In need of a Low Voltage elec Wizard!

kayaker97321 · 01-24-2005, 08:43 PM

I am in need of someone that can point me in the right direction. Back in High School a few years back I was in electronics and I remember using a transistor in a situation simular to using a relay.

See first pic.

What I need is a cheap transistor that can be used to swicth on and off a L.E.D. that is running on a 2 - 5 volt D.C. source. Part number and where I can get say 20 of them.

Also if it would work to use them directly off the 120 volt source with a resistor or diode?

See second pic.
Thanks.

Tyler

davidc · 01-24-2005, 09:29 PM

It won't be quite as simple as your diagram.

The 120V AC won't be good for a transistor. The average transistor would very quickly fry in that situation

Normally you would use a transistor to switch the LED when the thing that should be doing the switching isn't strong enough to power a LED (or many of them) on its own. For example, I might connect a pin of a microcontroller to the transistor which powers the LED because the microcontroller can't provide enough current to power the LED directly. Think of it as an amplifier - a weak input makes a strong output. With 120V AC, you already have a *very* strong input from LEDs perspective.

What are you trying to do? Maybe I can point you in the right direction with more info.

davidc · 01-24-2005, 09:36 PM

I think I missed your second diagram!

That would actually work, but you might get some flicker that way. You'll need at least a 220K resistor for a Blue LED. I'm not sure if the LED can handle the negative cycle, even with that resistor there.

davidc · 01-24-2005, 09:39 PM

I think this will provide your answer:

http://www.discovercircuits.com/PDF-...cwhiteleds.pdf

kayaker97321 · 01-24-2005, 09:50 PM

Hey thanks for the input. I wasn't sure myself if the led coud handle the ac current. Maybe with a diode?

Tyler

kayaker97321 · 01-24-2005, 09:59 PM

Can someone tell me if I can only use one led instead of two in the top pic?

Quote:

Originally Posted by davidc

I think this will provide your answer:

http://www.discovercircuits.com/PDF-...cwhiteleds.pdf

davidc · 01-24-2005, 10:04 PM

Quote:

Originally Posted by kayaker97321

Can someone tell me if I can only use one led instead of two in the top pic?

Should be able to. They just use two so that an LED is lit up during the positive and negative AC cycles.

kayaker97321 · 01-24-2005, 10:05 PM

Thats what I was thinking just wasn't completely sure.

Thanks again
Tyler

Clownfishman · 02-01-2005, 02:30 PM

Why would you want the flickering effect definitely use a current limiting resistor though as you have there.... but the flicker shouldn't be that noticeable since the frequency is 60Hz... so it should flicker 60 times a second..

Also for your diode be sure and get on that can handle 120VAC

kayaker97321 · 02-02-2005, 10:19 PM

Thank you all. I decided to make everything 12 volt and swicth the 120volt items on and off with relays..

Tyler