# Help with simple LED driver circuit

Discussion in 'The Projects Forum' started by Mike Bailey, Dec 31, 2014.

1. ### Mike Bailey Thread Starter New Member

Dec 31, 2014
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0
I have an existing 0-5v circuit that provides dimming control of 5v lights. I would like to replace the incandesent lights with LEDs.
The LEDs that I have dim from 2.5v to 3.3v
I plan on supplying constant 5v power in addition to the 0-5 control voltage.
The panels I am working on have anywhere from 5 to 70 lights on them wired in parallel.

I would like to restrict the output voltage to 2.5v - 3v.
The controlling voltage is 0-5 volts.
With up to 70 LED's I doubt that a 555 would be able to handle the load so it would need to account for this.
Do I still need to wire a resistor in series with each LED?

I am fine with assembling circuits but do not "speak the language" well enough to feel comfortable adapting a circuit to work so I appreciate any help that I can get.

Thanks.
Mike

2. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
1,457
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LEDs require current drive rather than voltage so you really need a variable current source to control the brightness of the LEDs.

What is the purpose of the 555 you refer to?

3. ### Mike Bailey Thread Starter New Member

Dec 31, 2014
4
0
I wondered about the variable current drive but with all of the LEds wired in parallel I was not sure if that would work.
As far as the 555, I only mentioned it as I have seen them used in simple drive circuits.
Mike

4. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
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The simplest solution is just to connect each LED across the supply with its own 100 Ohm series resistor and use your variable 0-5V supply to control the brightness. The downside is that the control will not be linear i.e. nothing will happen for the first 2.5V but if this is acceptable then you need look no further.

Last edited: Dec 31, 2014
5. ### Mike Bailey Thread Starter New Member

Dec 31, 2014
4
0
That is the problem, I need the full range to be 2.5 to 3v.
Mike

6. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
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In that case consider using PWM to control the brightness of the LEDs.

This circuit claims a duty cycle range of 5-95% and will work from 5V. Replace the LOAD with your paralleled LEDs, you will still require a 100R resistor in series with each LED.

Or, are you saying that you have a lamp controller already that only outputs 0-5V and you need to convert this variable voltage to a variable current to drive the LEDs? You cannot really drive LEDs from 2.5-3V as this is not how they work.

Last edited: Dec 31, 2014
7. ### Mike Bailey Thread Starter New Member

Dec 31, 2014
4
0
First, thank you for your help.
It sounds like I started with the wrong question.

What I have is an existing dimming control that outputs 0-5v.
I want to replace the existing 5v incandesant lamps with LEDs.
The existing lamps are wired in parallel and I would like to maintain this.
I need the LEDs to dim using the 0-5v to control them.

I am ok with putting a resistor in series with each LED.
Sounds like I need to convert the 0-5v to PWM and then use that with the constant 5v and PWM to dim the LEDs.
Mike

8. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
1,457
369
This can be done. Essentially, you need an oscillator that produces a 0-5V triangular wave and an analogue comparator. You feed the 0-5V control into one input of the comparator and the 0-5V triangular wave into the other. The output will be a PWM signal proportional to the control voltage.

The same principal is used here:

http://jeuch.us/html/pwm_rgb_controller.html

It should be possible to use this as the basis for your design.

Last edited: Dec 31, 2014
9. ### wayneh Expert

Sep 9, 2010
12,373
3,225
That's a good plan. Each LED will need a resistor to protect it when it sees 100% duty cycle, ie. 5V steady. A single, logic-level N-channel MOSFET rated to 2A or more can accomplish the PWM switching when driven by a 555 oscillator, for instance as in post #6.

Are you using "normal" LEDs, that have a 20-30mA max current rating and a forward voltage of ~3V?