Only partially related to question - using a hi side NPN as shown is somewhat unusual. It's certainly doable. You gate drive needs to swing from V_relay+ to typically 5 to 10 volts higher. If relay is say a 12V relay you will need say 17 to 20+ volts to drive gate (exact level depends on FET Vth etc). Use of a high side P channel FEt or a low side N channel FEt would be somewhat more usual. But, again, this is workable as is if needed.
D1 should be mounted as close to inductive load as possible. ON relay across coil contacts (or relay socket if used) is desirable. Any distance from inductive source gives a nice radiating loop.
Moving D1 around against the relay coil contacts will non-trivially reduce the radiating loop in the PCB track.
RC or RL decoupling the feed to the relay helps.
If same power source is used for thermocouple and relay then filtering in the thermocouple feed adequate to deal with any couple transients is needed. An active regulatopr will give typically 30 - 60 - ++ Vin noise rejection depending on model and design. Any RL or RC filtering in power circuit will add more filtering.
It's not obvious on the PCB how ground gets from the "Power" connector to the ground plane on the right hand side.
Any ground path commonality will help undo your good work. A milliohm of common ground return and a one amp spike will give you 1 mV of coupling. On a 5V supply that's 1:5000 coupling or about -70 dB. Not much, but it sets an upper limit to the isolation you can achieve. If you manage to couple that 1 mV directly into the sensor feed it will be much much worse.
A thermocouple usually has a thermal time constant of seconds at best and 10's of seconds often. You can get super fast response devices but they would be unusual. Low pass filtering the thermocouple or integrating over a few seconds will greatly diminish the effect of an occasional switching spike. If the spikes come thick and fast this will be less effective. A 10 mS spike in a 2 second integration period adds 10:2000 = 1:200 of it's magnitude to a signal or about -50 dB. Better not to be there at all BUT the effect will be small in many cases.
An ideal capacitor's voltage cannot change instantaneously. A good high frequency response capacitor on the relay side of the MOSFET will limit the inductive voltage spike while D1 is thinking about turning on.
This is basically a classic window comparator, with stuff around it to make is actually useful in the particular application.
PIR sensors report changes in IR accross the sensor area. C2 removes the DC bias, and the circuit around IC1D amplifies the result and also does some frequency filtering. This is probably in part to reduce frequencies that aren't relevant and therefore just add noise, and in part to get the response of the overall sensor+filter that is useful for detecting motion.
IC1C and IC1B are the window comparator. R7, R8, R9, and R10 are a divider chain making voltages for the output of IC1D to be compared against. Just from the topology without looking at any numbers, you can see that the threshold for IC1C is higher than that for IC1B. Also see that the input signal into the window comparator (outout of IC1D) is fed in to the two comparators (IC1C and IC1B) at opposite polarity. In the "window" region, which is the voltage range between the - input of IC1C and the + input of IC1B, both amps will be driving low. Below the window region, IC1B will drive high and IC1C low. Above the window, IC1C will drive high and IC1B low.
The two comparator outputs are averaged by R11 and R12, then the result compared to a threshold by IC1A. This threshold is set so that IC1A drives high only when both comparator amps are driving low, meaning the voltage is in the window region.
The digital signal that indicates whether the sensor output is within the window region is capacitively coupled into this HT2812 thing. I didn't look that up, so I don't know what exactly it does, but from the transistor and speaker it is probably intended to produce a beep when motion is detected.
I'm not sure what the point of the switch in series with the KEY input is. When the switch is open, the HT2812 block won't receive the motion signal. If that is the intent, then powering everything down would be the more obvious approach, so there is probably some additional feature it supports. I don't know why you'd want to only sound a beep due to motion when a button is pressed, but that appears to be what what this circuit will do.
Best Answer
No potentiometer with moving parts has any significant high-frequency output. So, just filter the 'received' voltage. The more important issue may be the ground connection, the '0' volt reference level. Are there multiple ground connections in the system? A ground loop will pick up any local magnetic field fluctuations.
Best connection scheme for a three-terminal potentiometer uses shielded three-wire cable, with the shield connecting to the metal shaft and case of the potentiometer, and grounded in ONE place. The three wires connect to the potentiometer terminals, of course.
The wiper contact of a potentiometer can be very reliable and low resistance with careful choice of materials. The least expensive potentiometers (carbon film) are not as good as those with ceramic and conductive-plastic elements.