Electrical – Cascade Wheatstone bridge sensor to increase sensitivity

Another reason (apart from being a symmetrical signal) for not getting a reading is that the voltage across the sensing resistor is too low. A 60W bulb at 230V gives 250mA. Across a 0.1\$\Omega\$ resistor this results in just 25mV. You'll need an amplifier, which can be as simple as a basic opamp circuit.

Another solution would be to use a Hall effect sensor. Not only can the Hall effect sensors do with mV/A losses, but sensors like Allegro ACS711 can handle high currents and have amplification built-in.

For the ACS711 sensitivity is 110mV/A. For currents like several amperes you can supply this signal directly to the ADC, if you want to measure also lower currents you need an external amplifier.

PT100 can be used as 3-wire or 4-wire based on the RTD you have. There are different options available and which one you need to use depends on the precision required.

As far as Wheatstone bridge is concerned, the idea lies between balanced and unbalanced bridge. Check wiki link for more details on bridge.

I don't know how to connect the Pt100 wires in order to get my Wheatstone Bridge.

The variable resistance part R2 shown below is your PT100

Image source: Wikipedia

You should use R1, R3 and Rx and 1% 100 ohm resistors for good results. The voltage change is measured along the terminals D and B. When the bridge is balanced you get almost zero voltage between terminals B and D. And when there is variation in the temperature, the resistance value of PT100 changes which in turn makes the bridge unbalanced and you get few mV between terminals D and B.

The important part here is the excitation i.e. the voltage applied at terminal A of the Wheatstone Bridge, it must be constant current source to give good and precise results, you can use darlington pair or opamps to do constant current source design. Refer this thread from TI which talks about constant current source and measurement techniques.

For a good and robust design and a signal conditioning circuit must come after the Wheatstone Bridge and ADC which converts you analog temperature to digital useful data. See here application note from Microchip which talks about signal conditioning for RTDs.

And even there are integrated options available from different Silicon manufactures for temperature sensing using RTD or thermocouple like LMP90080 and other are available from Analog Devices.

And at last please don't build sensing circuits on breadboard as it will add lead inductance to circuit and will surely affect performance.

Hope this helps you.