PEAC: Practical Electronics Analogue Computer
A low cost. general purpose analogue computer of modem
design,
, intended for the amateur or student.
A usef~l toOl which is capable of solving.complicated problems
at high speed.
,
Can be used as a model to simulate mechanical systems and
electronic networks.
.
Extends enormously the scope of the amateur experimenter.
This series of articles will eX'P'aln In detail the desl,n. construction.
and o'P8rGtion of PEAC.
M
OST of the publicity afforded to computers favours
digital equipment. However, digital methods
tend to be disproportionately expensive for sptaU
installations. On the other hand, although analogue
equipment is ideally suited to limited, low-cost applications, it was not until the silicon transistor had become
firmly established, and enough practical published
information was available, that a start could be made
on designing analogue computing equipment to yield
a reasonable standard of performance in the lowest
possible price range.
A WORTHWHILE PROJECT.
No doubt mallY readers will think that construction
of a true computer could involve them in a great deal
of time, money, and effort. They might also believe
that an average understanding of mathematics would
not be sufficient to equip them to operate a computer
effectively. However, the amount of time and money
spent building PEAC need be no more .than is consumed by a home constructed hi fi outfit of normal
proportions and performance, and the computer will
solve even simple problems a great deal faster than the
human mind or slide rule, once if has been programmed
to do so;
In fact, a general purpose computer can find application in almost every sphere of technical activity, and is
particularly useful in the electronic workshop, to the
point of becoming indispensable after a short period
of use.
UNIT CONSTRUCTION
PEAC is arranged in the form of units, and is
organised in such a way that reasonably advanced
computations may commetice upon completion of the
first unit, UNIT "A". The cost of building UNIT."A",
based upon typical retail prices at the time of writing,
will not be much above £25, and yet it will solve
algebraic polynomial equations, simultaneous linear
equations, simple differential equations, and can also
be used to simulate the behaviour of many elementary
mechanisms and electronic networks.
UNIT "A" is designed primarily to satisfy a minimum user requirement, for experimental and educational work, but it also serves as a convenient starting
point for the addition of further units to- expand the
computer to almost any desired degree of capability
and complexity. The additional facilit. pu;vided by
the add-on UNITS "B", "C", and "D" are described
in the specification. See also the block diagram, Fig. 1. 1.
A comprehensive PEAC installation, equipped with
a function generator and multiplier, and with fu))
integrating facilities for the fast solution of a range of
differential equations, might finally cost around £60:
not a lot to pay for an item of workshop equipment
which can solve electronic formulae in IOms,' and
which may also be employed as a variable waveform
generator, 18 input high quality audio mixer, variable
characteristic high Q audio filter, large inductante or
capacitance simulatQr, d.c. or a.c. millivoltmeter, and
many other things besides. •
COMPARISON BETWEEN AtJlALOGUE
AND. DIGITAL COMPUTERS
Although popularly regarded as an inaccurate
machine of limited usefulness, the analogue computer
is to be found in the Polaris missile, spacecraft. aircraft,
large scale chemical processes, and many automated
production lines, quite apart from basic research work,
where flexibility and ease or working are often 'consid~red to be more important than extreme accuracy.
The analogue computer is, in most cases, very much
faster than its digital counterpart, and can offer far
more in the way of general facilities for a given outlay.
ANALOGUE METHODS '
The time taken to SQlve a problem on an analOgUe
computer is independent of problem le gth. All
The statement that an aeroplane is a machine for
circuits oPerate in parallel, simultaneously. ~ ty~ical
solving sets oC'Clifferential equations is not very far
solution might be arrived at in 2Oms? nd this solution
removed fronT' the truth. If the aeroplane did not
'solve its equations, correCtly it would not be able to
can then be repeated at the rate, say, of 2~ solutions
per second. In human terms the solution is virtuallf
fly at all. Almost all relationships or events ~ be
described mathematically, or in turn be represented by
immediate and continuo,!s, therefore: a'ny adJlllStments
'tnade to problem parameters (tenm! of an ~uation)
an analogy. A model aeroplane in a wind tunnel
solves, by analogy, roughly the same equations which
while the computer is WOl'king will be immediately
govern the behaviour of the real aeroplane, although
reflected in the solution readout. iThis rapid. response
/lllows the operator to quickly gain an insighj into the
in much simpler and less expensive fashion.
t iWQrKings and structure of a roblem.
An analogy of a physical or mathematical process
could be achieved by a system of gears, pulleys, and
In contrast, digital computers perfonn many mathematical operations in a pre-determined and cOmparalevers; or by the controlled flow of gases or liquids.
But in the last couple of decades electronic methods
tively lengthy sequence, which bears little obvious
'relationship to the structure of the problem, but _hey
of simulation and equation solving have become almost
universal, because of the accuracy, ai.'ailability, and
do offer the very high degree of accuracy essential for
calculations involving money or very precise data.
\ adaptability of standard electronic components.
lbe main purpose 'of the analogue computer is to
The computer of the future will undoubtedly combine the best of both worlds with analogue and digital
allow a model to be set up quickly a!)d easily, to
equipment in hybrid fonn.
simulate the behaviour ot a full scale s}}tem, and at
The Practical ElectroniC' Malo,.,.
