Choosing between an op-amp and a comparator

Question

I am a YouTube educated, complete newbie with three questions regarding my design (see the schematic). Because I'm self-taught, some of my terminology may be non standard.

This is a circuit powered by a 12 V battery (model 23A). Its only function is to compare the output of two voltage dividers and beep when the first divider's voltage exceeds that of the second. A 3-position switch allows me to change the value of the resistance on the + side of the second divider, allowing me to select a level at which the beep will sound.

A) the first divider is comprised of a force sensing resistor (FSR) from Interlink Electronics on the + side of the divider (hearder1), and a simple resistor (R1) on the - side. The FSR's resistance varies from infinite to about 1 kΩ, with the resistance decreasing as pressure is applied. For my purpose, I have measured 10 kΩ, 5 kΩ, and 2 kΩ as the three thresholds I am trying to sense and have trigger the beeper.

B) the second divider is comprised of a 3-position switch connected to 3 potentiometers (R4 5 kΩ, R5 10 kΩ, R6 20 kΩ) on the + side of the divider and a simple resistor (R2) on the - side. I'm using pots so that I can fine-tune the 3 levels.

I have bread-boarded the circuit as shown and it works. I have also ordered PCBs and soldered all of the components. This works also. In order for this device to be useful, the beep should sound within 10 milliseconds of the trigger, and it will only need to trigger once every few seconds or so. Again, all is working fine as shown. Note that the beeper gets louder as the voltage it gets increases and I'd like it to be as loud as possible.

Note that this is a sporting goods product I hope to sell (to a tiny niche market). I'm not asking for anyone to "fix" my design or anything like that, I just have a few finicky questions that I can't seem to resolve on my own.

My 3 questions are:

1. I am currently using a DIP8 op-amp (Texas Instruments LM358P), but I will be using the SMD op-amp as shown in the schematic from OnSemi. If it's working now, will there be any difference with between the two?

2. Would there be any difference/benefit if I used a comparator instead? Namely, the one shown floating in the schematic, also by OnSemi. The price is basically the same.

3. What effect (if any) would changing the value of R1 & R2 have on the voltage seen by the beeper? On my breadboard I see no difference but I am assuming that I'm doing something wrong.

Thanks in advance for any advice I get.

Update- I got rid of the Op-Amp and wired the comparator instead. Also added a resistor where i think a pullup resistor would go as needed to resolve the open collector on the OUT pin of the 393. thanks

Answer 1

Definitely that is a job for a comparator. Using an op-amp in that circuit would be like turning a screw with a knife instead of a screwdriver.

• Op-amp: analog in and out
• Comparator: analog in, digital out

That buzzer is either off or on: that's digital. Therefore, you want a comparator.

Answer 2

1. An LM358 from any manufacturer should work interchangeably. There should be no difference who is the manufacturer, if they are otherwise equivalent to the original LM358. But you need to look at the datasheets if they are equivalent, as some models may have been modified in some way.

2. There will be a difference. Op-amps are not intended to be used as comparators as they are very poor comparators. Comparators are also very poor op-amps. But it still does not stop people for using components for something they are not good or intended. If you change to a comparator, do note that their output is usually different and you need to rewire the buzzer to work with a comparator.

3. There should be no difference to voltage seen by the beeper if you change any of the 1k resistors. Your observation is correct. There is no mechanism how their value would change the LM358 output voltage, but you may provide the info why you think it will.