User manual

40
20 Temperature sensor
We will modify our last circuit just a little bit, but it will
behave totally different and will respond to different temper-
atures. The 1 kΩ resistor (brown, black, red) delivers base
current to the left transistor. The LED is neither fully on nor
fully off, but glows steadily and very dimly.
Now, if you touch the right transistor with two fingers, it
warms up a bit. The LED will glow a bit brighter. Touch the
left transistor and you will see that the opposite is happen-
ing; the LED is only dimly lit. The difference in brightness is
not huge and depends on the actual temperature differences
achieved. You will get better results by touching the left and
the right transistor alternately at intervals of half a minute.
For greater temperature differences, place some ice on a
spoon, hold another spoon to the heater and then touch the
transistors with the spoons.