This is only a preview of the August 1990 issue of Silicon Chip. You can view 57 of the 112 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "Digital Sine/Square Wave Generator; Pt.2":
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|
Universal safety timer
for irons & kettles
How many times has someone in your house
left the iron on or allowed the electric kettle to
boil dry and burn out the element? This circuit
solves those problems by automatically
switching off the power after 9 minutes unless
a reset button is pressed. An alarm warns of
impending power shutdown.
By JOHN CLARKE
Although specifically intended
for use with irons, this Iron Timer
can also be used with many other
appliances. It is an in-line device
which plugs into the power point,
while the controlled appliance (an
iron, electric kettle, lamp etc) plugs
into the timer itself. A Triac is used
to control the power of the appliance while CMOS ICs perform
the timing function.
When first switched on, the Iron
Timer supplies power to the ap-
pliance for 9 minutes and then
sounds an alarm. If the reset button
is now pressed, the timing cycle
restarts and the unit supplies
power to the appliance for another
9 minutes. However, if the reset
switch is not pressed, the power
switches off after a further 16
seconds.
Because mains voltages are involved and some heatsinking is required, the project is housed in a
diecast metal case which is secure-
The Iron Timer can be used with appliances rated up to 2.4kW. It is installed
in series with the appliance and automatically switches off the power after 9
minutes unless the Reset button is pressed.
20
SILICON CHIP
ly earthed. The two power leads
emerge from either end of the case
- one fitted with a 3-pin plug and
the other with a 3-pin mains socket.
The reset switch is on the lid of the
box, along with a LED power
indicator.
Before electing to build this project though, you should note that it
is unsuitable for use with inductive loads such as induction motors.
Apart from this limitation, you can
use the project with virtually any
appliance that presents a resistive
load and is rated up to 2.4kW.
It's also quite easy to vary the
time-out period over quite a wide
range. This involves altering an RC
time constant in the circuit to give
the required period - more about
this later.
How it works
Fig. l shows the circuit details. It
uses 3 CMOS ICs (ICl-3) for the timing function plus an optoisolator
(IC4) to drive the Triac (TRIACl).
Note that this circuit is potentially dangerous. If you touch components inside the box while it is
powered up you could be electrocuted. That means you could be
DEAD! If you are not confident
about building a purely mainspowered circuit with no isolating
transformer, then give this one a
miss.
Having scared off half our
readers, let's look at the circuit in
more detail.
The low voltage supply for the
circuit is derived directly from the
mains via the 0.47 /.lF capacitor
which drops most of the mains
potential via its reactive impedance. A bridge rectifier (Dl-D4)
rectifies (what else?) the incoming
AC and the output is limited to 15V
DC by zener diode Dl and filtered by another 470/.lF capacitor. The
470!1 1W resistor limits the initial
switch-on surge current to a safe
Fl
10A
0.4 7
250VAC
S1
RESET
+15V
I
680!!
1W
3
014 1"-_.,--J--k--l--'
10k
01
BC328
IC2
4020
.056!.
OSCILLATOR
1M
IC1
7555
MASTER
OSCILLATOR
DIVIDER/COUNTER
B
EOc
VIEWED FROM
BELOW
CASE
value for the rectifier diodes and
the 470µ,F capacitor.
Note that the + 15V line on the
circuit is positive with respect to
the Neutral line. If the wall outlet is
wired correctly, you could safely
touch any part of the low voltage
circuit and not get a shock. But if
the wall outlet (power point) has the
Active and Neutral transposed,
then all the low voltage portion of
the circuit will actually be sitting at
the full 240V AC and will therefore
be extremely dangerous. So don't
-consider doing anything to this circuit while it is powered up. OK?
Having said that, it is possible to
safely test the circuit by powering it
from a low voltage supply. We'll tell
you how to do that later in the
article.
Timing oscillator
ICl is the master oscillator for
the circuit. This stage consists of a
CMOS 7555 timer which is connected for astable operation. Its
output frequency is set to 15 .2Hz by
the lMO resistor and .047µF
capacitor on pins 6 & 2 and so these
components determine the period
for which the appliance is on.
The 555 timer stage works like
this: Initially, the output, pin 3, is
LOAD
IRON TIMER
Fig.1: the circuit uses a 555 timer (IC1) to clock a 4020 binary counter (IC2).
