THE DEVELOPMENT OF A MODEL AUTOMATIC ELECTROMECHANICAL VENDING MACHINE

Abstract

A low cost automatic electromechanical vending machine has been designed and constructed using components available in the Nigerian local market. It consists of an aluminium beam which was loaded at the free end with N1 coins while the other end was fixed. The strain on the beam was measured by means of four strain gauges arranged in a Wheatstone bridge circuit configuration and permanently bonded to the aluminium beam. An instrumentation amplifier was designed to amplify the output voltage of the bridge circuit. A comparator circuit was then used to compare the output voltage of the instrumentation amplifier with a reference voltage of 5.1V. When the output voltage of the instrumentation amplifier exceeded the reference voltage, a switching and control circuits were triggered which supplied the product being sold automatically. An electromagnetic mechanical system with a power circuit was incorporated to empty the coins from the strain gauge system for the next user while a speed control circuit employing an infrared opto-coupler was also incorporated which ensured that the electric motor maintained a constant speed of rotation regardless of the torque due to the applied load. In addition, a lighting system was incorporated in the upper and lower chambers of the machine that switches on whenever any or both doors of these chambers are opened to illuminate the respective chambers during loading or retrieval of the money sold for the day. Finally, the machine was completed with an alarm system was designed to switch on as soon as the product being sold is exhausted. The items sold with this machine were soft drinks, biscuits and toiletries.

Full text

1
THE DEVELOPMENT OF A MODEL AUTOMATIC ELECTROMECHANICAL
VENDING MACHINE
1Vincent A. Akpan* & 2Michael T. Babalola
1. Department of Electrical & Computer Engineering,
Aristotle University of Thessaloniki, GR-45124 Thessaloniki, Greece
2. Department of Physics, Federal University of Technology, Akure (FUTA), Nigeria
*Corresponding Author,1. avincent@auth.gr & 2. mtbabalola@yahoo.co.uk
Abstract:
A low cost automatic electromechanical vending machine has been designed and constructed
using components available in the Nigerian local market. It consists of an aluminium beam which was
loaded at the free end with N1 coins while the other end was fixed. The strain on the beam was measured
by means of four strain gauges arranged in a Wheatstone bridge circuit configuration and permanently
bonded to the aluminium beam. An instrumentation amplifier was designed to amplify the output voltage
of the bridge circuit. A comparator circuit was then used to compare the output voltage of the
instrumentation amplifier with a reference voltage of 5.1V. When the output voltage of the
instrumentation amplifier exceeded the reference voltage, a switching and control circuits were triggered
which supplied the product being sold automatically. An electromagnetic mechanical system with a power
circuit was incorporated to empty the coins from the strain gauge system for the next user while a speed
control circuit employing an infrared opto-coupler was also incorporated which ensured that the electric
motor maintained a constant speed of rotation regardless of the torque due to the applied load. In addition,
a lighting system was incorporated in the upper and lower chambers of the machine that switches on
whenever any or both doors of these chambers are opened to illuminate the respective chambers during
loading or retrieval of the money sold for the day. Finally, the machine was completed with an alarm
system was designed to switch on as soon as the product being sold is exhausted. The items sold with this
machine were soft drinks, biscuits and toiletries.
Key Words: Electromechanical Vending Machine, Naira coins, Strain Gauges, Instrumentation
Amplifier, Comparator, Electromagnet control, Motor Speed Control, Lighting System
and Alarm System.
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ISSN:1596-9647
Pages: 121 - 131

