What does 1n4148 do in these circuits?
1st schema: the 1N4148 is a temperature sensor.
The forward voltage is dependent on temperature.
source: Vishay datasheet @farnell.
The voltage at the R1-D1 node will be will be around 0.6 - 0.7 V, but varies.
The current through the diode will be (15 V - 0.6 V) / 15 K = almost 1 mA (953 µA).
So it will follow the second-lowest trace and have 0.6 V at 40°C.