Bipolar led color shifting. - Page 1
Forum section: electronics / beginners

author: DBoulanger
author_rank: Regular Contributor
timestamp: January 08, 2016, 04:20:16 am
content: Hi everyone,I got some bipolar (red-green) leds, which have 2 leads, not to be confused with the bi-color ones, with 3 leads, and I found a schematic, although almost 5 years old, but fairly interesting.It uses readily available components, such as the NE556 and the basic 1458 opamp, but I do have a bunch of 555 timers and other opamps that I can use.The article describing this circuit is located athttp://freecircuitdiagram.com/2011/01/28/color-shifting-led-show/.What type of modification/adjustment could be done in order to accommodate more leds ? By nature, opamps aren't able to drive much current, so what could be done in the actual case ?I'm joining the schematic in this post, but for a more detailed explanation on how it works, I'll suggest you to consult the article above.BTW, I'm opened to alternative circuits as long as we're dealing with bipolar, 2 leads leds, without the use of a uc.Thank you in advance for your input.

author: matseng
author_rank: Frequent Contributor
timestamp: January 08, 2016, 04:55:58 am
content: Use the OPA4xx / OPA5xx-series :-) They are high-voltage, high-current opamps. For instance the OPA549 is specced at 8 Amp continuous, 10 Amp peak and voltages up to 60 volts.

author: DBoulanger
author_rank: Regular Contributor
timestamp: January 08, 2016, 05:34:10 am
content: Quote from: matseng on January 08, 2016, 04:55:58 amI didn't that such things were out there ! Anyhow, for the amount of leds I was aiming for, that opamp would be way above what I need. I'm looking at something that would handle 1.0 to 1.5 amps at the very maximum.I'll check for something a bit more conservative, spec wise, but now knowing that I may get something that could handle it, makes the overall thing more appealing.Anyhow, prior to make a final decision/move, I'll wait a bit more and see if with an alternative design I could achieve a similar result.Regards.

author: mikerj
author_rank: Super Contributor
timestamp: January 08, 2016, 10:36:30 am
content: You need a high current push/pull driver on each output. You could go for a discrete solution with a pair of complimentary transistors, but a simpler (though more expensive) solution would be one of the "half bridge" power drivers designed for driving motors etc. that include over current/temperature protection e.g. the MP8040 is good for ~4amps continuous and only costs around $1.

author: DBoulanger
author_rank: Regular Contributor
timestamp: January 09, 2016, 12:23:56 pm
content: Quote from: mikerj on January 08, 2016, 10:36:30 amI have to admit that I have very basic knowledge of the "push/pull driver" and the "half bridge" topologies. Perhaps the availability of a schematic and most importantly, an indication on how to use it with the sample I posted, would help me understanding more clearly. Searching on the net for the suggestions you provided, helped me having a better feeling about them, but not enough to be at ease and implement it. Globally, if I would need an exemple that I can refer to or at least follow the concept. Starting from scratch, not knowing what to expect, is pretty much like shooting in the dark and the odds to hit the target are rather slim.Regards.