2
1. Introduction:
Over the years, drinking parlours, snacks bars, business centres, faculty and departmental offices
often sell items such as soft drinks, biscuits, toiletries, beverages and other related items and these services
normally require sales personnel. However, the operation is bedevilled with many problems. Where the
level of sales is low as in a faculty office, it is inconceivable to employ a full staff for such business. But
in large centres, where the level of sale is high, quite often these services are inefficient due to the
incompetence or the shortage of sales personnel. Also dishonest staff may misappropriate monies
collected from customers. The above shortcomings have led to the invention of automatic vending
machines. A vending machine is an electro-mechanical control device that can be operated by the user and
it is employed in business centres and shops to sell variety of goods. The machine supplies a particular
product whenever it is activated by stipulated the amount of money, either in coins or currency notes.
The advantages of using a vending machine are many. The sale of products is accurate, fast and
efficient as long as there is mains electricity supply. No money is lost to the sales assistant since only the
owner of the machine has access to the money section of the machine. In addition, the introduction of
machines into a business centres will result in more profit, as no personnel is required to operate the
machine. The only activity requiring sales personnel is the loading of the machine with its various
products, a task which can easily be performed by an attendant. This paper presents the techniques
required for producing a vending machine using locally sourced components and parts.
2. Design of the machine:
The machine is divided into two parts namely: the mechanical part and the electronic part. The
electronic part has three sections: the machine control section, the lighting section and the alarm section.
The machine control section has a direct link with the mechanical part of the machine while the lighting
and the alarm systems are two independent sections.
2.1 The mechanical part of the machine:
The mechanical structure is shown in Fig.1. A plastic bowl has twelve holes of equal radius at its
base. The holes have been drilled at equal distances to each other and are equidistant from the centre of the
bowl. Each hole accommodates a single item to be dispensed which included canned and bottled soft
drinks, canned beer, tissue paper, sachet and bottled water and packet biscuits. These holes allowed the
items to rest on the smooth surface of a wooden plywood sheet covered with formica to reduce friction
during delivery. The wooden plywood also has a hole at the same distance from the centre of the shaft as
the holes on the plastic bowl. The shaft is an iron rod R upon which the plastic bowl rest. When the
permanent magnet direct current electric motor (PMDC) is energised, its spindle rotates in a clockwise
direction. This rotation is conveyed to the shaft R through a gear system A and B. As the shaft rotates, so
does the plastic bowl rotates such that whenever one of the holes on it coincides with the fixed hole on the
wooden surface, the item slides out of the vending machine.
Also shown in Fig.1, is a thick aluminium beam which is fixed at one end L and resting on its
other end is a coins receiver. The aluminium beam, separated by a wooden slab (WS) as shown in Fig.2,
was mounted on a fixed wooden beam A (See Fig.2 for an expanded diagram of the aluminium beam,
wooden beam A, aluminium beam and the coins receiver). The coins receiver is a small rectangular
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ISSN:1596-9647
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3
aluminium container opened at the top and on its rear side is a door which can slide in the vertical
direction. The upper part (a quarter) of the door is made of Γ-shaped steel while the lower (three quarter)
end is made of aluminium since aluminium are non-magnetic but the Naira coins were attracted by
magnets. A slanted aluminium sheet was placed inside the coins receiver with its lower edge touching the
lower edge of the sliding door. When coins are dropped into the box, they rest on the slanted aluminium
sheet. If the sliding door is pulled up, the coins slide down the slanted sheet into a plastic cylindrical coins
collector with foam at it base to reduce noise during the dropping of coins. The coins receiver was able to
receive up to fifty N1 coins without distorting the beam.
Fig.1 The mechanical and structural parts of the vending machine
y
Gear A
Opaque
Segmen
t
Item to be
Dispensed
Shaft R
Gear B
Machine
Control
Panel
Electric
Motor
(PMDC)
Wooden
Beam B
Wooden
Beam A
Coins
Collector
Item
Delivery
Tray
Plastic Bowl
With
Holes
Infrared
Opto-Coupler
Electro-
magnet
Small
Transparent
Gaps
Delivery
Hole on
Plywood
Plywood
gummed
with
Formica
.
.
.
Γ–Shape
Sliding
Door
Coins
Receiver
Aluminum
Beam
Strain
Gauge
Nuts and
Bolts
Fixed End L
With screws
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ISSN:1596-9647
Pages: 121 - 131

