# Reading 13 FSR over a multiplexer

I've made a design for reading 13 FSR with only 1 channel ADC. I want to apply the DAC voltage to only one of the FSRs using MUX. But when I choose one, I see voltage in others. For example, when I apply 1.5volt, I see 1.5V in the selected sensor, but in others, there are different values ​​like 0.4.0.5.0.7. this makes my value with ADC wrong. Other sensors need to be 0 volts.
For example, when I select the sensor at point A, the result is not only A dependent also B, C and others dependent.

Is there something I've overlooked with the internal structure of MUX?(Leakage etc.)
Do you recommend a circuit structure where 13 sensors can be read with a single ADC and a single MUX?

Notes: FSR:Force sensing resistor
MUX:74HC4067

I'll try this one

I think you mean something like this:

(I don't really understand what you need the DAC for in your schematic.)

R1//R2 create a voltage divider. The value of R1 depends on what FSR you have, and what voltage Vcc is, and what kind of ADC you used. See this link for more information.

For the MUX: The not-E pin (15) needs to be low for the MUX to work. You can select which Yn port will be connected to the Z port with the Sn inputs. So you'll need to copy the R1//R2 part of the schematic 16 times.

You could move R1 to the ADC input, like you did in your schematic. You'd save 15 resistors on the board. But the MUX has an internal resistance as well (somewhere in the 50-100 Ohm range, depending on Vcc). If that will work depends on your FSR, and what resistance it has, and what pressure levels you want to measure. It also depends on your Vcc and how much accuracy you need. E.g.: If after calculating R1 it ends up >10x larger that the internal resistance of the MUX (let's say R1 > 1k), and you don't need high precision in the lower resistance range of the FSR, it could work.

The advantage of the schematic as it is now, is that the only current through the MUX is the input impedance of your ADC, which is good for precision and stability.

If this will work correctly depends on your ADC and FSR. I can't guarantee things will work like this not knowing the specs of the other components. But I think that's something for you to figure out anyway :-).

• My guess is the OP means to use the DAC as the source, and the FSR+100k divider would then be used to set the maximum voltage the output (input to the ADC) would reach. In that case, the schematic should be mirrored to make this more obvious. Although then the last lines of the question ask about 13 sensors read through an ADC. Guess I am confused, too. – Richard the Spacecat May 17 at 10:23
• The reason I used the DAC was to adding the flexibility and adjusting resolution more precisely. Forget it can be VCC instead. No space for additional 15 resistance board. I will try your suggestion. – Berker Işık May 17 at 12:09
• at first, I accidentally uploaded the photo on your post.::)) – Berker Işık May 17 at 12:25
• the results are bad. I installed the circuit shown above, I select a sensor with MUX, the MUX output changes when it is pressed on any sensors. For example, I chose the 13th sensor, Z output of MUX vary with also 1,2,3..10,11,12. sensors. only needs to change with 13. where is my fault? :((((( – Berker Işık May 17 at 13:10
• Interesting. It's hard to tell where your fault is. The only thing I can think of is: Did you power (and GND) the MUX properly? Assuming you did: Please post the full schematic. – MartinF May 17 at 18:02