TIle
basIc equ f:lmell't I. UNIT <tAU.
Computln, facilities mjlf be e.xtencle#
by the addition of further units: "S",
"C" and "D". In accordance witH.
....r rfHJulremenu.
Thl. Pltotoff'flfjh .hows UNIT "A"
'ItGnCfIn, on UNIT ·'S"
Compu&er Is of flexible des',n.
STABILISED
POWER
SUPPLY
205V
-245V
50H.-
Fig. 1.1 Block
diagram of
PEAC
INTEGRATOR MODE SWITCHING
51
SPECIFICATION
r!----r:----r!
:J 01
52 ....
S3:l
fl!!!!
L ____ -'_____ J
o
NUUI
+10
0
I N P--'---1
W UIN:,PUT I
-g>
0
VOLTS
~
~ .. --v
READOUT AND
NULL METER
0----1
c---..., 1"....
•• ~ ••
I
Ir"U
It
I
MASTER
POTENTIOMETER
UNIT "A"
POWER SUPPLY
Input 205V-245V 50Hz. Output ± 12·5V d.c.
Voltage
regulation better than 1% for loads of 0-200mA. and 2%
for 0-300mA. Total ripple 2mV. Complete short circuit
protection.
o---~
0----1
I
I
............
'>----\)
0----1 I ,,/
'>---;.-y/ ,
OPTIONAL UNREGULATED
POWER SUPPLY FOR
COMPUTER RELAYS
MULTI-PURPOSE OPERATIONAL
AMPLIFIER AVAILABLE WHEN
MULTIPLIER IS NOT IN USE
UNIT "B"
MASTER POTENTIOMETER
' 25 kilohm 300°' wirewound; 25 watt. Two-voltage measuring
ranges ±O-IV and ±O-IOV. Scale length 14in. Accuracy
better than ±0·5% of full scale.
READOUT METER
Centre zero 1000000-IOOJLA. calibrated O-O·3V. O-IV.
0-3V. and O-IOV. Accuracy better' than ±2% of full
_
'
scale.
AMPLIFIERS
Three multi-purpose operational amplifiers. each with five
silicon transistors. Open loop voltage gain greater than
5.000. Output ± 10V at 5mA. Current demand (average)
4OmA. ' Equivalent input drift under normal room conditions better than ±0·5mV per hour. Unity gain frequency
response within 1% for O-IOkHz. and 5% for 0-25kHz.
Typical noise and hum at output 3mV.
'INTEGRATOR SWITCHES
Provision for three or more Integrating amplifiers. Compute
times ranging from 10ms-Is. Single shot,or repetitive
mode with "hold" facility. Current demand around 65mA.
VOLTAGE SOURCE
Five independent outputs. each continuously variable in
three steps giving ±O-O·IV. ±O-IV. and ±O-IOV. Dial
setting accuracy better than 3% of full scale between dial
divisions 1-10. Total current demand SOmA.
UNIT "e"
FUNCTION GENERATOR
Diode function generator for parabolic and other functions.
Typical accuracy 2%. Frequency response to several kHz.
COEFFICIENT POTENTIOMETERS
Four 10 kilohm 270° potentiometers. Dial setting accuracy
better than 5% of full scale between dial divisions 1-10.
UNIT" 0"
MULTIPLIER
Four quadrant multiplication of two or more variable voltage
inputs. Also incorporates an operational amplifier which
may be used on its own to supplement the amplifiers of UNiT
"A". Frequency response generally better than 0-50Hz.
Approximate current demand around 75mA.
SUMMING' NETWORKS
Three five-input summing networks provided with voltage
check sockets. and plug-in computing components.
38
the same time solve the equation which represents the
System. Sometimes the computer will be used just for ·
solving equations or, alternatively, as a working model
only, depending on the nature of the problem. The
advantage of the electronic computer is that it will do
each, or both at the same time, with ease.
The computer is set up, or "programmed", for a
particular task by inserting computing components,
i.e. resistors and capacitor~. into sockets on the front
panel. This procedure will be described in full detail .
in due course.
computing element art arranged to be very close to
earth potential in the absence of an inp\1t voltage, it is
feasible to take the earth rail for · granted and regard
all circuits as having only two terminals, instead of
the usual four .
. Although the symbol and function of cac::h of the
elements of Fig. 1.2 are common to all analogue computers, the actual circuit design and' choice of components will naturally vary from one computer to
another. For example, the time-division multiplier of
Fig. 1.2e is only one among many possible circuit
configurations for achieving multiplication of independent variables. Alternative approaches include the
Hall effect, the servo, logarithmic, and quarter square
mUltipliers.
ANALOGUE COMPUTER CIRCUITS
In the electronic analogue computer, the analogy is
created fundamentally by manipulating sets of d.c.
voltages. There is nothing to prevent
a.c. voltages being used-in fact they
often are-except that a.c. measurement techniques are generally less
INPUT~
accurate at low levels than d.c.
