User manual

21
Input
Output
0
1
1
0
3 Contact Switch
This experiment uses a gate as inverter with open input. The input
receives a protective impedance of 100 kΩ and may be touched with
the finger. If they are strongly charged electrically, the protective
resistance limits the discharge current.
The output condition of this circuit cannot be pre-determined, since the
input has an extremely high impedance and may carry accidental
charge. If the input voltage is clearly above half the operating voltage
(4.5 V), the condition is deemed 1, clearly below as 0. In fact, there also
is a medium input voltage where the output is also in the mid-range.
The digital circuit then works similar to an analogue amplifier. This
conditions should be avoided in emergency because the IC then
requires much more current. Undetermined input conditions also may
impair the function of a digital circuit. Open inputs must be avoided.
This experiment does, however, convey an impression of how open
inputs may behave.
Often, light contact with the finger is enough to change the condition.
Usually, your body has a certain alternating voltage because of low
capacitive coupling with the surrounding mains lines. At the output, you
will see a quick 50 Hz flicker of the LED. Releasing the input causes
the last condition to be retained for a while.