4
Fig.2 Mechanical arrangement of the strain gauges on aluminium beam and the coins receiver.
2.2 The electronic part of the machine:
2.2.1 The strain gauge sensing system and the instrumentation amplifier circuit:
The electronic section of the vending machine starts with four strain gauges that are permanently
glued to the surface of the aluminium beam (two on the top surface and two on the lower surface). The
strain gauges were the 120-SRA foil type and each had a resistance of 170Ω [1]. The strain gauges were
arranged in a Wheatstone bridge circuit configuration as shown in Fig.3 where R
B1
, R
B2
and R
B3
forms a
signal conditioning network for the strain gauge system; and with R
B1
, V
out
was tuned for better
performance during the design process as described in [2]. As N1 coins were dropped into the coins
receiver, the aluminium beam became strained and so were the strain gauges. The output voltage, V
out
,
of
the bridge circuit [3] is given by Eq.1:
E
R
R
Vout
∆
= (1)
where E is the supply voltage, ∆R is the change in resistance of the strain gauges, and R is the resistance
of the strain element without strain. The output voltage, V
out,
from the strain gauge system was very small
as shown respectively in Tables 1, 2 and 3 for each N1 and 50 kobo coins as they were dropped into the
coins receiver. To make V
out
useful for control, it was given further amplification. This was provided by
the instrumentation amplifier [4] shown in Fig.3. The instrumentation amplifier has four op-amps: IC1,
IC2, IC3, and IC4. IC1 and IC2 forms the differential input stage with a voltage gain, A
1
, is given by Eq.2:
( )
GBGA
RR
R
A+
+=
1
1
2
1 (2)
IC3 is the output buffer. It has a voltage gain, A
2
, is given by Eq.3:
+56V as shown and
described in Fig.6
+
–
Bottom of Plywood
gummed with formica
(See Fig. 1) Electromagnet
2.5cm
1.0cm
0.5cm
0.7cm
25.0cm
7.0cm
7.0cm
5
6
3
4
Wooden Slab (WS)
Strain Gauge (G1)
Open Cavity
(length = 17.5cm)
Aluminum
Beam
Slanted
Aluminum
Sheet
Γ–Shape
Sliding
Door
Coins
Receiver
Screw
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Pages: 121 - 131

5
2
3
2R
R
A= (3)
This overall voltage gain, A, of the instrumentation amplifier is given by Eq.4:
( )
















