nents present within
the circuit operate as switches being alternately cutoff or
driven into saturation. These circuits have now largely
been replaced by timer ICs like the 555. (See Kit 111.)
However, a basic understanding of them is still essential
since they are still used in many circuits.
This kit builds each of these three circuits and allows you
to experiment with them. To understand how these circuits
work will also make sure you have an understanding of
resistors, capacitors, RC characteristics, the transistor as a
switch and the light emitting diode (LED).
ASSEMBLY INSTRUCTIONS
Components may be added to the PCB in any order. It is
usually easier to add the smallest height components, the
resistors, first. All the small-signal transistors are the same.
Solder a 1/2" (10mm) length of wire which has been cut
from the leads of the other components into the three holes
marked Trigger, Set and Reset. Add two flying leads to the
positive rail and to the negative rail using any spare
insulated wire you have in your junk box. You will use
these flying leads to touch the trigger, set & reset points to
see what effect they have on the operation of the MV's.
CIRCUIT DESCRIPTION
Most basic electronic text books give a review of these
three MV's. We suggest you use a text book as well as our
explanations below in order to get a good understanding of
these basic topics.
When the 9V battery is connected, the astable MV should
flash from one LED to the other. A diode on the input
protects the MVs if the battery is connected wrongly. One
LED should be on for about twice the time of the other.
The LED in the monostable MV should remain off. In the
RS flip flop one LED should turn on and stay on. Play with
touching the flying wires to the trigger, set & reset points.
Try to follow what happens on the circuit diagram when
you touch a lead. What you see is all to do with transistors
acting as switches and capacitorscharging and discharging
with a time constant determined by an R and a C in the
charge path. You cannot do any harm to the components by
playing with the flying wires. If you have access to a CRO
look at the changes of the base/emitter voltages of the
transistors as you touch trigger, set & reset.
1. The Flip Flop.
Computer memory elements (the group of circuit
components in a memory IC which stores each 'bit' - binary
digit) use the flip-flop principal. Play with the flying wires
onto the set and reset wires. You can very quickly see what
this circuit does; it remembers information about which
was the last LED to be make to be turned on or off. Of
course, you have to define a convention: which flying wire
you are using, which pin is called what, etc.
When the power is connected to the circuit one or other of
the two transistors will turn on. Both transistors will try to
turn on as the base of each tries to go high. But due to
slight differences in component values one will be quicker
than the other. Suppose it is T5. This means that T5
collector voltage is low (below .65V), which means that
the base of T6 is also low (since the two are connected)
and T6 is off. Now when the set lead is touched by the
positive rail, T6 is turned on because its base potential is
raised over 0.6V. So T6 turns on and its collector potential
drops which drops the base potential of T5 to below 0.65V
and so T5 turns off. The circuit has flipped into its other
state. Touch the reset with the positive lead and the circuit
flops back to T5 turned on again.
We called one LED the set, and the other reset but these
names are quite arbitrary. The flying negative lead also
causes the LED's to turn on and off but in the opposite way
to the sequence caused by the positive flying lead. Study
what is happening with the schematic in front of you.
2. The Monostable Multivibrator.
Now we introduce an RC network into the flip flop circuit
just described. An electrolytic capacitor replaces one of the
base bias resistors of the flip flop circuit. And the biasing
which was supplied by this resistor is provided by a 56K
resistor to the positive rail. When the power is turned on
the circuit will settle into a stable state in which T4 is on
and T3 is off. Use a multimeter to measure the base/emitter
and collector/emitter voltages of T3 & T4 to show this.
The capacitor will have about 6V across it. We have put on CK1100 - OSCILLATOR BUILDING BLOCKS
the schematic above the voltages we measured. It is the
capacitor in the circuit which determines that T3 will be
off, and T4 will be on in its stable state.
A high applied to the trigger point turns T3 on. Then after
a few seconds T3 turns off and returns to the stable state.
What has happened is that as soon as the trigger goes high,
T3 is turned on and the positive end of the capacitor is
taken to zero. This immediately drags the negative end of
the capacitor to below zero potential. This turns T4 off
since the negative of the capacitor is connected to the base
of T4. The LED turns on because T3 is on. But then after
the pulse is removed C3 starts to charge up with a time
constant determined by C3 and R1. Then when the voltage
on the negative end of C3 reaches 0.65V, T4 starts to turn
on, T3 turns off and so does the LED. The state is then
stable again.
Play with this over & over until you understand what is
happening. A low pulse delivered to the trigger point when
the LED is off does nothing because it is already almost at
ground.
3. The Astable or Free Running
Multivibrator.
In this MV we replace the second base bias resistor by an
RC network. You can think of it as two monostable MV's
connected together; the output of one feeds the input of the
other First one LED is turned on, then the other. The
output is a square wave. Its mark/space ratio and its
frequency is determined by the values of the R & C
components. The time that the collector of T2 is low (& T1
high) is determined by the time constant R2 & C2.
Similarly, the time the collector of T1 is low (& T2 high) is
determined by the time constant of R1 & C1. We made R1
about twice the value of R2 to highlight this. The time
constant is: t=0.693 RC. Work them out for yourself to
check what you observe.
WHAT TO DO IF IT DOES NOT WORK
Poor soldering is the most likely reason that the circuit
does not work. Check all solder joints carefully under a
good light. Next check that all components are in their
correct position on the PCB. Thirdly, follow the track with
a voltmeter to check the potential differences at various
parts of the circuit particularly across the base, collector
and emitter of the two transistors. Is the battery OK? Are
the LEDs in the correct way?
WHAT TO LEARN FROM THIS KIT
It is quite amazing when you think about it - you apply a
constant voltage to a simple circuit consisting of 2
transistors, 2 capacitors and 4 resistors and what is
produced is an oscillation. Just alter the component values
of the astable MV and you have a square wave generator
for whatever frequency and mark/space ratio you wish. Try
changing some of the resistor and capacitor values. Try to
work out the RC values and correlate to the times the
LED's stay illuminated.
See our website at
http://
www.ElectronicKits.com
COMPONENTS
Resistors 5%, 1/4W:
1K brown black red
5
2K2 red red red
1
22K red red orange
2
100K brown black yellow
2
56K green blue orange
1
47K yellow violet orange
1
47uF electrolytic capacitor
3
BC547 small signal transistor
6
5mm bright red LED
5
1N4004
1
Kit 9 printed circuit board
1
9V battery snap
1