User manual
When using an electrolytic capacitor, the installation direction must be observed. The negative terminal
is marked with a white stripe. If one applies a current to the electrolytic capacitor the wrong way round
for a long time, it can be destroyed and in the worst case even burst. Here the positive terminal is
placed at Q12, where the voltage is alternately +9V and 0V. Two LEDs must be connected in the back
direction so that the electrolytic capacitor can alternately charge and discharge.
23 Four-way flashing light
Behind the 23rd door you will find a white LED. Four LEDs should now flash in sequence such that two
LEDs are never on at the same time. Despite that, only two counter outputs are used for this test. The
outputs Q12 and Q13 generate very slow changes. So that it does not take too long, the oscillator is
again set to a higher clock frequency with the smaller 10 nF capacitor.
The circuit forms a 1-of-4 decoder, which decodes single states from four possible binary numbers at
two outputs. Normally, one needs additional logical circuits for this purpose, but none are available
here. The circuit functions only with a trick and relies on the fact that the different LEDs operate at
different voltages. The red LEDs light up with less than 1.8 V; by contrast, the green and the white LED
require significantly more than 2V. When Q13 is switched on, the 10-kΩ series resistor supplies power
for the green LED. However, if Q12 is in the zero state at the same time, the red LED is practically
parallel to the green LED and draws current away from it because of the lower LED voltage. Q12 thus
determines whether the green or the lower red LED lights up. In the one state of Q13, by contrast, either
the white or the upper red LED lights.
24 Firelight and falling stars
Behind the last door you will find another 1kΩ resistor (brown, black, red). It is used for a festive light
that can be used to decorate the Christmas tree at the end of the experiments. Two red and two yellow
LEDs stand for a wood fire, which generates a relatively constant light and flickers only slightly.
Sometimes, however, a falling star lights up the night for a brief moment, represented by a green or a
white LED.
The base brightness of the red and yellow LEDs is predetermined by two 1kΩ resistors. Larger resistors
with 4.7kΩ and 10kΩ switch smaller currents from various counter outputs to the LEDs and provide
for a slight and seemingly irregular flickering. It has a calming effect and is also nice to look at for a
long time. The infrequently occurring light flashes are generated with output Q10 and a 100µF
electrolytic capacitor. It is easy to make changes. Experiment with other counter outputs and different