+
+=
2
3
1
2
1R
R
RR
R
A
GBGA
(4)
IC4 forms a bootstrapping compensation network [5] that kept the output of the IA at a stable value
regardless of the little vibration of the strain gauge system due to the changes in the load conditions as the
coins were added. Resistance mismatch was corrected by adjusting R
3B
to zero the output of IA during no
load condition.
The IA was designed so as to accommodate up to fifty N1 coins. With the resistors values shown
in Fig.3 and R
GB
= 2.2kΩ, the overall gain of IA was 214. But when R
GB
= 0Ω, the overall gain of the IA
became 1255. Hence, it was possible to vary the overall gain of the IA between 214 and 1225. The IA was
able to accept up to fifty N1 coins (N50) during the experiment (Table1) with by adjusting R
GB
to a value
of 2.12kΩ which gave the IA a gain of 220. Finally, R
GB
was adjusted to 192Ω to give the IA a gain of
approximately 850 to accept only six N1 coins or an exact equivalent of twelve 50 kobo coins before an
item could be dispensed. The IA was calibrated as follows: the required number of coins decided for a
particular item to be dispensed (for example six N1 coins) was place in the coins receiver and R
GB
was
tuned to set the output voltage (V
01
) to a desired value before the comparator (discussed in the proceeding
sub-section) was calibrated. In this work, the output voltage (V
01
) of the IA was specified by adjusting R
GB
to produce 1.02V for every N1 coins dropped into the coins receiver and the weight of 50 kobo coins
produced 0.51V. The results of this calibration are shown respectively in Tables 2 and 3.
In order to specify the minimum number of coins that must be dropped into the coins receiver
before the product would be delivered, it was necessary to add a comparator.
Table 1: Output voltage for different number of N1coins.
N o . of N 1 . 00
c oi n s
S t ra in Ga ug e Sy s t e m O u t p ut
v o l t ag e ( V o u t , m V)
I n s t ru me n t at io n Am p l i fi er
O ut p u t Vo l t a g e ( V 0 1 ,V )
0 0 .0 0 0 .0 0
2 2 .0 4 0 .4 6
4 4 .0 8 0 .9 1
6 6 .1 2 1 .3 6
8 8 .1 6 1 .8 0
1 0 1 0 . 2 0 2 .2 5
1 2 1 2 . 2 4 2 .7 0
1 4 1 4 . 2 8 3 .1 4
1 6 1 6 . 3 2 3 .6 0
1 8 1 8 . 3 6 4 .0 4
2 0 2 0 . 4 0 4 .4 9
2 2 2 2 . 4 4 4 .9 4
2 4 2 4 . 4 8 5 .4 0
2 6 2 6 . 5 2 5 .8 3
2 8 2 8 . 5 6 6 .2 8
3 0 3 0 . 6 0 6 .7 4
3 2 3 2 . 6 4 7 .1 8
3 4 3 4 . 6 8 7 .6 3
3 6 3 6 . 7 2 8 .0 8
3 8 3 8 . 7 6 8 .5 4
4 0 4 0 . 8 0 8 .8 0
4 2 4 2 . 8 4 9 .4 3
4 4 4 4 . 8 8 9 .8 9
4 6 4 6 . 9 2 1 0 . 3 3
4 8 4 8 . 9 6 1 0 . 8 8
5 0 5 1 . 0 0 1 1 . 2 3
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6
T a b l e 2 : O u tp u t v ol t a g e f o r d if fe r e nt n u mb e r o f N 1 c oi n s.
N o . of N 1
C o i n s
S t ra in Ga ug e Sy s t e m O u t p ut
v o l t ag e ( V o u t , m V)
I n s t ru me n t at io n Am p l i fi er
O ut p u t Vo l t a g e ( V 0 1 ,V )
0 0 .0 0 0 .0 0
1 1 .0 2 0 .8 7
2 2 .0 4 1 .7 3
3 3 .0 6 2 .6 0
4 4 .0 8 3 .4 7
5 5 .1 0 4 .3 4
6 6 .1 2 5 .2 0
T a b l e 3 : O u tp u t v ol t a g e f o r d if fe r e nt n u mb e r o f 5 0 k ob o c o i ns .
N o . of 5 0 k
C o i n s
S t ra in Ga ug e Sy s t e m O u t p ut
v o l t ag e ( V o u t , m V)
I n s t ru me n t at io n Am p l i fi er
O ut p u t Vo l t a g e ( V 0 1 ,V )
0 0 .0 0 0 .0 0
1 0 .5 1 0 .4 3
2 1 .0 2 0 .8 7
3 1 .5 3 1 .3 0
4 2 .0 4 1 .7 3
5 2 .5 5 2 .1 7
6 3 .0 6 2 .6 0
7 3 .5 7 3 .0 4
8 4 .0 8 3 .4 7
9 4 .5 9 3 .9 0
1 0 5 .1 0 4 .3 4
1 1 5 .6 1 4 .7 7
1 2 6 .1 2 5 .2 0
(To Fig.4(c))
01
V
-15V
3
R3
47.1k
-15V
-15V
-15V
+15V
+15V
+15V
+15V
6
out
V
10k
R
Gauge
5
Gauge
+12V
+
4
IC4
OP07
+IC3
OP07
+
8
IC2
OP07
+
7
IC1
OP07
3B
R
2.2k
R
B1
Gauge
1
R2
1.57k
5k
R2
1.57k
2
R4
10k
3A
R
45k
Gauge
R410k
B2
R
100k
Gauge
R1 7.96k
GB
R1 7.96k
GA
R390R
Activ e G4
Dummy G2
Dummy
G3
B3
R
240R
Active
G1
Fig.3 Electrical arrangement of the strain gauges in a bridge network connected to an instrumentation amplifier
2.2.2 The working of the comparator circuit:
The comparator circuit was built around the LM339 integrated circuit, which contains four
identical separate comparators in a dual-in-line (DIL) package [1] but only three of these comparators
(IC5, IC6 and IC7) were used in this work. The symbol for one of the comparator is shown in Fig 4(a).
Each comparator has an NPN transistor of the open collector type at the output stage. An external pull-up
resistor R
L
was connected between the output terminal and the positive terminal of the supply voltage. V
in
(V
01
) is the differential voltage between the inverting and the non-inverting inputs of the comparator.
When V
in
is negative, the output transistor switches ON and the output voltage is 0V. If V
in
is positive, the
transistor is switched OFF and the output voltage is +V
cc
[6].
The characteristics of the comparator circuit [7] are shown in Fig 4(b). V
LT
and V
UT
are the lower
and upper threshold voltages respectively defined as:
+V
sat
= 12V if V
in
> V
UT
and –V
sat
= 0V if V
in
<V
LT
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7
The comparator circuit of Fig.4(c) was used in this work and was used to compare the IA’s output
voltage (V
01
) with a reference voltage. V
UT
and V
LT
were selected as follows: Setting V
in
= 0V, the output
voltages of IC5, IC6 and IC7 were 12V, 0V, and 0V respectively and R
UT
was adjusted to place the upper
threshold voltage (V
UT
) at 5.2V. When V
in
= +12V, the outputs of IC5, IC6 and IC7 became 0V, 12V, and
12V respectively and R
LT
was adjusted to put V
LT
at 5.1V. Therefore, for this comparator, the reference
voltage was set to 5.1V.
Thus, V
02
= 0V, when V
in
< 5.1V and V
02
= +12V, when V
in
> 5.1V. IC8 is an operational
amplifier configured as a voltage follower with unity gain and serves as a buffer amplifier to the output of
IC7. However, for the fifty N1, R
UT
was adjusted to place V
UT
at 11.40V while R
LT
was equally varied to
put V
LT
at 11.10V and the results are shown in Table 1.
rail of IC9
suppl y
10k
-12V
+12V
in Fig.5)
V
+IC8
CA314 0
01
(From V
in
set
V
+12V
02
10k
(To the power
10k
10k
10k
UT
R
LT
R
of Fig.3)
V1/4
IC7
LM339
1/4
IC6 LM3 39
1/4
IC5
LM339
Fig.4(c) Circuit for independent setting of upper and lower threshold voltages VUT and VLT respectively
2.2.3 The switching circuit:
The output of the comparator circuit (V
02
) served as the driver to the switching circuit (Fig.5)
which has two NE555 timers wired as astable and monostable multivibrators respectively [8]. However,
for NE555 timers integrated circuits configured as astable multivibrator circuit with a duty cycle of 50%,
(provided R
A
= R
B
as in the circuit of IC20 and IC21 of Fig.8), the output is a perfect square wave of
frequency f given by Eq.5:
ABA
dc CRR
f×+
=)(
44.1 (5 )
But, if BA RR
≠
, as with the circuit of IC9 in Fig.5, the output waveform is not a perfect square wave and
the frequency of the astable multivibrator is give by Eq.6:
ABA CRR
f×+
=)2(
44.1 ( 6 )
Moreover, for NE555 timers configured as monostable multivibrator as in the circuits of IC10 (Fig.5) and
that of IC16 and IC17 (both of Fig.7), the output timing interval t
high
is given by Eq.7:
A
B
V
0
+V
sat
-
V
sat
V
in
V
LT
V
UT
C
D
+
_
V
in
¼ LM339T
V
out
+V
cc
= 5V
1k
Ω
RL
E
d

