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 Forum index » DIY Hardware and Software
A Collection of LED Drivers
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Paradigm X



Joined: Feb 15, 2011
Posts: 281
Location: Null and void

PostPosted: Tue Nov 22, 2011 9:45 am    Post subject: Re: A Collection of LED Drivers Reply with quote  Mark this post and the followings unread

Tim Servo wrote:
...snip...

Of course, here’s the classic. An NPN transistor actually supplies the current to the LED, and the amount of current required from driving circuit is minimal. This circuit works well with digital or analog inputs, and is dirt simple. R1 sets the current to the base of the transistor - I’ve used a wide range of values from 10K to 150K. R2 sets the current through the transistor and into the LED. Most LEDs can take up to 20mA (0.020A), but you can usually get away with MUCH less than this and still have a very visible indicator. Use Ohm’s Law (I = V/R) to figure out how much current you’re putting through your transistor and LED. ...snip

Tim (Captain Blinky) Servo


Hi Tim / Anyone Smile

I breadboarded this (the first) circuit last night, and it worked a treat. But to get anything like reasonable brightness, i had to use a 500ohm resistor for R2, giving about 24mA on 12v supply. The LFO im monitoring has a positive only waveform (tri) from about 1v to about 9v (its a very basic opamp LFO), which is arguably (?) similar to +/-4v, so not far off whats drawn on the schematic you provided. (EDIT for clarity; what i mean is i think its expecting a similar P-p input voltage. I think!)

R1 was 100k. Although its positive only, its also got a diode to ensure this, as detailed in the TH Drum cookbook.

The LEDs seemed to run fine for about an hour or so, but just checking if ive either misunderstood something, or if the 24MA is far too high and theyll blow eventually.

The Blue LEDs required something like 50k (!) to get anything vaguely sensible (viewable!) and i could never get them to match so sacked them off.

Many thanks, ben
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Paradigm X



Joined: Feb 15, 2011
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PostPosted: Tue Nov 22, 2011 4:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hmm actually using 100 ohm; that's 120ma that can't be right ? Somethings wrong here.
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JingleJoe



Joined: Nov 10, 2011
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PostPosted: Wed Nov 23, 2011 3:56 am    Post subject: Reply with quote  Mark this post and the followings unread

Thats far too much current for the average LED.
If you are having trouble lighting them with the stated resistor values then I'd suggest it is your transistors that are to blame.
Check thier datasheets and lower the value of R1, it's value will depend on how much current the transistor can handle accross it's base/emitter and emitter/collector.
(I'm sure you know ohms law but here are the apropriate formulas I'd be using anyway; I=V/R, R=V/I)
some suggested resistor values for greater LED brightness:
R1: 10 k
R2: 680 ohms
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Paradigm X



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PostPosted: Wed Nov 23, 2011 9:24 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks.

I thought the 100k was to prevent the LED getting loaded and vice verca, wont reducing it mean itll be affected? (its so annoying not quite understanding things sometimes Sad )

Although i did try increasing the 100k to 220k as suggested by T Henry elsewhere, which made things worse, so youre probably right. Thanks, ill give it a go tonight. Didnt think to reduce it...

My POS multimeter decided to break last night, the hfe tester 'holders' spring appears to have died so cant test anymore. Sad But can easily swap it out.

Although i can measure the current going thru the LED (i think).

Thanks Joe. Think ill rip it apart and start again. The LFO its attached to is behaving strangely too. Some things to try out anyway.

Cheers
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Tim Servo



Joined: Jul 16, 2006
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PostPosted: Wed Nov 23, 2011 6:23 pm    Post subject: A Collection of LED Drivers Reply with quote  Mark this post and the followings unread

Agreed, it sounds like you might have a bad driver transistor. Have you tried swapping out a different one? Also, what transistor are you using? One other thing - make sure you have the transistor pins correct. Double check against the data sheet to make sure you have the pin assignments correct.

Tim (new reality series: celebrity transistor swap!) Servo


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Paradigm X



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PostPosted: Fri Nov 25, 2011 3:16 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks, sorry limited interwebz access Razz

Ok, well it turns out the LFO schematic i was using is very sensitive to anything connected to it. I used another opamp buffer at the end and everything works perfectly as desribed. Smile

Bit weird as i kind of thought that the transistor buffer was supposed to do that, but there you go.

