Elenco Basic Electronic Experiments - Manual (Page 61)

61
EXPERIMENT #44: THE FLIP - FLOP
This circuit is yet another variation of the basic multivibrator configuration. Connect the circuit. One LED will be on, the
other off. Take the loose wire and touch it to the base of the transistor that is on (holes b15 and a27 will do, or you can
touch the resistor leads connected to these points). That transistor turns off and the other turns on. Do this a few more
times until you see that touching the “on” transistor base “flips” the transistors and the LEDs. You might say that the
transistor turning on “flips” and the one turning off “flops”. Notice that touching the “off” transistor base has no effect.
This circuit is called formally known as the bistable switch, but is nicknamed the “flip-flop” due to the way it operates. The
name flip-flop may seem silly to you at first, but variations of this circuit form one of the basic building blocks for digital
computers. This circuit can be thought of as a memory because it only changes states when you tell it to, it “remembers”
what you told it to do even though you removed the loose wire. By combining several of these circuits you can remember
a letter or number. By combining thousands of these circuits a computer can remember a small book. A typical computer
has many thousands of flip-flops, all in integrated circuit form. The operation of this circuit is simple. If NPN-left is on then
it will have a low collector voltage. Since this collector voltage also connects to NPN-right’s base, NPN-right will be off. But
if you ground NPN-left’s base then it will turn off and its collector voltage rises, turning on NPN-right. NPN-right will stay
on until you ground its base.
LOOSE WIRE
+9V
3.3k
Ω
1k
Ω
100k Ω
33k Ω