.
.
Fig. 4(a) The symbol of a single comparator Fig.4(b) Characteristic of the LM339T comparator
o
o
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Pages: 121 - 131

8
BChigh CRt ×= 1.1 ( 7 )
Finally, for the NE555 timer, the time constant for a monostable multivibrator is given by Eq.8:
CDCRtconsTime =tan (8)
However, it is also necessary that the time constant be very small when compared to t
high
[8]. The
diode D
i
has been included to prevent the 555 timer from triggering on the positive-going edges of the
input clock signals [9]. All the resistors and capacitors values used for both the astable and monostable
multivibrators were calculated using their respective formula from the last four equations. The
potentiometers in some of the multivibrator circuits allowed for precise time critical adjustments.
The logic high output voltage (V
02
) from the comparator of Fig.4(c) acts as the supply voltage to
IC9 which is wired as an astable multivibrator. The output of IC9 (Fig.5) turned on the green LED (LD1),
notifying the user that the required number of coins have been inserted and that the product will be
released once the reed switch (RS) is momentarily closed. The output of IC9 was connected to the input of
IC10 which is configured as a monostable multivibrator. At the mere pressing of RS, the falling edge of
the first pulse from IC9 triggered IC10. The output of IC10 (V
03
) remained logically high for
approximately two seconds. During that period, this logical high signal simultaneously triggered both the
electromagnet (Fig.6) and the motor speed control (Fig.7) circuits; and the coins receiver was instantly
emptied while the dispensed item was automatically simultaneously delivered automatically.
0.01 uF
A
+12V
56k
R
+
B
C
33uF
Gnd
Trg
Out
Rst Ctl
Thr
Dis
Vcc
IC10
NE555
IN40 01
D
+12V
TIP41
Q1
C
C
0.1uF
B
A2
(RS)
Swit ch
Reed
Comparator
From
(LD1)
Green
LED
Gnd
Trg
Out
Rst Ctl
Thr
Dis
Vcc
IC9
NE555
V =+12V
0.01uF
100k
R
To Fig.6
C
0.1uF and Fig.7
V
A1 i
5.6k
D
R
100k
03
C
470R
02
R
56k
of Fig.4(c)
R
56k
Fig.5 The switching circuit
2.2.4 The electromagnet and its control circuit:
When the electric motor rotates, the product was delivered. It was necessary to empty the coins
box in order to set the machine ready for the next user. Thus, an electromagnet (EM) with 80 turns of
SWG 24 wound on a carbon-free cylindrical soft iron core of height 13cm having a radius of 2.5cm was
constructed. Carbon-free soft iron core (obtained from Delta Steel Company, Aladja, Delta State, Nigeria)
was used which ensured that magnetism was not retained by the electromagnet once it has been de-
energised. The electromagnet was energised with 56 V
dc
triggered by the signal V
03
from the monostable
multivibrator (IC10) via a SSR model 240D10 solid state relay (Fig.6) and arranged as shown in Fig.1 and
Fig.2. This action magnetized the carbon-free soft iron core of the electromagnet system which then pulled
(attracted) the steel top of the Γ-shape horizontal surface of the sliding door of the coins receiver upward
from a distance of 5cm below it. As soon as the sliding door moved up, the coins dropped into the coins
collector which was secured on wooden beam B (Fig.1) and the machine was ready for the next user.
Journal of Science and Technology Research, Volume 8, Number 2, 2009
ISSN:1596-9647
Pages: 121 - 131

