Switchless Mods - Understanding LED Colours - Help Needed

Hello.

I am not an electronics person but I am trying to understand how the LED in a switchless region mod works. I have been reading about this and looking at all the different ways people have been wiring it in their mods.

This is my understanding so far.

LED Types Used: Tri Colour Red/Green Common Cathode LED or some people use a Red/Blue Common Cathode LED.

Basic functionality

​

• Positive applied to the Red pin will produce a Red colour.
• Positive applied to the Green pin will produce Green colour.
• Positive applied to both Red and Green pins will produce an Amber colour.

I presume positive in the context of Mega Drive switchless mods means 5v is being applied to the pins to produce the colours.

Can some please answer these three questions

1 - What do the resistors do? My basic presumption is that 5v is too much voltage for the LED and the resistors brings this down to a value the LED can use.

2 - Can someone please explain the difference between the following that I have seen?​

• 2 x 220 ohm resistors can be placed individually on the Red pin and the Green pin.
• 1 x 220 ohm resistor placed on the common pin.
• 1 x 180 ohm resistor on the Red pin and 1 x 680 ohm resistor on the Green pin.

And last question

3 - If I choose to use a Red/Blue LED. How do I get Purple as the third colour, what resistor values do I need for this combination of colours? (I have seen this combination on a few MD consoles that are currently being sold on eBay)


Many thanks




Examples

2 x 220 Ohm Resistors



Single Resistors





Red / Blue / Purple LED



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1x resistor on the common is the same as 2 resistors on the anodes. But using 2x on anodes give you the ability to use different values for different intensities.

the resistors limit the current, too much current and the led will get too bright or pop.


higher value resistor = that colour dimmer, as its letting the led draw less current.

 

Just to precise what BadAd said and answer this specifically.

A diode has a I-V or Current-Voltage characteristic that's an exponential. Roughly speaking that means that passed a certain applied voltage on the diode, the current it needs to draw gets very high very fast. Most diodes have that voltage at around 0.6V, so at 0.6V you may draw a few milliamperes but at 5V you'd draw 2+ amperes and the diode heated and melted/exploded before you reached it.

The resistor R is there to take most of the voltage drop and roughly fix the current flowing in the circuit. If the diode is at 0.6V and its resistance is negligible comparatively to the resistor, the voltage across the resistor will be V = 4.4V and so the current through it will be I = V /R. The current through the diode will be the same because they're in the same circuit loop — just like the water current per second through a pipe is the same all along its length even if the pipe gets bigger or smaller at some points (otherwise you'd be creating/destroying water).

 

Ok. That's great.

Can you give this a quick glance and let me know if I haven't completely misunderstood it all.

• Resistors have no effect on the actual colour, just the brightness.
• higher value resistor = less current thus lower brightness.
  
• 220 ohm is a good starting value for each pin it seems from kits and tutorials available.
• 2 x 220 ohm resistors on Red and Green pin should give you Red/Green/Yellow and on Red and Blue pins should give you Red/Blue/Purple as default.

Calculating Resistor value and current (Not using maths)

A typical Tri-Colour LED at https://www.rapidonline.com/kingbright-l-59egc-5mm-red-green-led-tricolour-clear-56-0685



So if I use a resistor of 220 ohm it will result in 12.7ma on the Red LED. Which will give it lower brightness compared to its default of 30ma.

http://www.ohmslawcalculator.com/led-resistor-calculator

One last question.

What happens if you wire the LED backwards? The Mega Drive switchless kit I just did resulted in Red, Green and then another green variation. Is this a result of backwards wiring?
​

 

Led backwards just inverts the 2 colours. The "both" color remains the same, assuming the same value resistors on both lines.

Colours don't change, but a dimmer green with brighter red would show a different colour... So while both are still red or green, it does determine what colour you see.

 

Ah okay. Just trying to figure out why the switchless I just did on the Mega Drive 2 has resulted in Red and Green as the separate colours and then another Green for the combined state. This is with two 220 ohm resistors. I assumed this should give Red and Green and then combined to Amber.

So this indicates that the green resistor is too low and the red too high. Less green and slightly more red would result in a more amber colour as the third state. I will have to source some resistors.

Do you have any recommendations for resistor values other than the 220ohm you supplied that might get me closer to red / Green and then Amber?

I appreciate all the info and help so far. Thanks

 

off the top of my head - i think 330 for the green and 150 for the red should work

 

That's great. Thanks a lot.

 

You mean 330 for red and 150 for green, yes?

 

I thought that, but hes saying the mixed one is too green. But I think the LEDS I ship have changed in last restock - so who knows, it could have changed. Can only go on what hes saying. But ultimately, if one way doesnt work - do the other.

 

Red has a higher wavelength though.

 

Which has nothing to do with this.

 

I think lowering the green in the combined state with higher resistor value will do the trick.

But yes, decided to buy a bunch and experiment a bit.

Cheers.