I'm trying to wire my case up so that when my hard drive is active, the case sort of strobes. I've drawn up this circuit, and I need to know if this would serve the purpose.



The Blue wires lead to ground

that diode should be a base current limiting resistor.



using a generic NPN transistor like 2N2222, 2N3904, BC337, etc it, R2, is usually around 4k7 ohms to 10k ohms from a logic level signal. there are algebras you should could do based on the specific transistor's HFE and what not but it isn't super critical for light blinking. the load must be before the collector because of how electricity works. from a 5 volt supply for a blue led the current limiting resistor, for the led,R1, would be around 100 ohms. i'm no engineer and got a D in physics 135 so... Transistor Circuits

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something like this id get ahold of twizted about

in carbongraphite's diagram, R1 would be something like

resistance = ( Supply Voltage - LED's Forward Voltage ) / LED's Forward Current
resistance is in Ohms, voltages are in volts, and current is in amperes

where supply voltage is the voltage of the power source you're connecting the LED's to, LED's Forward Voltage is the forward voltage of the LED (for typical light emitting diodes their forward voltages range from 1.5 volts to 4.6 volts), and LED's Forward Current is the rated current of the LED (which would probably be around 20 milliamps, or 0.02 amps).

once you have that value, round it up to the closest value of a resistor you can get. the value that carbongraphite has would work (assuming that Vcc is +5 volts) for some blue LEDs, but might not work with some other leds.

you could also use an optocoupler to control the led. I think I saw optocouplers in some hard drive activity meters between the motherboard's connection, and the operational amp.

I think a transistor would be less expensive, and Im not sure if there'd be any benefits of an optocoupler over a transistor in this case

sorry this comment got so long; I seem to get carried away quite easily

There no Need to calculate R1. The transistor is acting as a switch. 1K or Less Would even be sufficient. You also dont need a diode. If you ran that circuit, your diode would probably blow and so would your transistor. The reason being is you have no external resistor to restrict current. This is why R1 is needed on the base of the transistor.

Quote: Originally Posted by Fire Hawk in carbongraphite's diagram, R1 would be something like ( Supply Voltage (volts)- Diode's Forward Voltage (volts) ) / Diode's Forward Current (amps) where supply voltage is the voltage of the power source you're connecting the diode to, Diode's Forward Voltage is the forward voltage of the diode (for typical light emitting diodes their forward voltages range from 1.5 volts to 4.6 volts), and Diode's Forward Current is the rated current of the diode (which would probably be around 20 milliamps, or 0.02 amps). once you have that value, round it up to the closest value of a resistor you can get. the value that carbongraphite has would work (assuming that Vcc is +5 volts) for some blue diodes, but might not work with some other leds. you could also use an optocoupler to control the led. I think I saw optocouplers in some hard drive activity meters between the motherboard's connection, and the operational amp. I think a transistor would be less expensive, and Im not sure if there'd be any benefits of an optocoupler over a transistor in this case sorry this comment got so long; I seem to get carried away quite easily

Your thinking of a Voltage Divider Biased Transistor.

Quote: Originally Posted by Nerdz Your thinking of a Voltage Divider Biased Transistor.

I am? sorry; I meant to talk about the resistor to put in series with a light emitting diode when connecting it to a power source? like, without a transistor, even. but I think I know what happened. I forgot there was a diode in the circuit with the light emitting diode, so I got lazy and referred to the light emitting diode as a diode.

Im going to go edit that post

edit: alright, I think I cleaned it up; but Im not sure how much cleaner. also, Nerdz, which diagram are you looking at? Im looking at carbongraphite's, and there is a resistor connected to the base of the transistor, R2, and R1, the resistor I was talking about in my previous post is restricting the current flowing through the light emitting diode, and the transistor