Need help reading basic pot circuit diagram

Ragtop

My circuit diagram comprehension just does not seem to be improving.

I am wanting to change the sensitivity of a EPIR motion sensor that seems to go crazy outside but works fine indoors. I currently have it set to max by connecting the txd/sense pin to ground.

I want to add a pot to vary the sensitivity but I don't understand the circuit inside the box.


The tutorials I have read run the wiper to a microcontroler pin and one of the outside pins to ground, but this seems to have three connections: power,ground, and EPIR pin.

Does the power go to wiper pin with the EPIR pin?

ElectricAye

What's inside the red box appears to be a classic voltage divider made with a pot (R4) and an external resistor (R2). If your pot is not labelled, then you will need to use an ohmmeter to connect to the pot pins and twiddle the pot knob to see how the resistance varies as you turn the knob. From that little experiment, you should be able to discover which wire pot lead is the wiper (which is the lead with the arrow pointing at the R4). From there, you need to connect your R2, and your voltage supply, 3.3 volts. In effect, R2 and R4 divide the voltage of 3.3 volts so it gets sent to your sense pin. This division is adjustable thanks to the pot.

Look up "voltage divider" on wikipedia, etc. for a more detailed explanation.

Ragtop

Does this look right?

ElectricAye

What is the blue wire connected to?

Also, I'm just guessing that the pot wiper (pot lead with the arrowhead in your schematic) is the middle lead (shown as green) and not the yellow wire. You need to check that with an ohm meter. Is the pot you are showing an adjustment for the delay input? I don't see two pots in your picture.

Ragtop

I am keeping the delay pin going to ground. I don't wish to vary that. I am only adding a pot to the sensitivity pin. I am assuming green is the wiper in the picture. Is that how the voltage divider should be wired to the wiper pin?

What should the blue wire be connected to?

schill

For a pot like the one in the breadboard picture (and most pots for that matter) the middle connection is almost always the wiper.

With that assumption, if you look at the pot connections - orange, green, and blue - you should connect them this way:

1) green to sensor pin (the wiper which is the output of the voltage divider)
2) orange to ground
3) blue to 82K resistor that then connects to +3.3V

The pot itself should be 100K. There is no other resistor required.

The orange and blue connections are interchangeable. The only effect of switching them will be to change the direction you turn the pot to reduce (or increase) the voltage.

Ragtop

Like this?

schill

That should work.

It's worth looking up how voltage dividers work so that you understand what's going on here. The original schematic you posted has three different voltage dividers on it. All are used to adjust the voltage level at different connections on the sensor.

Voltage dividers are very common in electronics. If you look it up on wikipedia as suggested above, you should be primarily interested in simple resistive dividers. The description starts out talking about general impedances, then discusses resistive dividers, and then rapidly gets more complicated than you need right now.

Ragtop

Thank you. I was missing that the pot was one of the resistors.

schill

It's helpful to think of the pot as two resistors - one on either side of the wiper.

If you turn the pot all the way to one end or the other, the corresponding resistor will effectively have zero resistance.

The extra resistor in this circuit is used to prevent the maximum voltage coming out of the wiper from going all the way to 3.3V.