incrementally brighten LED with multiple switches and resistors

Thread Starter

David Tonkin

Joined Nov 21, 2018
6
Hi there, I'm hoping someone can confirm if this will work... I'm trying to create what I'm calling a "Giant Mechanical Pixel" (Basically 3 LEDs, Red, Green, Blue close together like a pixel in PC monitor). Each LED would have 8 switches, representing bits, and as you turn on each of the switches it adds a little more brightness to the LED, crudely simulating the 256 binary combinations of a 24bit image.

(I hope that made sense, Image attached)

Anyway, I have done a diagram on TinkerCad and it seems to work, but I'm not sure about the resistors... Its a bit weird, because every switch turned on needs to "add less resistance" to the point that when all 8 switches are on, the LED would be at its maximum brightness without burning out....

Can someone confirm:
1) would my circuit work?
2) what Ohms should my resistors be for each LED (diff color, diff voltage?)

I am using water clear Red, Green and Blue LEDs (2x5x7mm)

Thanks in advance!
 

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AlbertHall

Joined Jun 4, 2014
12,344
Yes it would work.
You could make each resistor twice the previous one and that would make the LED current (roughly) follow the binary value of the switches.

[EDIT] If you do the doubling resistor values then you don't need the chageover switches shown on your diagram, just connect or disconnect the resistor.
 

Alec_t

Joined Sep 17, 2013
14,280
As the human eye is not particularly sensitive to small brightness/colour changes you may find that fewer switches per LED (e.g. 4 rather than 8) would suffice.
 

Thread Starter

David Tonkin

Joined Nov 21, 2018
6
Thanks @AlbertHall.
Not sure I'm completely following... if I remove the switches then I'm not able to show the physical idea of turning a bit on and off... ?

Also with the doubling idea, would that not make it so I can't just pick random sequence and get a semi dim LED? My thought was that in my crude example, 11001100 would technically be the same brightness as 11110000, because both combinations have 4 switches set to on, meaning they both recieve the same wattage.... are you saying if each switch had a higher level of resistance than the last, I would able to hyperthetically look up the binary value for a particular level of brightness and set my switches to the same and it would (very crudely) set my LED to same? I was just going to be happy with, "look 1 switch on, its dim, and wow, all 8 on and the LED is bright!"
 

Thread Starter

David Tonkin

Joined Nov 21, 2018
6
As the human eye is not particularly sensitive to small brightness/colour changes you may find that fewer switches per LED (e.g. 4 rather than 8) would suffice.
You're right, I am a little concerned about the tiny changes between 1 or 2 switches, but I'm really wanting the students to see a physical example of 8 bits per color channel=24 bit image, etc....
 

Thread Starter

David Tonkin

Joined Nov 21, 2018
6

AlbertHall

Joined Jun 4, 2014
12,344
Not sure I'm completely following... if I remove the switches then I'm not able to show the physical idea of turning a bit on and off... ?
your diagram shows each switch connecting the resistor to either the supply or ground. It only needs to either connect the resistor to the supply or disconnect it.

I was just going to be happy with, "look 1 switch on, its dim, and wow, all 8 on and the LED is bright!"
To do that simply make the resistors equal values.
 

oz93666

Joined Sep 7, 2010
739
This is something you have to do in practice .... start with a resistor which gives the faintest possible discernible light .. probably around 1 M Ohm .. then the next size is determined by the lowest increase that gives light clearly different from the first , so on
 

Thread Starter

David Tonkin

Joined Nov 21, 2018
6
This is something you have to do in practice .... start with a resistor which gives the faintest possible discernible light .. probably around 1 M Ohm .. then the next size is determined by the lowest increase that gives light clearly different from the first , so on
Thats an awesome idea. Just do it by eye rather than math... that way I'll see every change. Awesome. Thanks for that.
 
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