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Author Calculating LED resistor (Turing Machine Pulses)
hirada
 Hello, being new to DIY I've thought, I start with a simple project and change the LEDs from the Turing Machine Pulses expander from red to blue. According to the BOM the current (standard) red LED have a forward voltage of 2V and usual 20mA current. The blue ones in question have a forward voltage of 3,2V, so I need to lower the resistor LR4. Having so far only found this schematic: I do not know the operating Voltage of Bit 7. Is it 5V, 12V, something in between? So, given the formula R=(Ub7-Ud)/I I wanted to find out the feeding voltage, since R is given with 2.1K. And that result came out with something like 44V operating voltage for Bit 7. Which of course does not make any sense at all. I've also tried an online calculator for LEDs and it confirmed, that for something between 5-12V, LR4 should be around 150-560 Ohm. But not 2100. But it is working, the lights are blinkung. Why? Can someone please shed some light on this? Above picture is an excerpt of the schematic found on the turing machine website: http://musicthing.co.uk/modular/?page_id=323
nigel
 It looks like the source chip for that signal uses the 12V power, so the input voltage is probably close to that. LEDs don't need to be run at 20mA though. (Regular high-intensity ones will try hard to blind you given that much current.) 2.1k should give around 4-5mA, which is more than enough.
Synthiq
 The light output from a LED is more or less proportional to the diode current over a wide range and the 20mA is just a common reference current to make it easier to compare voltage drop and light intensity for different LEDs. The light output from the LED in the Mouser BOM is 30mcd @ 10mA, so with 12V and 2.1kohm, the light output will be around 14mcd. This is a reasonable brightness. When you replace the LED with a blue one, you need to check the spec for the new LED to make sure it outputs the same light intensity as the old one or you have to change the series resistor to compensate for any differences if you want to keep the brightness the same.
hirada
 Thanks very much! So I need to learn to read the specs right. Alternatively I'll just get a breadboard, a 2k pot and see, what brightness fits me most and then read the value. Cheers again!
Synthiq
 The lowest cost 3mm blue LED I could find on Mouser has a 520mcd output at 20mA, so you would need quite a bit more than 2kohm in series to get the brightness you want. So yes, it is a good idea to learn to read a datasheet so you calculate the series resistance directly or have the right starting point if you want to test it on a breadboard.
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