PNPs; you need a NPN.

Crap, , dyslexia strikes again. Thanks.