_OUTPUT
However, when simulating dynamic
T
.
processes . with d.c. voltages, the
computer will be handling a voltage
POTENTIOMETER
which varies·with time. In this context
it is more appropriate to regard a
waveform, even if it is a pure
OUTPUT
RZ
sinewave, as a d.c. voltage varying
-EZ,o-~,.,......-+-t
with time according to a formula
E3o-R3JWIr--'
Rf=Rl=RZ"'R3
which describes the nature of the
VIRTUAL EARTH
waveform.
The main computing element is the
SUM~ING AMPLIFIER
"operational amplifier". As far as
Eic o--"'NIr--......~"""'--,
operational amplifiers are concerned,
the decibel is much too coarse a
unit to use for the measurement of
frequency response, so amplitude
linearity is usually expressed as a
OUTPUT
percentage variation . over a fairly
restricted range of audio frequencies.
In some cases, for example, an operational amplifier and its attendant
SUMMING INTEGRATOR
circuits will be expected to respond to
inputs from d.c. to 5kHz with an
accuracy ora fraction of 1 per cent,
and up to 10kHz at no worse than 1
per cent.
,
r
t
COMPUTING ELEMENTS
The majority of problems can be
solved by the varied application of
only five analogue elements, but the
size of the problem to be handled will
in turn depend on the qmmtity of
elements available, and hence on the
overall size of the computer.
The five computing elements are
.s.h own in Fig. 1.2, together with their
. conventional symbols and generalised
Junctions. The symbols are used as
a kind of shorthand when drawing
up a computer programme.
. The first thing to note about the
simplified circuit diagrams of Fig. 1.2
is that the <:;ommon earth return is
often completely ignored. Computer
supply voltages are usually positive
and negative in relation to an
earthed centre tap. Since the input
and output terminals of each
xZ
OR iO=f(X)
I
I
I
I
DIODE FUNCTION GENERATOR
xo----t
I---oxr
r o - - -........
TIME DIVISION MULTIPLIER
Fir. '.2 Ancdorue computlnr element.
39
It is proposed to examine computing elements in
greater detail when they are dealt with individually at .
a later stage, but in the meantime a brief survey
will suffice.
COMPUTING POTENTIOMEl'ER
The potentiometer of Fig. 1.2a may be used ~or
multiplying a variable voltage (often called a machIne
variable) by a constant of less than unity.
Example: potentiometer input l' 5 volts. Slider
set exactly half way along resistance track, corresponding to a constant of 0'5. Output voltage Eo therefore
equals 1·5 x 0'5, or 0·75. As set, the potentiometer
will multiply any input voltage by 0·5.
When incorporated in the feedback loop of an
operational amplifier, the potentiometer will divide a
machine variable by a constant smaller than 1. The
fact that potentiometer constants are less than unity
is no real disadvantage. It is a simple matter to
either increase input voltages by a factor o~ ten, or
increase the gain of an operational amplifier ~en
times, to bring the potentiometer constant abov~ ~ruty.
Like the slide-rule, it is simply a matter of decidIng In
advance where the decimal point should be.
SUMMING AMPLIFIER
The summing amplifier of Fig. 1·2b uses a high gain
operational amplifier with several inputs . to achieve
addition and subtraction of machine variables. When
the operational amplifier has a voltage gain equal to.
several thousand, input voltages will be accurately
summed together, without unwanted interaction. The
summing junction SJ is at "virtual earth", a :-v~y of
saying that SJ will never be more than a few milhvolts
above or below earth potential, and is also, to all
intents and purposes, shunted by a resistance of only
a few ohms. Compared with input resistors RI-R3,
the SJ shunt resistance is very low indeed, a condition
necessary for accurate summing of voltages.
.
A definite relationship exists between resistors
RI-R3, and feedback resistor Rr, and if these resistors
are arranged to plug into the amplifier, many problem
conditions can be met by "ringing the changes" on
preferred values of fixed resistor, including multiplication by a constant as well as addition.
If a voltage El is applied via resistor RI (in Fig. 1.2~)
ot the summing junction SJ, the output voltage Eo will
This "hotograph shows UNIT "A" being used to simulate
a tuned Le circuit, consisting of an Inductance of 5H In
series with a ca"acltance of 5/LF. The oscilloscope Is displaying "hase shift within the simulated circuit at the
resonant frequency of 31Hz, and the trace also giYes an
Indication of the damping factor or "Q" of the circuit
be - El ;:. The operational amplifier is designed to
invert an input voltage, hence the minus Sign in front
of this expression. The ratio between input resistor
and Rr holds good for each input.
Example: apply three input v~ltag~s ~l = ?,
E2 = - 3'5, and Ea = 2 to the summmg Junct1~m via
RI = 10 kilohm, R2 = 2 kilohm and Ra = 100 kilohm.
Let the feedback resistor Rr = 10 kilohm. The
relationship between voltages and resistances will be
Eo =
-
(El ~ -
£2 ;: + E3 ;:) Substituting valu~s
Eo =' (510 - 3·5!.Q + 2JQ.) = (5 - 3·5 x 5) + 0·2),
10
2
100
therefore Eo = 12·3.
.
In the above example, the summing amplifier has
not only summed negative and positive inputs, but has
also multiplied E2 by 5, and Ea by a constant of 0'1, .
merely by selection of appropriate values of input .
resistor.
