Elenco 130-in-1 Electronics Playground - Manual (Page 42)

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What is a flip-flop? It is a kind of circuit that changes
back and forth between two states (on and off) at
specific intervals. It flips into one state and flops into
another and so on.
Two transistors, two capacitors and four resistors are
used by the flip-flop to turn on and off the LED. Each
of the transistors are always in the opposing state of
each other; when transistor Q1 is on, transistor Q2
is off; when Q2 is on then Q1 is off. The change from
on to off or off to on, happens quickly (in
microseconds). Note the effect on the flashing rate
of the LED when adjusting the control.
To see how this circuit works, look at the schematic.
Remember when voltage is applied to the base of a
transistor, it turns on. On the negative side of the
batteries you have the two PNP transistor connected
through resistors. You may think that both transistors
would always be on however, there are two
capacitors connected to the bases that aid the cause
of the flip-flop action.
In order to explain the circuit, you should assume that
transistor Q1 is off. The 100
μF capacitor will be
charging and discharging through its base, so we
can say that Q2 is on. Transistor Q2 is kept on after
the 100
μF capacitor has discharged due to the 47kΩ
resistor and the control. Now the 10
μF capacitor has
received a charge and is discharging through the
4.7k
Ω resistor, the battery and the Q2. (Remember
that current can flow through the collector to the
emitter when transistor Q2 when it is on.) As long as
the charge on the 10
μF is high enough the Q1
transistor remains off.
Transistor Q1 turns on when the charge drops to a
specific point, the negative voltage from the 47k
Ω
resistor. Once Q1 turns on, and 100
μF quickly starts
charging and transistor Q2 turns off. With the Q2 off,
its collector voltage rises toward the 9V of the battery
supply and thus the LED turns off. The Q1 turns on
fully through the fast charging of the 10
μF. This flip
occurs very fast.
The circuit will eventually flop back to the original
state to repeat the above action due to the 100
μF
discharging through the Q2 transformer.
Look back at the previous projects and try to locate
where you have used this sort of circuit.
Notes:
EXPERIMENT #27: “FLIP-FLOP” TRANSISTOR CIRCUIT
Schematic
Wiring Sequence:
o 21-23-41-84
o 75-81-87-25-27-124
o 28-79-82
o 40-115-80
o 45-42-119
o 43-88-83
o 44-116-76
o 121-122
This circuit is a delayed timer that uses an
operational amplifier and the RC time constant. RC
stands for resistor/capacitor. A circuit that delays an
operation is a time constant.
Through resistors RA and RB the negative (–)
terminal of the operational amplifier receives a
voltage of about 4.5V. This is the comparator’s
reference voltage. Connected to capacitor C1 is the
positive (+) terminal of the comparator. This capacitor
receives its charge through the series resistance of
R2 and the control. The charging speed is slower
when the resistance is large, and faster when the
resistance is small. This charging speed set the delay
time for the timer circuit.
Now turn the control fully clockwise to position 10.
Set the switch to position A to turn on the power. LED
1 lights first; LED 2 lights about 5 to 7 seconds later.
This 5 to 7 second time difference is the delay time
that is set by the CR time constant.
Now, turn off the power, set the control fully counter-
clockwise to position 1, and see what happens when
you turn on the power again. LED 2 lights later than
LED 1 again, but how many seconds later?
Notes:
EXPERIMENT #99: RC DELAY TIMER
Schematic
Wiring Sequence:
o 81-31-63-27-131
o 28-87
o 83-33-36-70-116-135-121
o 34-67
o 68-82-84
o 88-69-115-136
o 119-124
o 122-132