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RTD Pt-100 Four Point Alarm - del90001

Here 4052 is used as an analog multiplexer, U3A TL062 opamp is wired as a 1mA Constant Current Source. It pumps 1mA into  U2  4052  pin 13  X . Depending on BCD code on inputs A, B of 4052  the current is routed to any one of the four RTD 100E, whose one end is connected to X0-X3. The current then flows to GND thru the Platinum 100E resistance. The Y0-Y3 monitors the mV developed on RTD in tandem with X0-X3  positions.  Like a  ganged  rotary switch. The Output mV at Y is the mV of Active Channel as selected by the BCD of 4029.

See another way 4052 is used to route analog signals to DPM - Analog mV Switch for Digital Meters

The BCD is provided by 4029 counter which is clocked by a 555. U5A cancels out the 100E mV (1mA * 100E) of a cold RTD and Amplifies the differential mV. This output of U5A is in proportion with measured temperature. U5B compares the mV that was obtained with a  preset mV of  POT1, which is a user setting called setpoint. The difference is amplified by U5B which is saturated by U3B comparator which adds a little hysteresis too. R7-C2 further dampen and slow the response. This finally drives Q2 to provide a Logic Signal indicating if temperature is below or above setpoint.

To view the circuit below, Click the Link of PNG or PDF and view the Circuit, PNG can be Scrolled with  Drag-Drop Left-Click-Mouse and you could use the PDF to take a paper print for reference only.

PNG-Schematic     PDF-Circuit
RTD Pt-100 Four Point Alarm - del90001
Below is a Preview Image Linked to the Larger Circuit
RTD Pt-100 Four Point Alarm - del90001

It is important to understand process control theory and the material technologies that go into these sensors too.

Here is a instrument i used to make in a similar need - Multizone Process Scanner

Four points in a Closed Loop Temperature Control System is Monitored and a Alarm set to go off when the temperature goes beyond the set limit.