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Simple MUX circuit

Barsto

Feb 12, 2023
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Hey all
Ive been trying to design a simple MUX circuit for my project.
My circuit ia built from a controller (arduino), which controls two 2:1 MUXs (74lvc1g157). The output of each mux is connected to a multimeter, and the inputs are connected two different loads, meaning I0 for mux 1 and I0 for mux 2 are connected to Resistor 1, and I1 for mux 1 and I1 for mux 2 are connected to resistor 2.
I controll both Mux via the same arduino digital pin.
I want to switch the measuring between both loads. Muxs receive gnd and vcc from arduino.
So I always get 20ohm resistance, even if I remove the loads from the muxs inputs. When I remove the vcc from the voltage, the multimeter doea show overload (when no load is on the muxs).

I dont understand whats wrong. Should have I used demux instead ?
Why does the multimeter show this resistance ? Please help
 

Alec_t

Jul 7, 2015
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Welcome to maker.pro.
Can you post a sketch/schematic of your setup so that we have a better understanding of your project?
 

Alec_t

Jul 7, 2015
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So I always get 20ohm resistance
If you are using the MM on a resistance range, don't forget the meter itself includes a current source to pass current through the load under test. Also, are the loads floating, or ground-referenced?
 

Barsto

Feb 12, 2023
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The loads are floating
But when the loafs are disconnected shouldnt I get OL reading ?
Is there something wrong with the wiring
 

Barsto

Feb 12, 2023
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is it okay that I used the same digital pin to control both mux?
 

Harald Kapp

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Seem you misunderstood the concept of multiplexers.
The 74LVC1G157 are digital multiplexers. You can use them to select between digital signal only. These are not suitable for multiplexing analog signals or "loads".
You will have to use analog switches, but note that analog switches have a comparatively high resistance and are not well suited for exact measurements. When you want to switch between two resistors and measure their values, the resistance of the switches should be much much smaller than the resistance to be measured. Or you'll have to include some corrective math into your measurements to account for the resistance of the switches.
 

Barsto

Feb 12, 2023
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Thanks for you response Harald !
So MUX are only used for logic signals?
Because I know that measurement systems with multiple channels use a MUX to combine/read the different signals, same goes for audio applications.
Aren't there analog MUX ? or are they essentially the analog switches you referred to
 

Barsto

Feb 12, 2023
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understood
So I previously used CD74HC4067, but I wanted to use a smaller chip. Now I understand that I chose the wrong component for the job.
Do you reckon that CD74HC4067 is fitting for my application? see attached
Ben
 

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  • cd74hc4067.pdf
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Harald Kapp

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Do you reckon that CD74HC4067 is fitting for my application?
Describe your application.
  1. much too big in terms of physical size and number of channels. This is a 16:1 analog mux, you need a 2:1 only.
  2. max. on resistance is 80 Ω. You don't state what resistance you are going to measure. the on resistance may affect your measurement.

Perhaps it would help if you were to describe what you want to do, not how. There may be other options than an analog multiplexer.
 

Barsto

Feb 12, 2023
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Right, that would be better
I think I got the gist
I used the CD74HC4067 only cause I had it on my shop
My device is supposed to measure different alternating loads, ranging from 3-10kOhm. I'd like to control the measurement channel with the press of a button (software ).
So what I think I should do now is look for a low resistance (as possible) and appropriate physical size mux.
Sounds sensible?
 

Harald Kapp

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Sounds sensible?
Yes.

Have you considered using a relay? A dpdt relay (double pole, dual throw) has a very low on resistance compared to an analog semiconductor switch, is robust and is easily controlled by a single pin of the µC.
You will need a driver (transistor) to control the coil of a relay. The output pins of an Arduino are not powerful enough.
 
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Barsto

Feb 12, 2023
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I wasn't familiar with these components, I'll check it out
Thanks so much for your help
 

crutschow

May 7, 2021
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Have you considered using a relay?
It likely should be a relay designed for low current (dry circuit) switching, usually having gold plated contacts, to avoid problems with high contact resistance due to contact oxide formation.
 
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