Common anode vs common cathode for PRIZM 3.5 Crystal Chamber LED

**Darth Nurse** · 02-21-2016, 01:03 PM

I am reading through the PRIZM 3.5 manual and trying to decide how to do my crystal chamber LED.

My understanding is that if I want to run a R/G/B LED to mimic blade color, I would use a common anode and just use the same pads as the main LED (with a resistor for each leg of the CC LED).

I saw mention of using a common cathode LED, but I am not seeing how that would be wired, or the advantages/disadvantages of going that route?

**NanoRex** · 02-21-2016, 01:10 PM

You can wire a common cathode LED to the PWM driver pads. It's in the manual somewhere :P

The advantage is that you won't need a large resistor as they are 3.3V.

**Darth Nurse** · 02-23-2016, 04:05 AM

Originally Posted by NanoRex

You can wire a common cathode LED to the PWM driver pads. It's in the manual somewhere :P

The advantage is that you won't need a large resistor as they are 3.3V.

Will this also mimic blade color?

**NanoRex** · 02-23-2016, 06:30 AM

Yes. Basically the three colour channels each have a PWM signal generated by the MCU, which is only 3.3V and low-current, and can't drive a high-power LED. So they drive three PExs which you can see at the top edge of the board, that you wire your high-power LEDs to. So you have the choice to wire your CC LED to the PWM signal pads (3.3V pad, common cathode LED type) or the PExs (3.7V pad, common anode LED type), however the former is recommended by the manual, on pages 28-29.

**Darth Nurse** · 02-23-2016, 07:43 AM

I am going to have to sit down and diagram it out. I have read that section of the manual a few times and am still missing something.

Per the diagram, exactly where are you soldering the accent LED leads? The picture is fuzzy. Does anyone have a picture of their PRIZM wired this way?

**NanoRex** · 02-23-2016, 08:04 AM

You'd have to solder a common cathode LED right to the legs of the transistors: the legs that are closest to the edge of the board.

**Darth Nurse** · 05-05-2016, 04:13 AM

Originally Posted by NanoRex

You'd have to solder a common cathode LED right to the legs of the transistors: the legs that are closest to the edge of the board.

So to be clear, if I go this route, I still need a resistor for the CC LED?

For the sake of cram fu, I am trying to figure out how to mirror the blade color when it changes with the least number or size of resistors.

**TheSilverDark** · 05-05-2016, 08:30 AM

I am following this as well since I need to do the same. Are the PWM "pads" actual pads? Even better does anyone have a pic they're willing to share?

**Forgetful Jedi Knight** · 05-05-2016, 05:20 PM

Originally Posted by Darth Nurse

So to be clear, if I go this route, I still need a resistor for the CC LED?

For the sake of cram fu, I am trying to figure out how to mirror the blade color when it changes with the least number or size of resistors.

Yes, you would still need to use resistor(s) for the CC LED.

**NanoRex** · 05-05-2016, 07:59 PM

Originally Posted by Darth Nurse

So to be clear, if I go this route, I still need a resistor for the CC LED?

For the sake of cram fu, I am trying to figure out how to mirror the blade color when it changes with the least number or size of resistors.

You'd still need a resistor, but to be honest there is so little current going through the circuit that you'll probably end up using the same size of resistor if you use the blade pad or the transistor legs.

Originally Posted by TheSilverDark

I am following this as well since I need to do the same. Are the PWM "pads" actual pads? Even better does anyone have a pic they're willing to share?

I say "pads" but they're not actually pads, no. Check the manual section on crystal chamber wiring.