SparkFun Electronics Forum

I found an article on instructables and it gave me an idea. is it possible to hook a rotary potentiometer up to this circuit without adding a micro controller and not over powering anything? Thanks.

What is the pot. supposed to do?

overall brightness of the LED's. one pot for all the LED's. I was thinking it would go between the power source and the transistor.

Use PWM with an MCU.

I was trying to avoid a microcontroller... they are beyond my skill level.

You won't get proper control with a pot.

hmm any suggestion for an alternative other than a pot with a microcontroller? thanks for the quick replies!

There's always the variable PWM 555 circuit somewhere... this looks about right...

If it's only a low current, you could always have an adjustable constant-current circuit. If you've got enough voltage "overhead" you can use a 3-terminal voltage regulator as a constant current source. Otherwise, there are plently of other simple circuits that would do the trick.

Liencouer wrote:There's always the variable PWM 555 circuit somewhere... this looks about right...

Perfect!... for now... lol.

MichaelN wrote:If it's only a low current, you could always have an adjustable constant-current circuit. If you've got enough voltage "overhead" you can use a 3-terminal voltage regulator as a constant current source. Otherwise, there are plently of other simple circuits that would do the trick.

Hmm interesting. seems this would be more for an RGB LED project... which is where this project will probably end up.

Thank you both!

MichaelN's suggestion will work. Look up how to use an LM317 as an adjustable current source in the data sheet or in an app note.

If you do this, use a logarithmic (audio) pot. Human vision is very nonlinear: our vision is very sensitive at low light levels. If you use a linear PWM or current control, you will find that very small current changes near the bottom will cause huge apparent brightness changes and vice versa near the top of range.

At least using a microcontroller allows you to tailor the PWM to any curve you like.

lyndon wrote:If you use a linear PWM or current control, you will find that very small current changes near the bottom will cause huge apparent brightness changes and vice versa near the top of range.

thanks I didn't know that.

waltr wrote:MichaelN's suggestion will work. Look up how to use an LM317 as an adjustable current source in the data sheet or in an app note.

Your right. It looks like it might work a little better. Not sure which setup im going to use yet but I ordered some parts to test the original circuit and ill go from there.

thanks for the replies, I'll post back later with some more questions/updates.

Silent77 wrote:

How would you modify the above to work with this... http://www.sparkfun.com/commerce/produc ... cts_id=678

I tried hooking it up but I ended up smoking a TIP... So with four left I figured I better ask.

Thanks.

Its an NPN transistor so you did put the LED bar on the collector side not the emitter side yeah? And add a 10k resistor to the base to limit the base-emitter current to 1.2mA and the minimum 25 DC gain will give the circuit close to 30mA (even though it will be limited by the LED bar anyway but doesn't hurt).

Other than wiring it wrong I can't see how you would of smoked a TIP they are rated at amps not mA so it won't be from over-current surely.

What I think you may of done is wired the LED bar so that the + is connected to the collector, but the Gnd is to the battery directly, and then the collector to the battery Gnd also, effectively creating a dead short for the TIP. For example I think you may of done this:



When really you want this:



Luke