Come to think about it,
it’s a bit strange that range hoods in our kitchens don’t switch on and
off automatically. After all, a simple temperature sensor under the hood
can detect whether a burner is on. The circuit described here goes a
step further and compares the temperature under the hood with the
temperature just outside the hood. At a certain (adjustable) temperature
difference, the hood will be switched on, possibly along with the lamp
under the hood.
After the burners are shut off, the hood fan and lamp will switch
off again by themselves. The advantage of using two sensors is that the
hood will have the same switching characteristics in the summer as in
the winter. When building the circuit, it’s important to ensure that IC1
is located beneath the hood in the middle and IC2 is located next to
the hood or above it. If the temperature under the hood is higher than
the temperature outside it, the open-collector output of IC3 will be
pulled up to the supply voltage by R6.
The combination of IC3 and R7–R10 forms a Schmitt trigger, which we
need because the output of IC3a does not change immediately from 0V to
the supply voltage (or vice versa) in the transition region. The output
of the IC3b will thus be at the supply voltage, which will switch on T1
via R10. That causes the relay to engage and switch on the fan and lamp
of the range hood. P1 can be used to adjust the output voltage of IC1
over a range of approximately 0.1 V, which corresponds to around 10 °C.
It’s a good idea to use a supply voltage that matches the operating
voltage of the relay. It’s also convenient to fit the relay in a small
box with an electrical outlet and plug so it can be easily and safely
inserted between the plug and outlet of the range hood. The circuit
works best with a gas cooker, because the heat rises immediately after a
burner is lit. With a ceramic or inductive cooking top, it takes a bit
longer for the relay to be actuated.
Author: Heino Peters
Copyright: Elektor Electronics