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I'm using 5 different LED to indicate water level. I used a 330ohm resistor to all LEDs. Some LEDs glow brighter some are less bright.

How can I calculate an exact resistor value to my LEDs to get equal brightness?

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    \$\begingroup\$ You never mentioned if these LEDs are all the same LEDs (i.e. same colour) or different LEDs (different colours). But easiest way is to equip each one with a potentiometer and adjust to your eye since individual LEDs of the same type will vary so you can't calculate anything and the eye perceives different colours differently. If you're making just a few, do that. Put a minimum fixed resistor in series with the pot so you don't burn something out by accidentally turning the pot too low. \$\endgroup\$ – DKNguyen Mar 22 at 7:51
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    \$\begingroup\$ Show your schematic diagram. If your LED + resistor are fed from different voltage sources then that would explain your problem. \$\endgroup\$ – Transistor Mar 22 at 9:58
  • \$\begingroup\$ Schematic. Are you using ONE resistor to 5 LEDs? \$\endgroup\$ – StainlessSteelRat Mar 22 at 13:41
  • \$\begingroup\$ Normaly I keep resistors of all values, at least 10 of each values in labled packets. Resistors are cheap. You can just go to the next value. \$\endgroup\$ – upali Mar 23 at 12:22
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The trick is not in matching resistors but in matching current, since even the same model of LED has differents Vf among parts and the resistors are computed over that.

This, of course, if you have the same model of LED. If these are different (like different colours) you need to look up the curves for the light emission. If you are lucky your leds come from a 'companion' family (i.e. red/yellow/green leds build to go together in a scale).

How to give them all the same current (or the right current)? Well that's depend on your circuit topology.

Using comparators there a neat trick involving all the leds in series and a single current source. Look for "THAT Corporation Design Note 112 LED Bar-Graph Compression Indicator", it's for audio but you can trivially adapt it to your water level.

If you are using some kind of digital control there are dedicated ICs to match the current of indicator leds. Some of these even have some programmable register to match different LEDs. Look in the catalogues, TI and NXP make a lot of these.

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How can I calculate an exact resistor value to my LEDs to get equal brightness?

You can only calculate this if you have bought LEDs that have been 'binned' (selected, sorted) to particular brightness bands.

Randomly bought LEDs can cover such a wide range of brightness that all you can do is adjust the resistors until they look matched.

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  • \$\begingroup\$ Yup. That's about the only way. And manufacturers may offer binned LEDs. Those I've worked for in the past will bin on both perceived wavelength and intensity. \$\endgroup\$ – jonk Mar 22 at 11:24

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