Both of the temp sensors have finally arrived, so I could finally spend some time with tinkering the display of the engine and ambient temperature.

Engine temp

I ordered this sensor from Aliexpress, it was around 3 dollars. It's 10mm and has a 1/8 thread. The seller claims that it is a 50K thermistor, that's why it only has 2 wires. I measured a 4.68K resistance at room temperature, which is pretty strange. 

It's basically a thermistor, so it changes its resistance when the temperature changes. The conversion from resistance to temperature requires some data about the sensor, which the seller didn't provide. Without these, it's almost impossible to make it accurate through the whole temperature range, but it's gonna be used mainly the 90 - 110 Celsius range.

I found a random thermistor conversion code for Arduino on the web, using this and with a few experiments with resistor I managed to get it quite accurate in the given range. I made a voltage divider in which the first resistor is a 35K resistor, the second one is the thermistor. The Arduino measures the given voltage and from that and the given resistor, it calculates the resistance of the thermistor, which can later be converted to temperature.    

The sensor is going the measure the temp of the water coolant with this bracket on the picture. A sensor can be screwed into that. 

Ambient temp

The second is also from Aliexpress, but it works differently. 

A sensor is a DS18B20, which is one the of most accurate sensors in my opinion. It needs 5V Volts, and the temp value can be read with a digital pin on the Arduino. According to its datasheet, it has an accuracy of 0,5 Celsius up to 80 Celsius. It doesn't need any calibration or calculation, it's easy to use. The probe has a 1 meter long wire, so it can be put anywhere on the motorcycle.

On the screen, the user can switch between the Engine Temp and the Ambient Temperature instantly with a simple touch.