IC2's Q outputs in turn control Schmitt trigger gates IC3a-d and these drive
transistor Qt and the piezo transducer. IC4 controls TRIAC1 which switches
the load current.
high and so the .047µF capacitor
begins charging via the lMO
resistor. When the voltage across
the capacitor reaches 2/3Vcc (after
time 0.695RC), pin 3 switches low
and the capacitor then discharges
via the lMO resistor until it reaches
1/3Vcc. This switches pin 3 high
again and thus the cycle is repeated
indefinitely to give a 15.2Hz square
wave output.
This 15.2Hz signal is used to
clock IC2 which is a 4020 14-stage
binary counter. When power is first
applied or the reset switch is pressed, the reset input (pin 11) of IC2 is
pulled high and this sets all the Q
outputs low. As soon as Sl is released, the 10µ,F capacitor quickly
charges via the 1ookn resistor and
this pulls pin 11 low to release the
reset. IC2 now begins to count the
clock signal from ICl.
Four Q outputs of IC2 control
three Schmitt triggers, IC3a, IC3c &
IC3d. Let's look at IC3a first. After
reset, Q9 and Q14 of IC2 are low
and pin 3 of IC3a is high. This
means that pin 4 (reset) of ICl is
also high and so ICl is enabled (ie,
it oscillates at 15.2Hz).
At the same time, pin 4 of IC3b
will be low and thus transistor Ql ,
LED 1 and the internal LED in IC4
are all on. The internal LED in turn
activates the optical Triac in IC4
and this drives the gate of TRIACl to
switch power to the load.
The low on Q 14 also pulls pin 8 of
IC3c low. Thus, pin 10 of IC3c remains high and no sound is produced by the piezo transducer. At the
same time, Q14 pulls pin 13 of IC3d
low via diode D5 and this prevents
IC3d from oscillating. This is done
to ensure that there is no low-level
feedthrough to the piezo transducer.
The circuit remains in this state
until Q14 goes high which does not
happen until 535 seconds, or about
9 minutes, have passed. When Q14
goes high, it pulls pin 8 of IC3c high.
At the same time, diode D5 is
reverse biased and pin 13 of IC3d is
pulled high by its lOOkO resistor.
IC3d then begins to oscillate at
about lkHz, as set by the 10k0
resistor and .056µ,F capacitor at its
pin 12. This is the signal that drives
AUGUST 1990
21
Fig.2: here's how to
wire up ·the Iron
Timer. Check that the
mains cords are
securely clamped and
be sure to use 240V
AC cable for all mains
wiring. The Triac is
bolted to the bottom
of the case and this
provides the
necessary heatsinking.
0
MAINS CORO
FROM
MAINS
\
CORO CLAMP
GROMMET
,J
~-------<
I
CORO CLAMP
GROMMET
1---------1
TO LOAD
0
0
0
3 4
AUK
LE01
0
0
RESISTOR CODES
□
□
□
□
□
□
□
No
1
2
Value
1MO
100k0
10k0
68on
4700
3900
2
1
1
1
the piezo transducer.
Actually, a flat lkHz tone produced by the piezo speaker is not
very noticeable. To make it more
noticeable it is modulated on and
off by the Q4 output of IC2. Q4 goes
high for 520ms and then low for
520ms continuously, and this low
frequency square wave modulates
IC3d via diode D6.
Now consider the Q9 output. It
goes high 16.7 seconds after Q14
goes high, during which time the
4-Band Code
brown black green gold
brown black yellow gold
brown black orange gold
blue grey brown gold
yellow violet brown gold
orange white brown gold
piezo oscillator is enabled. Actually, Q9 will have been changing state
every 16.7 seconds but it does not
come into play until this moment.
When Q14 and Q9 go high
together, IC3a's output goes low.
This signal is inverted by IC3b
which drives transistor Ql. With
Ql 's base now pulled high by IC3b,
it has no choice but to turn off. This
turns off the optoisolator (IC4) and
so the Triac turns off too.
The other result of IC3a's output
CAPACITOR CODES
□
□
□
□
22
Value
0.47µF
.056µF
.047µF
Alt Value
470nF
56nF
47nF
SILICON CHIP
IEC Code
470n
56n
47n
5-Band Code
brown black black yellow brown
brown black black orange brown
brown black black red brown
blue grey black black brown
yellow violet black black brown
orange white black black brown
EIA Code
474
563
473
going low is that it pulls pin 4 of ICl
low. This stops ICl and so the circuit stops operating altogether.
Reset
If the reset switch is pressed
before the load is switched off, IC2
is reset and mains power is maintained for another nine minutes or
so, when the circuit will again
switch off the power.
Alternatively, if the circuit has
already powered down and the
reset switch is pressed, the power
is restored to the load for another
nine minutes or so.