SUMMING INTEGRATOR
The summing integrator is used for th~ detailed
investigation of time dependent variables, and for the
solution of problems involving calculus.
The integrator of Fig. 1.2c is based on the inverting
operational amplifier, with capacitor er acting as the
feedback component. The output from a single integrator, in response to a steady voltage input, is a linear
ramp voltage which increases with time at a rate
dependent on choice of input resistor,. feedba~k
capacitor, and input voltage. Once agaIn, precise
relationships must exist between computing components and voltage, but now time is introduced as an
additional analogue variable.
The action of electronic integration is best explained
by a working example, and reference should be made
to the diagram of Fig. I.3a.
Example: a fairly sluggish motor car accelerates
from rest at a steady rate of 20ft/second/second.
Examine the progress of the motor car during the
first four seconds of its motion. The computer is set
up t.o operate in "real time", that is to say, the t!me
actually occupied by the motor car when acceleratIng.
The problem layout of Fig, 1.3a shows a computing
potentiometer "A" coupled to the input of Integrator
"I", which in turn feeds Integrator "2". Voltmeters
are connected into circuit to display the three parameters of interest. Potentiometer "A" is first adjusted
so that its dial reads 2, corresponding to multiplication
by the constant 0'2, to represent 20ft/s2 scaled down
to yield a voltage of appropriate magnitude for t.he
integrators to handle. The output from the potentlOmeter is a steady voltage analogue of a steady rate of
acceleration.
As soon as switch S3 is closed to the +V position,
the velocity and distance meter pointers will start to
move in a manner analogous to the motion of the
motor car. Velocity will increase linearly with respect to
time, while distance will be displayed as an accelerating
pointer movement. Integrator "2" computes distance (s) as a voltage function of the square of time, in
'
.
terms of s = tal 2 •
With the problem of Fig. 1.3a, acceleration; velocity,
and distance are immediately available to the computer
operator as dial and meter readings. He can vary
acceleration just by turning the dial of the potentio-
D.e. VOLTMETERS
ACCELERATION
a=ft/.ec1
DISTANCE
S=ft
VElOCITY
v=ft/~c
'.la
(left). T"e u•• of Inte,rator. I.
Illustrated In
dla,ram. In t"'. examp'.
t"e rate of acce'eratlon. ve'oclty. and
dlJtCJnce covered by a motor car are computed and can be read off t"e potentlometer
dla' and meter .ca'e.
Fig.
t"'.
/
-VOLTS
+VOLTS
10
10
8
8
::..I~
~
6
g
4
<:>
...>
STEADY VOlTAGE
OISTAHCE 5 AFTER
... 6
II)I~
<.J
O~~----~----------~--~~~~~
~--~Ir-~~------HOLO------~.~I··-'I--·
COMPUTE
RESET
Z
;:!
4
VI
Ci
FI,. , .3b. Arrestln, a computation to ,Ive a steady
1
Z
1
4
3
TIME SECS.
Z
3
TIME SECS.
meter. If switch S3 is moved to the -V position, the
car will decelerate and stop.
COMPUTE, HOLD AND RESET
It is obviously inconvenient to take readings from
. voltmeters when pointers are on the move, and it is
impossible to do so if time t is very short, as with fast
events, or when the computer is speeded up to some
fraction of real time. The sequence governing switches
SI and S2, in Fig. 1.2c, is therefore arranged to provide
three facilities, called "compute", "hold", and "reset".
The purpese of the "hold" facility is to allow a
steady meter reading to be taken at any point on the
voltage/time curve output Elf an integrator. The high
.gain introduced by the operational amplifier effectively
4
vo'tmeter reading
.
multiplies the capacitance of Cr when the integrator
input is disconnected from input resistors and reset
resistan~ R r .
With amplification, Cr becomes the
equivalent of a very large capacitor which is capable of
holding a charge for a relatively long time. In prac- .
tice, the ability of an integrator to "hold~' or store a
voltage will also deper1ti on low amplifier drift.
Fig. 1.3b shows graphically the effect of compute,
hold, and reset modes, when applied to the distance
curve of Fig. 1.3a. In this case, it is necessary to halt
the computation after an elapsed time of 2·5s, and
obtain a value for distance in the form of a steady meter
reading.
lZ~----------------------'---~
I
I
//I
9
I
I /
)...
VI
I
- ~I
I
I
8
!;:;
BIAS
I
10
11
/
7
OUTPUT
INPUT
BIAS VOLTAGE
I
'/ /
g
...........=> 6
/
/
S 5
'"
/
/
SlOPE OETERMIlEO
8YRj
",
l,// · ./\/./
3
2
././
./
././
./
'/
DIOOE NETWORK
OUTPUTS
/'
-----Z
INPUT VOLTS X
·1
L BIAS VOLTAGE
FI,. , .4a ('eft). illustrating flow a mathematlca' function
can be constructed from a .erles of stralg"t line cane_nu
3
F'g. , .4b (above). A .'n,'e diode network and Its output
c"aracterlstlc
41
The compute mode is initiated by opening SI and
closing S2 (Fig. 1.2c). After 2·5seconds, S2 automatically opens and the amplifier input-is left floating,
with Ct still connected between input and output and
holding a stored charge. A .meter couplep to the
integrator output will show the distance travelled after
2·5s of acceleration.