9
Transformer
Down
Step
of Fig.5
V
From
+
63V
4700uF
Supply
Power
Public (EM)
ac
40V
(SDT1)
ac
220V
Diodes
From
1N5401
Diodes
1N5401
Electromagnet
03
D
CP Q
_
Q
Fig.6 Electromagnet and its control circuitry
2.2.5 The motor speed control circuit:
The rotation of the electric motor was not timed but controlled by an infrared opto-coupler fixed
on a wooden structure and secured in such a way that the upper edge of the plastic bowl was in between
the opto-coupler’s sensor slot (Fig.1). The opto-coupler consists of an infrared transmitter and a receiver
positioned directly opposite each other in a single plastic package [1]. On the upper edge of the plastic
bowl are twelve equal opaque segments separated by small transparent gaps. During construction, the
plastic bowl was positioned in such a way that once the electric motor is stationary, the opto-coupler’s
sensors were always in between one of the opaque segments on the plastic bowl. The motor speed control
circuit is shown in Fig.7 and it took care of the starting torque, the rotary speed as well as ensuring a well-
controlled delivery of the product. It was desired that the electric motor be energised until the product was
supplied and the opto-coupler’s sensors repositioned; immediately after which the motor should be
automatically disconnected. When the required of coins has been loaded, V
03
goes high and the voltage at
point A goes high. At this instant, one of the opaque segments at the top of the bowl isolates the opto-
coupler’s transmitter and receiver; and thus the output voltage of the opto-coupler at point B is high. The
opto-coupler’s output was configured to always be at logic high state except during the delivery operation
when it momentarily goes low but its logic high output state is usually restored by the circuit of IC17.
Therefore, the output of IC12 went high and relay RC2 was triggered, the motor’s rotor was set into
motion and shaft R (Fig.1) on which the plastic bowl rested as well as the plastic bowl began to rotate.
This rotation continues until when one of the holes on the plastic bowl coincides with the hole on the
wooden plywood, the product rolls out of the machine.
At the instant where one of the holes on the plastic bowl coincides with the hole on the wooden
plywood gummed with formica below it, the opto-coupler also encounters the non-opaque gap (small
transparent gap) on the plastic bowl and the output voltage of the opto-coupler at point B immediately
drops to 0V. This negative (0V) pulse places the output of IC12 at logic low while the output of IC14 goes
to a logic high and places the output of IC15 (voltage follower) at +15V which turns on two monostable
multivibrators built around IC16 and IC17. The positive output voltage from IC17 re-triggers relay RC2
and thus causing the rotation of the electric motor for just a second to place the opto-coupler’s transmitter
and receiver in the next opaque segment on the plastic bowl; while the output of IC16 triggers relay RC1
to cut off point A from V
03
as well as the current to the electric motor. However, voltage switching on the
electric motor’s rotor between the actions of IC12 and IC17 was unnoticeable since it was instantaneous
and there was no break in the rotor’s rotation. The entire operation between when RS was closed and the
item’s delivery was less than 2 seconds. D1 and D1 were included to prevent current flow-back either
when IC 12 or IC14 switches on. Hence, the product delivery was independent of neither the motor rotor’s
+ –
SSR
Model
240D10
~ ~
Journal of Science and Technology Research, Volume 8, Number 2, 2009
ISSN:1596-9647
Pages: 121 - 131