Ive now got the thankless task of wiring up 23 for my midi to trigger converter!

Many thanks for your help/replies.

Ben (do i really need trigger leds?) Paradigm X Smile
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JingleJoe



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PostPosted: Fri Nov 25, 2011 5:28 am    Post subject: Reply with quote  Mark this post and the followings unread

Paradigm X wrote:

I thought the 100k was to prevent the LED getting loaded and vice verca, wont reducing it mean itll be affected? (its so annoying not quite understanding things sometimes Sad )

I know exactly what you mean, thats how I am currently with exponential convertors Confused

The 100k resistor (R1) only limits the current going into the base and out via the emitter, so in an npn transistor this will affect only the current amplified by the transistor and will not add to the collector current.
(I don't know if you need this explanation but it will help anyone trying to understand (including me))
For example lets say you have a tranny with a gain of 100 and input 1ma to the base, then 100ma will flow from collector to emitter and any load connected to the collector will only have 100ma flowing through it. One should also get 101ma flowing out of the emitter. (base to emitter + collector to emitter current)
You can saturate the transistor too (have it fully conducting) and add a current limiting resistor to the load on the collector to limit the current to whatever you get from the equation I = V/R Smile
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roman_f



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PostPosted: Sat Mar 31, 2012 3:47 am    Post subject: Reply with quote  Mark this post and the followings unread

should i lower the 100k if my amplitude is less then +/-5V?
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mubase



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PostPosted: Mon May 06, 2013 11:19 am    Post subject: LFO driver for green/red??? Reply with quote  Mark this post and the followings unread

Hi Tim. Thanks for the write up on LED drivers. I would like to ask how it would be possible to drive a bi-color LED..
I have a Weird sound Generator for which I have replaced my old simple tri wave LFO for the LFX 8K LFO. ( 8000 times better... Smile
The old LFO had a single red/green bicolor LED which takes positive V for red and negative for green. So it looked really good. However, I tried connecting it to the triangle wave output whilst modulating a wave on the WSG and just like you said, the wave shape got weaker...
I have looked at your drivers and I see one for 2 seperate LEDs but not one for a bi-color red/green....
Any suggestions?
All the best.
Steve.

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Tim Servo



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PostPosted: Mon May 06, 2013 6:00 pm    Post subject: A Collection of LED Drivers
Subject description: A driver for three-pin LEDs
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Hi All,

Circuits for three-pin dual LEDs? Well I just happen to have something. Wink

First off, here's a slightly different design using an op-amp this time. The first is essentially the same as the one presented earlier. However, I build on that for the second design, using a couple of 'steering diodes' to route the current properly through a three-pin bi-color LED. Note that since you have different current paths, you can use a separate current limiting resistor for the second LED. Very handy for those red/blue or green/blue pairs where the blue LED always seems too bright. Note that this design is for a common cathode dual LED. To use this with a common anode part, simply reverse the direction of the two diodes. Also, you can use 1N914 or 1N4148 diodes.

Now with the first two designs, we get back to the same old problem I originally wanted to address with the transistor based versions: I want something that shows the full range of the input waveform, including activity around the 0 volt region. To get around this, I created the third design. This one uses A1 as the standard buffer, and A2 is set up as a comparator. The signal from A2 is added to the signal to bias it up for the positive half of the input, and down for the negative half. This makes the LEDs come on a little earlier (normally they don't come on until you've reached the forward voltage drop - anywhere from 1.7 V for red up to a little over 3 V for blues). Play with the values of R1 and R2 to get the results you like (one example, 4.7 K for R1 gives a smaller region around 0 V where the LEDs are off, although the on/off transition is a little less smooth). Of course, you can use this design with two-pin duals or even two separate LEDs.

Of course, these are good drivers if you don't mind throwing an op-amp or two at the design. Most of the time that's not a problem, but I am working on a dual LED driver using transistors. With two transistors, it should be fairly easy, but I'm going to see if I can do it with one. Stay tuned (also, I usually post these on the Magic Smoke Facebook page first, so you can see them earlier there if you're really in a hurry). Smile

Tim (I'm a good driver a very good driver) Servo


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