.
The "hold" period can occupy several tens of seconds,
and is usually at the discretion of the operator. To
begin a new computer run, SI is closed, discharging
Ct through R r , thus resetting the integrator output to
zero. The input Elc in Fig. I.2c, is to allow an initial
condition to be applied to the integrator, as in the case
of a motor car which does not start from rest, but is
already in motion when it a~lerates. When computing
and resetting times are shorter than about Is, voltmeter answers will appear to be given at the instant of
pressing the button which initiates the SI, S2 cycle. .
The above description relates to a "single shot"
computer run, where the operator adjusts, takes a
reading, adjusts, and so on. In the repetitive mode,
the hold facility is ignored and the computer keeps on
repeating the answer curve, for display on an oscilloscope, chart recorder, or XY plotter..
DIODE FUNCTION GENERATOR
In many computer applications it is necessary to
generate a voltage which varies according to some nonlinear function not provided by normal operational
amplifier techniques. The diode function generator of
Fig. I.2d will allow a mathematical function to be
. constructed from a series of straight I.ine tangents, 'as
shown in Fig. 1Aa.
Each straight line characteristic is obtained from a
single diode-resistor network, and when the outputs
from several networks are summed together a complete
function will result. The shape of the final approximated
curve ' is determined by adjustment of the .network
r~sistors. Apart from powers of x, and other functions, roots are achieved by placing the function
generator in the feedback loop of an operational
amplifier.
A single diode network appears in Fig. lAb, and
the slope of its output characteristic can be varied by
adjustment of RI. The diode breakpoint (the voltage
at which the diode starts to conduct) is dependent on
the value of Rb.
MULTIPLIER
The computing potentiometer will multiply a variable
by a constant, but special techniques must be used to
mUltiply one variable by another variable. The
procesS' employed in modern computers i.s akin to
modulation, where the gain of a circuit is controlled
by an applied voltage.
The multiplier should yield a product of correct
,sign when multiplying negative or positive variables,
and this isrea.dily achieved with the self-excited time
division circuit of Fig. I.2e. The time division multiplier
operates on the principle 'of modifying the mark-space
- and amplitude of a square wave in accord with two
voltage inputs. The filter of Fig. 1.2e extracts the
mean level of d.c. from the square waveform. An
additional advantage of the Fig. I.2e circuit is that it
can be arranged to cater for more than two variables.
For example, inputs Xl, X2, and X3 multiplied by
input Y.
Next month: Commencing the construction of
UNIT ""A".
42
I
;4
EFORE embarking. on
I
-
constructional dl'taili, a few
words must be said concerning measuring and
\.0
test equipment required.
B
VOLTAGE STANDARD
It is necessary, at an early stage of omputer
construction, to establish a voltage standard for setting
up the PEACcircuits.
Si1'lce relative voltage levels are more important than
absolute levels, one particular voltmeter of proven
reliability can serve as a voltage standard, and this
might well be a reputable testmeter which has a large
scale conveniently calibrated in terms of 0-10 volts,
with a d.~. sensitivity of not less than 20,000 ohms per
volt. Even if the testmeter has an error of 2 per cent
of the indicated reading on d.c. ranges, it should be
capable of reproducing a given reading, from day to day
under similar room temperature conditions, with much
greater accuracy.
In addition to use as a voltage st<mdard, the testmeter can, of course, be employed for setting up
problems, answer readout, comparative resistance
checks, and for general testing of all circuits. There is
nothing- to prevent re-calibration of the computer to
laboratory voltage standards at a later date, a1)d this has
been allowed for in the overall design of PEAC.
COMPUTER INSTRUMENTATION
Analogue computer instrumentation has much in
common with electronic workshop equipment. Among
those instruments likely to be of use to the computer
operator are : ap oscilloscope, a small collection of d.c.
voltmeters, an audio oscillator, an a.c. voltmeter, and a
component measuring bridge.
\,.. The oscilloscope need not conform to a modem
specification, and could be a government surplus
However, it is often an advantage to have a large
area, and redund~.mt television sets can be converted
computer readout purposes with excellent results.
limited bandwidth of magnetic deflection is no
advantage at normal computer operating ~.
D.C. voltmeters with centre zero scales are
useful for rough checks on the terms of 'a COInPllteJr::
equation, where, for example, the wish is to see
varies ' in relation to x when manipulating a
taneous equation.
A sine wave oscillator, with attendant a.c. vo'ltn1letc~;
will often be employed for work on transfer fUllLCti~l)nsf '
and for general electronic circuit simulation.
Finally, the component bridge is a help i'~~-k:=~'~
up plug':in computing components, and for
possible SOUl'ceS of ~or.
n is assumed that special classes of equipment, such
as the XY plotter, will not be available to the am,ateur;
and they are therefore excluded from further mention.
