User manual

Mission: Press the switching contact so that both wires are only lightly touching or stroking each other.
This generates a scratchy sound in the loudspeaker of the kind often heard in the old telephones. The
contact is neither reliably closed, nor fully opened. The LED is flickering. However, much skill is required
to achieve this effect: 4 points.
5 Has the integrated circuit landed well?
Open the fifth door. Behind it, you will find the most important component of this calendar, the fourfold op-amp LM324. This IC (integrated circuit) with 14
connecting pins contains four separate amplifier units each with two inputs and one output. The amplifiers themselves are interchangeable, but both operating
voltage connectors may never be confused, otherwise the IC can be damaged. The plus connector is on pin 4, while the minus connector is on pin 11. However,
normally, the plus pole is on the top, and the minus pole - on the bottom of the experimental board. Therefore, the IC must be used in such a way that the label is
upside down.
Imagine that the fourfold op-amp is a spacecraft, which is going to land on Mars. Everyone is anxiously awaiting the first signs of activities proving that the
spacecraft has landed properly. In this case, light of the red LED shows that everything has worked out well. And the resistor prevents damage if an error occurs.
Unlike on Mars, anything can now be easily rectified and the entire circuit retested.
Info: An op-amp amplifies the voltage difference between its two inputs. If the difference is
significant, the output is either fully on or fully off. In this case, the voltage at the (+) input is
higher than it is at the (–) input, therefore the LED is on. Normally the IC is powered by the
full operating voltage, but this time there is also a protective resistor connected to the plus
lead.
Mission: Modify the circuit so that another of the four potentially used amplifiers will be used
(1 point). Or test the same with each of the other three amplifiers (3 points).
6 Contact sensor
Behind the door number 6, there is a 330 k

resistor (orange, orange, yellow). Make a circuit with an open op-amp input. Two wires with bare ends lead outward.
In normal state, the LED is on. It goes off if the two input wires are connected. It is enough to touch both wires by the finger, as the conductivity of the skin is
sufficient to change the state. If only one wire is touched at the (+) input, the LED can be on or off and sometimes it may flicker. If electric cables are in close
proximity, humming or buzzing may come out of the loudspeaker.
Info: The voltage between the two inputs is amplified about 100,000-fold. However,
in this case the inverting input (–) is directly connected with the output. The negative
feedback thus created reduces the voltage amplification to one (1). At the output is
thus almost always exactly the same voltage as at the non-inverting input (+).
Although the voltage amplification is only 1-fold, the op-amp provides very strong
current amplification. Therefore, the amplifier is sensitive to weak interference
signals.