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Using different colour LEDs
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Using different colour LEDs
OB1
Bit of a nooby question...

Is it possible to switch out LEDs from a kit for a different colour? Or will the difference in forward voltage screw things up?

e.g. I'd like to build the Turing Machine kit, but I'd like to replace the red LEDS with yellow ones. Is that possible? Is there anything else I'd need to do to compensate?
das_Produkt
The only problem you could face is a difference in brightness. Depending on your LEDs, you could change the 2.2k resistors next to the LEDs to adjust the brightness.

In my turing machine I used orange LEDs and I choose 4.7k resistors, because I like my LEDs a bit dimmer.
OB1
Ok, so if I kept the resistors as specified on the BOM but switched out the red LEDs for yellow, it should work, but they might be a bit dimmer? Because to be honest, that sounds ideal.
euromorcego
OB1 wrote:
Ok, so if I kept the resistors as specified on the BOM but switched out the red LEDs for yellow, it should work, but they might be a bit dimmer? Because to be honest, that sounds ideal.

usually it will be brighter, not dimmer. But the yellow and red are probably reasonably close (if it is not some clear superbright, then you have to go >10k or so).

Best is to take a led and try, all you need is a resistor and a power supply (a 9V battery is fine) and check if the yellow is brighter/dimmer than the red and then adjust the resistor accordingly.
julian
As above, I've found the best way with leds is just to check.

There's so much going on, from Planck's constant to sensitivity of the eye to different wavelengths that, whist I'm sure it's do'able with maths, simply testing is always going to be easier unless you properly know what you're doing (which I certainly don't!)
mskala
Difference in forward voltage could be an issue if the LED is being driven from a low constant-voltage source through a current-limiting resistor. Different voltage across the LED means different voltage across the resistor and therefore different current through both. Between that and the differences in LEDs' efficiency (brightness per unit current) you may end up with a considerable difference in brightness when you swap for a different colour. In an extreme case a driver for a red LED might not be able to light up a higher-voltage blue or violet one at all. A constant-current supply (used in some circuits for more precise LED drive) will show this issue less, as will a higher-voltage driver that drops more voltage across the limiting resistor (which means differences in the LED voltage will be proportionally smaller on the resistor voltage).

Also worth thinking about is the current capability of the LED... swapping in one that can only handle 10mA for one which was being driven at 30mA, may cause the new one to burn out. On the other hand, driving one at far less than its rated current will be safe but will make it dimmer (and can be useful to do on purpose if you want it dim).

As the previous two commenters said, the easy way is probably to just build it and see what happens. The more complicated way would be to compare the specs of the two LEDs, look at the circuit, and calculate.
OB1
Thanks all cool
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