10
speed nor the number of items in the plastic bowl (load torque) but was controlled by the opto-coupler’s
sensors positions during and after delivery.
56k
R
56k
NC
+12V
+12V
of Fi g.5
From V
+12V
-15V
-15V
+15V
+15V
i
i
5.6V
Diode
Zener
Zener
Diode
5.6V
CC
100pF
CR
0.01u F
+
B
C
33uF C
C
100pF
+
B
C
33uF
C
0.01u F
+
LM107
IC15
+
LM107
IC13
TIP31 Q4
Elect ric
Motor
Opto-
Coupler
OP4N2 5 BC108
Q3
TIP41
Q2
IN540 1
D2
PMDC
IN400 1
D
IN400 1
D
IN540 1
D1
12Vdc
Gnd
Trg
Out
Rst Ctl
Thr
Dis
Vcc
IC17
NE555
Gnd
Trg
Out
Rst Ctl
Thr
Dis
Vcc
IC16
NE555
Relay RC2
12V
Relay
RC1
12V
B
IC14
TC4011BP
IC12
TC401 1BP
AIC11
TC4011BP
DC
5.6k
( )
Magne t
5.6k
D
R
100k
D
R
100k
Permanent
3.3k
Point
2.2k
Infra red
470R
03
1k
Point
3.3k
NC 2.2k
Fig.7 Motor speed control circuit
2.2.6 The automatic lighting system:
The lighting system is shown in Fig.8 and consists of two 12V 1A white bulbs and its control
circuit. The power supply for the lighting system was built around IC18 (LM317) whose output is give by
Eq.9 with an additional current provided by Q5 [10]. IC18 and Q5 were properly secured on heat sinks.








+=
fix
adj
R
R
V12.1
04 (9)
The bulb WB3 was fixed at the bottom of the top cover of the machine (marked x in Fig.10(b))
while WB4 was fixed to the bottom of the plywood gummed with formica (marked y in Fig.1). The two
switches, SW1 and SW2, were each fixed in between the exterior rear edges of the two doors (Fig.10 (b))
and the right side of the machine in such a way that whenever any or both of these doors are open, the
respective switch completes it’s circuit and illuminates the chamber that has been opened.
on heat
sink
adj
on heat
V04
+
25V
4700 uF
Supply
Powe r
Publi c
ac
15V
(SDT 2)
ac
220V
Diodes
From
1N540 1
Diodes
1N540 1
Transformer
Down
Step
IN
COM
OUT
IC18 on
LM317
IN400 1
+
25V
0.1uF
+
25V
10uF
Q5 2N3 055
+
25V
100u F
2.2k
R
For t heSW 2
Lower
For theSW1
Uppe r
fix
R
240R
(WB4)
Chamber
12V 1A
(WB3 )
Chamber
12V 1A
Fig.8 The interior upper and lower chambers lighting system
2.2.7 The alarm system:
The alarm system is shown in Fig.9 and consists of two 12V 500mA small white bulbs (WB1 and
WB2) and a photodiode (PD), a tone generator (IC20 and IC21), an audio power amplifier (IC22) and an
8Ω speaker. The alarm sensors are made up of WB1 and WB2 as the transmitters while PD serves as the
receiver. The receiver was positioned directly below the last hole just before the fixed hole where the item
being dispensed is delivered to the user while the transmitter was also fixed (suspended) directly above
this last hole. When the item to be dispensed is in this position (above the receiver), the photodiode can
Journal of Science and Technology Research, Volume 8, Number 2, 2009
ISSN:1596-9647
Pages: 121 - 131

