drive two led's with only two pins (total with power) with no microcontroller

Thread Starter

johnny411

Joined Mar 3, 2020
31
hello!

so here is my problem. i have a pushbutton (controlling power on/ off toggle circuit using ltc2951) with the following led arrangement:

1280px-2-pin_Charlieplexing_with_common_resistor.svg.png
from my understanding this is called "charlie plexing". this would be simple enough to drive with a micro controller, but i have no idea how to control it without one "simply" (i started working out a design but it was overly complex, so i thought there must be a better way).

essentially the led's need to be controlled by 1 pin that when it is high turns led 1 on (2 off), and when low turns led 2 on (1 off). this pin is the EN pin on the ltc2951.

any suggestions for how this might be done?
 

djsfantasi

Joined Apr 11, 2010
6,838
I’m don’t understand your question...

The circuit you posted needs two pins to control. But with a digital NOT gate, can do what you describe with only one pin.

Feed the pin to the LEDs resistor. Also, feed the pin into a NOT gate and connect the gate output to the other side of the LEDs.
 
Last edited:

Thread Starter

johnny411

Joined Mar 3, 2020
31
The circuit posted is the interior circuit of my pushbutton (minus the resistor). My question is how do I control these two leds which only two pins in the above formation? (No microcontroller). The pushbutton only has two pins for two leds. Sorry if my wording in my original post was confusing.
 

djsfantasi

Joined Apr 11, 2010
6,838
The circuit posted is the interior circuit of my pushbutton (minus the resistor). My question is how do I control these two leds which only two pins in the above formation? (No microcontroller). The pushbutton only has two pins for two leds.
See my other post. You can use a transistor (NPN recommended) as the NOT gate.
 

eetech00

Joined Jun 8, 2013
1,992
hello!

so here is my problem. i have a pushbutton (controlling power on/ off toggle circuit using ltc2951) with the following led arrangement:

View attachment 217161
from my understanding this is called "charlie plexing". this would be simple enough to drive with a micro controller, but i have no idea how to control it without one "simply" (i started working out a design but it was overly complex, so i thought there must be a better way).

essentially the led's need to be controlled by 1 pin that when it is high turns led 1 on (2 off), and when low turns led 2 on (1 off). this pin is the EN pin on the ltc2951.

any suggestions for how this might be done?
Here is one way.
If you want it to toggle, you can use a FF to drive the transistor base.

1600035071691.png
 

Thread Starter

johnny411

Joined Mar 3, 2020
31
@djsfantasi


thanks. your suggestion has helped me quite abit. here is a rough simulation i did of your proposed circuit. ill probably be replacing the not gate with a transistor as you proposed aswell.

dual led annot.PNG

@sghioto

no, my part number is correct it is the LTC2951-2. should have linked in the product page to begin with, so sorry for the confusion.

@eetech00

thanks, but if you look at my first post i am locked into that led configuration, so your proposed circuit wouldn't work for this application.
 

SamR

Joined Mar 19, 2019
2,597
The diagram in the OP is directly from the WIKI on Charlie and other plexing. Read the WIKI. It notes that two uC pins & two diodes are not needed to be plexed. Plexing is used to uC drive multiple diodes very fast ON/OFF so that eye persistence makes them all to appear ON. Remember that pins can be used to both source and sink. One pin with a diode to ground and a diode to Vcc will flipflop them ON/OFF. That is one concept of plexing and if done quickly both diodes appear to be constantly on. This is for microprocessing, not a simple switch circuit.
 
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