Conrad Course material 3964 14 years and over 3964 Manual De Usuario
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3964
11
10.
Step: On and off
Now it goes digital: while more or less current flows in an analogue circuit, a digital circuit is either completely on or
completely off. The states On and Off are also designated as One and Zero. The circuit depicted here can be seen as
a basic module of computer technology.
completely off. The states On and Off are also designated as One and Zero. The circuit depicted here can be seen as
a basic module of computer technology.
A circuit with two stable states is called a trigger circuit or also a flip-flop circuit. An LED is either on or off, but never
“half-on.” Fig. 28 shows the typical circuit of a simple flip-flop. In principle, the circuit consists of two coupled amplifier
stages with closed feedback.
Fig. 28: A bistable flip-flop (Schaltung9.jpg)
The circuit flips into one of two possible states. If the right transistor conducts, the left is blocked and vice versa. The
conductive transistor has a low collector voltage and thus switches the base current of the other transistor off. For that
reason, once engaged a switching status remains stable until it is changed by one of the key switches.
Fig. 29: The flip-flop (F_Flipflop1.jpg)
Switch the operating voltage on. You will discover that one of the two LEDs shines. But it cannot be predicted which
side is switched on. Usually the unequal current gain of the transistors decides which side the circuit flips to.
Now use a jumper to block one of the two transistors. The engaged state remains after removal of the jumper. The two
states are also designated as Set (S) and Reset (R), thus the name RS flip-flop.
11.
Step: Firing and erasing
A bistable circuit can also be constructed with an NPN and a PNP transistor. The collector current of one transistor
becomes at the same time the base current of the other transistor. Thus either both transistors are blocked or both are
conductive. After being switched on, the circuit is at first in the blocking state. Brief activation of the switch at S1
switches over into the conductive state. This state is now stored and remains as long as the supply voltage is present.
Only by switching the operating voltage off do the transistors return to the blocked state.
becomes at the same time the base current of the other transistor. Thus either both transistors are blocked or both are
conductive. After being switched on, the circuit is at first in the blocking state. Brief activation of the switch at S1
switches over into the conductive state. This state is now stored and remains as long as the supply voltage is present.
Only by switching the operating voltage off do the transistors return to the blocked state.
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