UNIT "A" CONSTRUCTION
The general form of construction adopted for
is based on a series of boxes made with laminates
white Arrnaboard or Formica and hardboard. The
resulting box is rigid and durable, with a surface which
easily takes panel transfers and. lines drawn in Indian
ink. With such a construction, it is possible to achieve
a professional appearance using only simwe WOOd1"'0l~k~i;'"
ing t o o l s . '
,
It is advisable to start with the UNIT '~A"
panel and case. This slightly unusual procedure,
building the box before starting on internal cm::uits.
o
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t
~
'~C'-1/2
12
~~
+11V;
DRILL TO TAKE
6 B.A. SCREWS .
DRILL AND FILE
TO TAKE SWITCHES
~"
1/;
'. 1
·1
D.C. VOLTAGE SOURCE
1"
T
@1"
:iI+' ;~' ~l- . 0
·1
318"
~
10
~~
L@--------~@r--------~@@~---------~
LINES DRAWN WITH"
INDIAN INK
SUMMER 1
o o o
BLACK
@WHITE
RED
O'.,: ./'T' VELLOW
Fig. 2.2. Left-hand portion offront panel. Drilling details,
layout of components, and panel engraving. (Below the
broken line, there are two further sections, each a replica
of "Summer I")
BLUE
@GREEN
/ / - '\
I
)1
/--"I (
/ - ........ \ / -
........
I1
,
', _/1 \ _/1 ',,_/ ', _ . /
\
IT
I
5"
COEFFICIENT POTENTlOMETERS
VOLTAGE SOURCE
I
,
~- _+- -O-PE-R-A-Tl-ON-A-L-A-M-PL-'F-IE_R_1 ~~'
~L1LNE_S_!.D_R_AW_N_r--------------------~~1/2"
r-__________SU_M_M_E_R_1______
SUMMER 2
WITH INDIA~ .
.,..
__
OPERATIONAL AMPLIFIER 2
__________________ INj
1SUMMER 1
OPERATIONAL AMPLIFIER 1
11
i~==========================~
~1.~----------------------11~;----------------------~.1
Fig. 2.1. UNIT" A" front panel. Overall dimenslorrs and sectional dlvldln,lInes
106
"'~'" "'~'" "'~'" "'~'" ",~s.
SK3
SK3
WHITE
SK3
SK3
BLUE
BLACK
REO
e
8SK4
8SK4
SK4
SK4
SK4
Fig. 2.3. Reverse side of front panel. left-hand portion
(Fig. 2.2). showing components and wiring. Summer 2 and
3 are wired exactly as Summer I shown here. The three
terminals TU. 2 and 3 are mounted on the side of the box
J_ '- . ,.!'!t.,~
.
~
!
. " . e . ' ,;'
•• ~: ~:~
. :.,: .~~ .
• t- . ,.~i ~ .f:,:.~ . ~e. ... .:. .-. -J1
..... ' 0.-._"..
,. ,. ,. ,". ....-- --.
~-tJ~ •
,.
,.
,.
....
r, ,w
I
..
. .~ .
~• ..,.-~~
•
t.:.
."."",-,·o()'1
•
•
•
•
f:' i'='
•
•
0
0
•
•
•
•i
' 1 "-• • • • • "
"".0;:1
•
"'"
-
"'l
-.-~.-.. Y
.....
+."'\,1 .
.
•
, ~-.
o , 0<1' i CII'''~::;.;.,
i'='
0
SK3
YELLOW
SK5~
.
-
-o. "
-
,'",-.",
UNIT "A" front panel
-
.
€)
may be justified on two counts: firstly, the front panel
really forms a circuit which is designed to be accessible,
and is an important part of the unit; secondly, the
method of construction chosen brings economy by
dispensing with a self-supporting internal chassis
assembly, and much of the internal gear is actually
mounted on the front panel, or to the box itself.
UNIT "A" FRONT PANEL
To prepare the front panel, a sheet of white plastic
laminate, slightly larger than its finished size of
13in x 17i-in, is glued to a sheet of hardboard of the
same measurements with Evostick or a similar adhesive.
When firm, the panel edges can be planed, rasped, or
sandpapered down to size, while making sure that all is
square. Next, taking Fig. 2.1 and the photograph of
the front panel as a guide, mark out the main dividing
lines with a pencil.
The positions of all holes and slots may be found by
referring to panel drawings Fig. 2.2 and Fig. 2.5.
Establish hole centres by first marking with a pencil,
then indenting with a sharp spike. Note that all
drilling should be carried out from the plastic laminate
side of the panel, to avoid chipping the white surface.
It is important to handle tools carefully, and prevent
them skidding across the plastic surface and scoring it .
When all holes have been drilled, deburr them on the
reverse side of the panel with sandpaper, and check that
components will fit correctly before applying a coat of
clear varnish to the hardboard backing.
107
,I
I
YELLOW
1\1 1
SK6
I
SK5
SK4
SK3
G~~
BLUE
SK14
GREEN
1
/1I
TO OP-AMPS
2 AND3
~
TO OPERATIONAL
AMPLIFIER PANEL
Fig. 2.4. Right-hand portion of front panel viewed from
rear. showing components and wiring. Operational Amplifiers 2 and 3 are wired exactly as "Operational Amplifier
I" shown here
To finish the panel, draw in all lines and symbols
with a nib pen and Indian ink. If any mistakes are
made, the ink can be removed-when dry-with a
typewriter eraser, and surface shine restored with metal
polish. Lettering can be applied by the "rub-on" or
"stick-on" transfer methods, and should be protected
by a thin layer of clear varnish.