11
not receive light rays from the transmitter and hence the photodiode is off and consequently, the alarm is
also off.
However, when the last product being dispensed rolls out of the machine, the photodiode then
conducts and the result is an output voltage of +12V at the output of transistor Q5, via IC19, which serves
as the supply voltage to the circuit of IC20. IC20 is a 555 timer used as an astable multivibrator. The
output taken at pin 3 is high for one half of a cycle for one second and goes low for the next half cycle [8].
When the output is low, IC21 is inhibited and the loud-speaker is off. During the next half cycle the output
is high. Thus, the output of IC21 oscillates at 1 kHz and this oscillating signal was amplified by a 25
Watts audio power amplifier built around IC22, TDA2050 [11], and passed to the speaker situated on the
far right bottom of Fig.10(b). When this alarm is on, it however notifies the sales personnel to reload the
machine. The choice in the alarm volume adjustment was possible with the volume control potentiometer
(R
Vol
) situated on the front of the machine in Fig.10(a) and available to the sales personnel. The sensitivity
of the photodiode was adjusted for various items with help of the 1MΩ potentiometer (R
M
) which also
available but only to the owner of the machine.
1M
R
+
uF
R
470
A
C
0.1uF
A
R
0.01u F
Gnd
Trg
Out
Rst Ctl
Thr
Dis
Vcc
IC20
NE555 1 00k
Gange d
100k
PotA
+15V
TIP31
Q6
+
LM356
IC19
+12V
-15V
Diode
Photo
(PD)
-2V
+12V
+
B
0.1uF
+
uF
1000
+
uF
1000
+
A
R
22uF
+
100uF
A
C
0.01u F
0.01u F
Gnd
Trg
Out
Rst Ctl
Thr
Dis
Vcc
IC21
NE555
(LD2)
RED
LED
100k
PotB
100k
Gange d
+
22uF
+
TDA20 50
IC22
+
0.47u F
100k
Vol
5W
2.2R
(WB1 & WB2 )
12V 5 00mA
White Bulb s
M
5.6k
5W
2.2R
Speak er
35W
22k
680R
5W
2R2
22k
R
470R
22k
B
R
22k
Fig.9 The alarm system.
x
Door of
Upper
Chamber
Door of
Lower
Chamber
(a) (b)
Fig.10 Photographs of the (a) front and (b) interior views of the vending machine.
Journal of Science and Technology Research, Volume 8, Number 2, 2009
ISSN:1596-9647
Pages: 121 - 131

12
3. Conclusion and Discussions:
An automatic electromechanical vending machine (Fig.10) has been developed with components
sourced from Nigerian local markets and the machine satisfied the objectives of its design. The machine
was designed to accept only coins and is suitable for dispensing items such as canned beer, canned and
bottled soft drinks, sachet and bottled water, packet biscuits and toiletries (soaps and tissue papers). The
IA, comparator and R
M
are adjustable depending on the amount of coins decided by the owner of the
machine for a particular item to be sold.
The following are some of the improvements that can be made on this machine. The inclusion of
heating and cooling systems will make it possible to dispense beverages such as warm snacks, hot coffee
and tea as well as cold drinks. However, the incorporation of a multiprocessor field programmable gate
arrays (FPGAs) into the vending machine to accept Naira note currencies, dispense multiple items, issuing
of change for items bought with higher Naira note denominations as well as changing higher
denominations to lower denominations of the Naira notes currencies for users not buying any of the
dispensed items will be a novel achievement for a future work.
REFERENCES:
[1]. T. Melling. RS Catalogue. East Sussex, U.K: Enigma Corporation Limited, 2003.
[2]. M.T. Babalola and V.A. Akpan, “Experimental Determination of the Poisson’s Ratio (µ ), and the Gauge Factor (k)
Values of a Practical Strain Gauge (120 – SRA Foil Type)”, Ife J. of Sci., Vol. 8, No. 1, pp. 47-58, (2006).
[3]. R.F. Coughlin and F.F. Driscoll. Operational Amplifiers and Linear Integrated Circuits, 4th Ed., New Delhi,
India: Prentice–Hall, 1992.
[4]. S. Franco. Design with Operational Amplifier and Analog Integrated Circuits. 2nd Ed., Singapore: McGraw-Hill
Books Company, 1998.
[5]. P. Horowitz and W. Hill. The Arts of Electronics, 2nd Ed., London: Cambridge University Press, 1997.
[6]. A. Mottershead. Electronic Devices and Circuits. 1st Ed., New Delhi–110001, India: Prentice-Hall, 1981.
[7]. M.H Jones. A Practical Introduction to Electronic Circuits. 1st Ed., London: Cambridge University Press, 1979.
[8]. D.V. Hall. Digital Circuits and Systems. Singapore: McGraw–Hill,1989.
[9]. W.D. Stanley. Operational Amplifier with Linear Integrated Circuits. 2nd Ed., Singapore: Macmillan Publishing
Company, 1990.
[10]. A.R. Ronald. Digital Electronics Through Project Analysis. 2nd Ed., New York 10022, U.S.A.: Macmillan
Publishing Company, 1991.
[11]. Maplin Professional System. A Product Handbook (Project and Modules). Rayleigh, Essex. UK: Maplin
Components, 2003.
Journal of Science and Technology Research, Volume 8, Number 2, 2009
ISSN:1596-9647
Pages: 121 - 131