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I am trying to control 64 leds through a number of 8:1 analog multiplexers. The input is from pin 11 on the Arduino which is giving 40mA (maximum) and it is pulsed at 38kHz. Then I use other pins to select one of 64 leds to control. For testing, I am just trying one of those leds. The problem is that the current across the led is just 2mA, which means it is very dim. Using a multimeter, the current after the first multiplexer seems to be 4mA and after the second just 2mA. How can I get the current to increase? Thanks in advance!

  • Please add circuit. Guess you are using 4051. The internal resistor is approx. 70 ohm. Please see ti.com.cn/cn/lit/ds/symlink/cd74hc4051-ep.pdf – Mikael Patel Aug 23 '16 at 22:21
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    Analog multiplexers are a silly choice for a digital signal. Be sensible. – Majenko Aug 23 '16 at 23:03
  • Don't depend on a digital output to provide you 40mA. That is the maximum current it can provide before getting damaged. – Gerben Aug 24 '16 at 8:18
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Analog multiplexers are not a good design choice for switching digital signals.

You should instead choose a digital solution. Maybe the 74HC138 would be a better choice? That's a 3-to-8 decoder with 3 enable inputs.

You could use 9 of them to reduce pin usage on the Arduino, or 8 of them if you have pins to spare.

One of the enables on the first-stage can be a "mater enable" - or tie one of the enables of all 8 of the second-stage together for a "master enable". Another enable to all the 8 second-stage muxes can be the PWM signal. The third is driven by the Arduino or the first stage as a MUX selector.

Something like this arrangement:

enter image description here

Note that the outputs are active low, so you would connect the anode of your LEDs to +5V and the cathode, through a suitable resistor, to the output.

Another option would be to use a set of shift registers that have an output enable pin. Drive that pin with the PWM signal and you can have any combination of LEDs enabled at once.

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