When the panel decor has dried, mount all sockets,
potentiometers, knobs with dials, switches, and the
neon mains lamp. Dials may be lined up on potentiometer spindles later.
UNIT "A" BOX
Rear view of UNIT "A" front panel
108
This time, the box is first constructed of hardboard
on a wooden frame, and is later covered with plastic
laminate. See fig. 2.12.
Cut and finish the four hardboard panels to size, and
cut the various lengths of softwood. The manner of
assembly could be as follows: attach wood lengths
A and C to top and bottom panels with panel pins or
countersunk woodscrews, gluing all joints. Attach
lengths B to side panels, bring panels together and
secure. Next, position D, E, and F. Note that there
is no length D at the back portion of the top panel so the
slotted amplifier mount F should be lined up vertically
with its companion E . All drilling must be left until
the plastic laminate is in place.
o
I
1
/1"-..'
1\ \ \
,-/1
Y
I
I
I
I
I
•
t""""-- 11/8- - - 1 - _ I
I
I
I
I
I
IlOkIl
I
II
0
------o------~-----~
--v~
------
0
------
-0- ---~--~
~
I COEFFICIENT POTENTlOMETERS
COMPONENTS • • •
Fig. 2.5.
Right-hand portion of front panel. Drilling
detalls,/ayout of components, and panel engraving. Below
the broken line there are two further sections, each a
replica of "Operational Amplifier I"
UNtT "A" FRONT PANEL AND BOX
Resistors
RI-R5
9·lkO (5 off)
R6-RIO 9100 (5 off)
RII-RI5 1000 (5 off)
All 5%, !W carbon film
Pre-set Potentiometers
VRI-VR5
2500 miniature wirewound slider
type (5 off)
VRIS-VRI7 500 wirewound panel mounting
type (3 off)
Potentiometers
VR6-VRIO IkO 3W linear wirewound, ±20% or
better, 270 0 effective rotation (5 off)
VRII-VRI4 10kO 3W linear wlrewound, ±20%
or better, 270 0 effective rotation (4 off)
Switches
SI-S6 Double-pole, on/off slide switch (Radio.
spares) (6 off)
Plug
PLI
3 way panel mounting mains plug and cable
connector
Fuse
FS I 1·5A cartridge fuse and 20mm fuseholder
Lamp
LPI Neon indicator lamp (Radiospares "miniature
200-250V panel neon" with self-contained
resistor)
Sockets
21 Red, 15 Black, 15 Blue, 15 Yellow, 15 White,
12 Green (painted green, see text)
48 miniature sockets, black or red to choice
Terminals
Insulated screw, to take 4mm stackable plugs
(Radiospares). I Red,1 Green, I Blue
Miscellaneous
Material for panel and box: Hardboard: 2 off 13in x
Sin, 2 off 18in x Sin, I off 13in x 171in. White
plastic laminate: 2 off 13in x Sin, 2 off 18in x5in,
I off 13in x 171in. Softwood: 52in x tin square,
4in x _in x lino
20 s.w.g. tinned copper wire. Insulated sleeving
Dials and knobs
Nine 0-10 2700 dial knobs (Bulgin type K400),
. black or grey
109
~ +12·5_ TU
~~------~~~~------~~~------~~--~------~~~------~)
~RED
O~~~---o--~~----~~~+---~~~~----~~~~--~~
-12·5~ TL2
) ~BLUE
TL3
• GREEN
10
Fig. 2.6.
Circuit diagram of Volttlge Source section
=:E E'"
10
.
10
SK2 VR13
VR12
10kll
0
10~:
E~
SK4
8RE£N
SK4 GREEN
CP1 (RED)
CP2 (BLACK)
E'"
10
VR14
SK2 10kll
SK3
SK2
0
SK3
SK4 GREEN
SOCKET IDENTIFICATION
The following abbreviations will be used in the
programming instructions for PEAC. Applied
as prefixes to socket (SK) numbers. they clearly
establish the identity of the particular socket
referred to. For example. "VS2/SKI"; "CPI/
SK3" etc.
SK3
VS
CP
S
I
OA
SK4 GREEN
CP3 (BLUE)
CP4 (YELLOW)
~
Fig. 2.7. Circuit diagram of Coefficient Potentlometers section
SKI
INPUT 1
(RED)
SK2
SKt . SQ
INPUT Z
(BLACK)
SKI
SK2
SK1
SKI
SK2
INPUT 3
(BLUE)
Voltage source
Coefficient potentiometers
Summer
Input (Summer)
Operational Amplifier
SK2
INPUT 5
(WHITE)
Miniatun
Acluol si.e
soekets
Fig. 2.8. Circuit diagram of Summer I, 2, and 3
Fig. 2.11. Method of bending leads to
make ,,'ug-/n "rogramming resistors
INITIAL
CONDITIONS
::lVRI5
50n
3
\
SK14
GREEN
7m~
INPUT
SKB
WHITE
Grltn
r-I
I
Bloc\( JTO TI Pow.r
. Puc\( Transformer .
