Force Sensitive Resistor Design Question

Thread Starter

ponas.jonas

Joined Feb 28, 2023
36
Hi everyone,

I wanted to post in this forum to ask a question regarding a PCB I have designed. I have just received my PCB and am trying to program it, but I am not getting any values, so I immediately thought that something is wrong with my electronics...

I am using a force sensitive resistor (U53), maybe someone can spot an error I have done immediately

1707238459528.png
 

Thread Starter

ponas.jonas

Joined Feb 28, 2023
36
If pins 4 and 8 of your op-amp are its supply pins, get rid of C90 and C89.
Which op-amp is it?
Thank you for your reply. Why is having these two capacitors not necessary? The op-amp is LM358 (YW)

How about the placement of the variable resistor, I saw some other schematics, where it is placed in the feedback of the opamp:
1707393902425.png
 

atferrari

Joined Jan 6, 2004
4,766
Could you please add the pin numbers to your schematic. The correct ones as per datasheet so they would allow you to verify your PCB as well.
 

Thread Starter

ponas.jonas

Joined Feb 28, 2023
36
Because they are shown in series with the supply. If they are meant to be decoupling caps then they should be connected close to the supply pins but in parallel with the supply.
I have removed them and made a simulation in LTSpice to see what I am supposed to expect when I measure the voltage after the op-amp, as that is what goes into my ADC in the microcontroller. I only see a constant voltage in the simulation, but do not really understand why so. What am I doing wrong?

1708946248412.png
 

Papabravo

Joined Feb 24, 2006
21,223
The input to an opamp is very high impedance. Very little current will flow into or out of that input. If there is little to no current in the resistors there must also be little to no voltage drop. In post #3 you have an inverting configuration for the opamp and you switched to a follower in post #6. Why did you do that?

ETA: I'm not sure it is wise to have a single power supply of +4 VDC and an input signal from -1 VDC as shown in the diagram from post #3. Most opamps have a common mode voltage range limitation. That means no voltages on the inputs outside of the power supply range.

ETA2: Functioning opamp circuits need a DC path to GND from the inputs.
 
Last edited:

Alec_t

Joined Sep 17, 2013
14,312
For the LM358 opamp that you are using, the input voltage should be at least 1.5V below the +v supply voltage.
 

Thread Starter

ponas.jonas

Joined Feb 28, 2023
36
@Papabravo @Alec_t Thank you very much for your answers and directions where to look.

I realized that to have an output of 4V I needed to supply the rails with +6V. But nonetheless, my circuit worked when I de-soldered the capacitors on the rails. Is it because they are connected in series, as you have mentioned before? Why do they still not "charge up"? Does current now flow through them? I checked with a multimeter and got 0V on the output of the capacitor
 
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