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Topic: Question regarding the control of 24 individual switches (Read 556 times) previous topic - next topic

ThreeDMD

Hi Everyone,

I have a question that deals with sequentially controlling 24 SPST analog switch(ADG1412) for sourcing 24 different LEDs. I'm mostly concerned with the advantages/disadvantages of using a GPIO expander or simply running 24 different wires from my ATMEGA2560.

My goal is to maintain low power within my whole design (<4W) but I need to compare this to the ease of using 2 MCP23017 or simply turning the pins on and off from the ATMEGA. The 2 MCPs add about 10mW, this takes up a lot. I do have the option of increasing to about 6W but would rather not lower the bar.

Just looking for some advice or direction. Thanks in advance.

Grumpy_Mike

Using a multiplexed analogue switch to drive a LED is just plane stupid.
Use a port expander every time.

Paul__B

Using a multiplexed analogue switch to drive a LED is just plane stupid.
It seems he was having a flight of fancy. :smiley-lol:

ThreeDMD

Using a multiplexed analogue switch to drive a LED is just plane stupid.
Use a port expander every time.
My description is a bit off. My LEDs are driven by high current output op-amp and my switches simply direct to each one.

Can you explain why I should always use an expander?

I appreciate it.

PaulRB

My LEDs are driven by high current output op-amp and my switches simply direct to each one.
Why drive leds with op-amps? And what are the switches directing? Some signal to the op-amp inputs, or the op-amp outputs to the leds? What are the input signals to the op-amps?

Can you explain why I should always use an expander?
Its simpler. Right now we don't understand the reasons for all your op-amps and switches, so that approach sounds over-complicated.

Have you considered shift registers? There are high-current ones available like tpic6c595.

Paul__B

Have you considered shift registers? There are high-current ones available like tpic6c595.
TPIC6B595s or the highest current are TPIC6A595s.  But yes, for simple LEDs, the lower-rated TPIC6C595 would generally suffice.

These combine the function of "port expander" and current driver and are obviously the proper way to perform this function - only three port pins used to control 24 (or in fact, 128) LEDs.  Of course, the LEDs require current limiting resistors.

My LEDs are driven by high current output op-amp and my switches simply direct to each one.
Clearly absurd, but out of curiosity, just what "op-amps" were you proposing to use?

The 2 MCPs add about 10mW, this takes up a lot.
Mmm, no you got that wrong, it would be 10 ยตW.  Pages 1 and 26 of the datasheet, parameter D005; Standby current.

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