User manual

Mission: Touch only lightly the LED anode and the input. It will weaken the LED's
light, but it won't fully go off: 4 points.
7 Afterglow after a touch
Behind the seventh door, you will find a brand new component, namely a 100 nF (nanofarad) capacitor. It is a ceramic disk capacitor with the 104 (100,000 pF,
picofarad) label. Integrate it into your circuit. The LED this time is connected to plus and thus the idle mode is reversed. In normal state, the LED is off. However, if
the wires are short-circuited or touched by the finger, the LED switches on. It glows for a long time after that and only slowly goes off. Thus, a nightlight can be
made, which is switched on by simple touch and goes off again by itself.
Info: This circuit does not function with just any op-amp, but is dependent on a
specific feature of this op-amp type. The LM324 contains a bipolar operational
amplifier with PNP input stages with operational voltages down to negative ones
and even lower. The basic current of the input transistors is about 30 nA, i.e. 0.03
µ
A. Current this weak only slowly charges the capacitor at the input. The input
voltage increase rate is about 0.3 V/s. Therefore, it takes a bit longer than 20 s until
the voltage exceeds 7V and the LED goes off.
Mission: touch the inputs so delicately that the LED does not go off yet, but gleams
evenly weakly. You must find yourself how you create the proper resistance by
your fingers and get: 3 points.
8 Light control
Behind the eighth door, there is another LED. It is green and should be placed at the output of the op-amp instead of the red LED, while the red LED receives a new
task: it will be a photodiode and thus an effective light sensor. Please note the installation direction with the cathode at the input of the op-imp. If you expose the
red LED to a bright light of a torch, the green LED lights up.
Info: A photodiode has the same structure as any other diode or light diode. An
insulating barrier layer is formed in the locking direction, which prevents current
flow. However, if light penetrates the barrier layer, some single electrons are
released from their ties and can move freely. The current flows. Since an LED
crystal only has small surface, only little light gets into creating weak photo-
current. In this case, only 30 nA of it suffices to switch on the LED.
Mission: Integrate an additional capacitor as on day 7. Thereby, switching on is
decelerated, and afterglow of the green LED can be observed: 2 points.