A very simple presence detector can be made using this electronic scheme. This electronic circuit reacts to the presence of a conductive object within a particular field. Circuit sensitivity can be adjusted with potentiometer P1 for the desired distance. The electronic circuit consists essentially of a monostable oscillator and a tipping stage. Oscillator is built with transistor T1. This is why a flap that oscillator frequency is particularly stable, because the capacity of the transistor is in parallel with relatively large capacities C1 and C2 of the oscillating circuit.
A switch for temperature differences can be done using electronic diagram below. The electronic circuit can measure temperature in two different places and, depending on the difference between them, to connect or disconnect relay.
Both the desired temperature difference and hysteresis can be set completely independently and do not influence each other. In addition, adjustments are almost linear potentiometers so can be easily equipped with an accurate scale. An LED indicates the relay activity.