Im a complete noob at electronics

[JWJR]

I know this will be a noob question, i am a noob at electronics and a really noob at transistors. i am trying to wire an activity light into my 360s hard drive. i'm wanting to so that the lights are normally on and turn of when there is activity. so here is what i got so far.**broken link removed**
the 5 volt is my main source and the 2.5volt is a terminal on my hard drive connector that i figure was supposed to be used for an activity light anyways.
so i figured and transistor would be the best way to do it. so i got a N24401 from radio shack, but for the life of me i can not figure out how much of a resistor i should use on the base. could yall please help? ive done basic LED wiring but have never tried anything harder than that. also if yall need for info from me or if there is a better why i could do the circuit. let me know

Thanks

 

[MikeMl]

First, you need to put a resistor in series with each LED. The resistor value will be (5V - Vf - Vce_/20mA = (5 - 2.2 - 0.4)/0.02 = 120Ω.

The total collector current Ic will be 4x20mA=80mA. The base current should be about Ic/10, so ~8mA. The source is 2.5V, the Vbe is ~0.6V, so there will be 1.9V across the base resistor, so R = E/I = 1.9/0.008 ≈ 240Ω, or use a 220Ω or 270Ω if that is easier to get.

 

[JWJR]

I know your alot better at this stuff than me but just wondering but how does using a resistor in series each led differ than using one resistor in sereis with the parrallel branches? does it make the leds last longer or what?

 

[Diver300]

I know your alot better at this stuff than me but just wondering but how does using a resistor in series each led differ than using one resistor in sereis with the parrallel branches? does it make the leds last longer or what?

The current through an LED changes a lot if the voltage changes by a tiny amount. A very small difference between the LED voltages will make a large difference in the currents.

If you put an LED in series with each LED, a small difference will mean a small difference in the voltage across the resistors, so a small difference of the current. The result is the LED brightness will be much more even, and the LEDs are less likely to burn out.

 

[JWJR]

make since to me. i'm going to have to make another trip to Radio Shack. i got several different resistors but no 120s or 240s.

 

[Reloadron]

As a standard off the shelf 120 Ohms is no problem. You will need to settle for 220 or 270 ohms as 240 ohms is not standard. Using a 220 ohm should be fine.

Ron

 

[gabeNC]

make since to me. i'm going to have to make another trip to Radio Shack. i got several different resistors but no 120s or 240s.

If you are going to continue playing with electronics and have several other projects in mind, I wouldn't buy from RS. It's just way too expensive. There are many other places that are cheaper, better selection etc.

Mouser Electronics - Electronic Component Distributor
www.futurlec.com
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Electronic Components Distributor | DigiKey Corp. | US Home Page

 

[audioguru]

If you are measuring 2.5V now across the existing LED indicating hard drive activity then it might be less when loaded with the new circuit.
Then the 240 ohm resistor value must be reduced.

 

[4pyros]

If you are measuring 2.5V now across the existing LED indicating hard drive activity then it might be less when loaded with the new circuit.

Yes was that output from the HD meant to drive an LED directly?

 

[audioguru]

Yes was that output from the HD meant to drive an LED directly?

It probably feeds 5V through a series current-limiting resistor to the existing 2.5V LED. We don't know the original voltage nor the value of the resistor so we must just guess what will happen when you connect your parts.