I'm building a project with an ESP8266 which requires multiple analog inputs - but the ESP8266 only has one. I know of ways to multiplex with dedicated modules, but since I have lots of diodes lying around I've decided to multiplex with them. I've found lots of tutorials on how to multiplex (e.g. this one) but none of them say what size diode to use.
So which size shall I use? Does it even matter? I have 1N4148s, 1N4007s, 1N5819s, 1N5399s, 1N5408s, 1N5822s, FR107s and FR207s.
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2 Answers
By the time you finish mucking around powering different devices from I/O pins (which have a limit to how much current they can source) and then install different diodes for each one, you are probably better off (and more simply) to use a multiplexer like the 74HC4051 which I describe here.
As Gil mentioned in his reply, the 0.7V drop (or thereabouts) that the diodes will incur are going to affect the accuracy of the readings, plus you have the issue that the devices might take a few milliseconds to stablise if you keep turning them on and off.
To answer your question, though, any signal diode should do the trick as there will be very little current drawn, as the analog input is high impedance. For example the 1N4148.
You can connect 8 devices to the 74HC4051 with only 4 connections between it and the microprocessor. Three "selector" pins which therefore give you 8 possible combinations, and the signal pin which would go to your analog input.
You could even get a 74HC4067 and have 16 inputs, for the cost of only one more selector pin.
answered Mar 16, 2022 at 5:29
I do not think it is possible to truly multiplex analog signals with simply using diodes in an 'or' configuration. Your demo shows powering only one sensor at a time and isolating the sensors (pots) output with the diode "or". This demo works as long as only one sensor is powered and or the rest output voltage is below the one you are measuring. You also have a voltage drop across the diode and nothing to keep the signal stable, it will bounce around by the forward voltage of the diode or more. The diode will add about a 10% error and reduce your measurement range. To stabilize it you need to add some resistance between the analog input of the microprocessor and ground (load sensor output). This will introduce some error in your reading but you can compensate for that. Any diode will work, use one that you're comfortable with or you can check their characteristics and make a choice.