I
I
I
I
,-<)":-:o--~Or::.=::ce
I
I
:
:
'·SA
I
I
IL ______ JI
Fig. 2.9. Circuit diagrtlm of O"erationtll Am"lifier I, 2, and 3
110
Fig. 2.10.
Circuit diagram of mains sUI>I>'y
o
WOOD
2 OFF 173,~' X ,,~' X,,~'
® 2 OFF '2"x,,;'x1fz'
© 4 OFF 4" X',;' X";'
@ , OFF'2 3,~' X";' X";'
®' OFF 4" X"; X";' WITH SLOT
® ' OFF 4" X 7,~' X3,~' WITH SLOT
HARDBOARD PANELS
2 OFF 13" X5" (SIDES)
2 OFF 18" X5" (TOP AND BOTTOM)
Fig. 2.12. Constructional details of UNIT "A" Sox
-Cut plastic laminate to fit hardboard panels wifh
1\-in overlap, and glue to the box sides first. . Reduce
the overlap to size when the laminates are firm, before
fitting the top and bottom surfaces. When trimming
the top and bottom panels down to size, take care not
to scratch and score the side pieces. For economy, the
bottom plastic laminate layer can be omitted.
When satisfied with the laminated exterior, the
1·062in dia. hole can be made by a series of small
drillings and finished with a half-round file. The box
interior and wood may be varnished, but the raised lip
at the front of the box is best painted black, or some
dark colour, to contrast with the front panel.
The finished box is quite strong, and will support the
full weight of a normal adult when the front panel is in
place. However, it is recommended that this test
should not be applied too often!
FRONT PANEL WIRING
Attach the front panel to its box, which will act as a
convenient mount when wiring the back of the panel.
The bare earth wire linking all green sockets runs
along the top half of the front panel and down its lefthand side, looking from the back; this should be
soldered in place before embarking on the sleeved
wiring. (No matching green sockets were available
for the prototype, so odd coloured socIs-ets were
painted green with cellulose model aeroplane dope.)
The 4mm red, green, and blue terminal sockets on .
the side of UNIT "A" are designed to take stackable
plugs, and will make available the power supply outputs to external sub-units. Wiring can proceed from
the terminal sockets along the voltage source (see
Fig. 2.3) and then to the rest of the front panel.
Circuit diagrams for all the various "sections"
incorporated in the front panel are given in Figs. 2.6 to
2.10 inclusive. Wiring details are given in Fig. 2.3 and
Fig. 2.4.
The summer and operational amplifier sections are
triplicated-although only one of each of these sections
has been shown in the diagrams Fig. 2.2 to Fig. 2.5
inclusive.
The purpose of the miniature sockets, which appear
in the above mentioned diagrams, is to take the plug-in
programming components; explained by Fig. 2.11.
Resistor leads are preformed in the manner shown.
The distance between miniature sockets is standardised
at 1in, to allow the use of a special made-up two pin
plug to support the bulkier components, such as large
polyester capacitors.
When wiring up the operational amplifier sockets,
ignore for the time being the coloured flexible wires
shown in Fig. 2.4 as these are the flying leads from the
operational amplifier panel, and will be referred back
to when the time comes to mount the amplifiers.
Fit the mains connector PLl and fuseholder for FSl
to the side of the box. Wire up the neon lamp LPl
and the fuse FSI to PLl as shown in Fig. 2.10,
CORRECTION.
In Part 1, Page 40, last line of the equation in the example at
top of right-hand column should read:
10
1010)
+ 2100 = - (5 - . (3-5 x 5) + 0-2),
Eo = - ( 510 - 3-5 2
therefore Eo = 12-3.
Next month:
amplifiers
Power supply and operational
111
PEAC
By,
D.BOLLEN'
r
The current res.erve of the stabilised supply will just be
sufficient to cater for the needs of UNITS" A to 0". If
further expansion of the computer is contemplated,
beyond the inclusion of UNIT "0", a subsidiary
unstabilised supply can be added to the computer at a
late stage of construction, to power the relays of
UNITS, "B" .and "0", and thus make available some
extra current from the stabilised supply.
E main design target for the PEAC power supply
.
was a maximum voltage variarion of not more than
1 per cent under all normal operating conditions.
Several circuit configurations were tried, based upon
either .series or shunt regulation, but it was found that
shunt regulation invariably gave the best performance
for a given cost, plus the bonus of complete short-circuit
protection. It was not considered to be.a disadvantage
for computer work, where nearly everything is switched
on for most of the time, that a shunt regulated supply
would be wasteful of power ·under no-load conditions.
STABILISED POWER PACK
The circuit of Fig. 3.1 is based on a small, standard
type of rectifier transformer, with bridge rectification
fig. 3.1. Circuit diagram of the
stabilised power supply
+30V
+
r,--'
IL
i
1
12V
0
1
12V
1
1
1
1
DZ
51AR2
zov
11
1
1
1 1
1
1
I1
1
I
C2
03
SIAR2
20V
04
SIARZ
+
I
lE
...
1000I'F
+
I
200
Cl
1000pF
o
IN
1
I
I
I
r
+IZ·5V
C3
1000pF
245
Pl