Step 12V DC down to 3.5V DC?

[r1owner]

Anyone know how to do this using stuff from Radio Shack?

I want to convert an LED bicycle light to my rear brake light.

I know crap about schematics so any help would be appreciated.

TIA

[dwilson]

I use the transformers from RadioShack all the time.  They work fine, you need to make sure that the voltage doesn't accidentally get bumped up and you have to get the polarity right (+/-).  

Since it's a transformer with other electronics inside, it may have a little lag compared to your other brake lights, like a fraction of a second...  Depending on the draw of you light you could probably get away with a zener diode but that gets a touch complicated...

[cornercamping]

Anyone know how to do this using stuff from Radio Shack?

I want to convert an LED bicycle light to my rear brake light.

I know crap about schematics so any help would be appreciated.

TIA

Use a voltage regulator.  About $0.30 each.  Only  need one.  PIN 1 = Power Input, PIN 2= Ground, PIN3= Regulated power out to your LED.  Radio shack has 3.3V regulators in stock. Also, shrink tube the entire regulator.  They are about the size of a dime.  Make sure to not touch the chrome part cause it gets hot.

By the way all.. EE here.  ;D

[cornercamping]

Here's an adjustable Voltage Regulator at Radio Shack.  I lied, there $1.40 or something.  They have fixed voltage too. I'd say go with a 3.3v fixed.  They will all look like this though.

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=276-1771