Question for you diodeites

[rodekyll]

I'm planning 5mm LED dash lights for the trike. They will be hot from a variety of sources and grounding to a buss bar.  Can I wire them in parallel to a rotary pot on the ground side to act as a dimmer?  What should the potentiometer's ohm range be?

[Triple Jim]

If you connect all the cathodes together and then use one pot, it's likely the brightness will vary quite a bit from LED to LED.  This is because their brightness is current dependent, not voltage dependent, for the most part, and it's unlikely the current will be the same for all LEDs when connected that way.

On top of that, since the number of LEDs lit at any given time will vary, having a common resistor on the ground side means that each time an additional LED lights, the others will get dimmer.

I'd probably  just use a fixed resistor on each LED, on the supply side, and choose its value as a compromise between daytime visibility and nighttime irritation.  If you don't mind some complication, you could use a single pot to control the brightness of all the LEDs, but you'd need to do something like drive each LED with a transistor, with the transistors supplied by a regulated variable voltage, and the bases driven by the input that normally lights each dash light.

[rodekyll]

Thanks for the assessment, Jim.  My last dash was done having each LED with its own resistor to bring the various colors to the same brightness.  It was a PITA playing with resistors to find a good middle-ground, and I ended up with fixed brightness that wasn't quite right for either day or night.  So I was hoping for an elegant fix.  I guess some imaginings are not meant to be.   

[Kiwi_Roy]

I assume you are using 12 Volt LEDs with a built in resistor, they would be equal brightness.
As Triple Jim points out using just a pot the brightness would be effected by how many are on.
You could regulate the Voltage using a PNP transistor set up in an Emitter follower circuit that would stabilize the Voltage for one or many.

Are you sure it's practical to have all the LEDs referenced to ground though?
Likes of Oil pressure and Neutral switch pull the lamps to -12 Volts, you would need the pot in the positive lead so you would also need a supply with the positive regulated.

I think you could recon on 5 milliamps at low setting 10 milliamps for high, 1000 Ohms would be a place to start.

[Kiwi Dave]

LEDs are better off being connected in series.  That said, there will be a limit of how many than can be connected, depending on the color of the LEDs.  Red LEDs typically have a voltage drop of around 1.8v, green 2.2v, and blue over 3v.  There will be still be a requirement for a current limiting series resistor which can be calculated by the remaining voltage divided by the desired current.  I would not go higher than 20mA (for standard LEDs, superbright can handle around 30mA).  Then by introducing a variable resistance (rheostat, not potentiometer) in series as well, you can regulate the current down to say 5mA to achieve dimming.

There are obviously more sophisticated means of controlling multiple LEDs as witnessed by the LED arrays available that can run on 12v, but this is the simple solution.

[Wayne Orwig]

Busy again, so maybe this was covered.

You can not use a simple resistor/pot to dim them, because you are going to have a varying number of LEDS to load the resistor. Even with the pot on 'dim' with 4 LEDS on, when only one LED is on, that one LED will be bright.

Depending on the LEDs, the thing to do might be a Zener. As in, connect a 5 volt zener to ground. Run the LEDs to a switch. In one position, the LEDs go to ground and see a total of 12 volts. In the other position the LEDs go to the zener and see 7 volts (12 - 5). It would be dimmer. That assumes the LEDs are simple LED/resistors.

A fancy variable way to do it would be a pulse width modulator. As in the LEDs are grounded 100% of the time, or vary it so the LEDs are grounded ??% of the time. A simple 555 design would work. I can whip together for you if you want.