Note that by increasing the timing capacitor at pin 2 of ICl, the
overall timing period can be increased in proportion. Want the
circuit to run for ten times as long
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PARTS LIST
Because mains voltages are involved, the circuit must be housed in a metal
diecast case which is securely earthed. Don't substitute for any of the
specified parts and don't work on the circuit while it is powered from the
mains - it could be dangerous (see warning panel).
- ie, 90 minutes? Simple - just increase the .047 µ,F capacitor to
0.47µ,F. Want it to run for around 7
hours? increase the .047µ,F
capacitor to 2.2µ,F.
Construction
All of the circuitry for the Iron
Timer is assembled onto a PCB coded SC 10107901 and measuring 100
x 60mm. It is a tight fit in a diecast
aluminium box measuring 122 x 67
x 43mm.
The Triac connects to the underside of the PCB so that it can be
directly mounted onto the base of
the box for heatsinking. Note that
the specified Triac is an isolated
tab device - it can be mounted
directly onto the inside of the earthed case and no mica insulating
washers are required.
You can start construction by installing the oarts on the PCB. Install
the 10 PC stakes first , then the
low profile components such as
resistors, links, ICs and diodes.
Make sure that the diodes and ICs
are in the right way around, otherwise the circuit will not work.
Zener diode ZDl is installed with a
loop in one of its leads. This is to
24
SILICON CHIP
reduce mechanical stress as the
device varies in temperature.
Once the low profile components
have been installed, mount the fuse
clips, fuse, capacitors and the transistor. This done, mount the Triac
so that the lower face of the tab is
6mm below the PCB. The tab of the
Triac should overhang the PCB
edge by 6.5mm. This will allow the
Triac to sit comfortably on the base
of the box while the PCB sits on the
6mm standoffs. Also, the overhang
of the Triac allows access to the
mounting screw.
The PCB and Triac assembly can
now be temporarily installed in the
box so that the drilling locations for
the PCB corner mounting screws,
Triac tab and earth solder lug can
be marked. Locate the solder lug
position at the opposite end of the
PCB to the Triac and as close as
possible to the edge of the case so
that the screw is accessible with
the PCB in place. After drilling, all
holes should be deburred using a
larger drill, particularly the Triac
mounting hole.
The Triac is simply bolted to the
case with a smear of heatsink compound between the case and the
1 PC board, code
SC10107901, 1 00 x 60mm
1 Dynamark label for front
panel, 120 x 65mm
1 diecast aluminium box, 1 22 x
67 x 43mm
1 plastic momentary push on
mains rated switch ; DSE Cat.
P- 7 5 50 (do not substitute)
1 piezo transducer
1 red LED and bezel
2 mains cord grip grommets
1 metre 1 OA mains wire
1 mains line plug
1 mains line socket
2 PCB 3AG fuse clips
1 1 OA 3AG fuse
6 6mm standoffs
4 screws and nuts for securing
PCB
1 screw and nut for Triac
mounting
1 screw, nut & star washer
2 screws and nuts for piezo
transducer
1 solder lug
10 PC stakes
4 adhesive rubber feet
1 70mm-length of 1 0mm dia.
heatshrink tubing
Semiconductors
1 7555 CMOS timer (IC1)
1 4020 14-bit binary counter
(IC2)
4093 quad Schmitt NANO
gate (IC3)
1 MOC3021 optically isolated
Triac driver (IC4)
4 1 N4004 1 A 400V diodes
(D1-D4)
2 1 N914, 1 N4148 diodes
(D5 ,D6)
1 BC328 transistor (01)
1 15V 1 W zener diode (ZD1)
1 MAC320A8FP 20A 600V
insulated tab Triac
Capacitors
1
1
1
1
1
4 70µ,F 25VW PC electrolytic
1 Oµ,F 25VW PC electrolytic
0.47µ,F 250VAC capacitor
.056µ,F metallised polyester
.04 7 uF metallised polyester
Resistors (0.25W, 5%)
1 1 MO
1 6800 1W
2 100k0
1 4700 1W
2 10k0
1 3900
Miscellaneous
Solder, heatsink compound , tinned copper wire.
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• Similar performance
to 0S7020A
0S8020R $1045 ex tax $1243.55 Inc tax •
Supported & Serviced Australia-wide by
Elmeasco Instruments Pty Ltd
N SW (02) 736 2888 Vic (03) 879 2322 Old (07) 875 1444
S.A. (08) 344 9000 W.A. (09) 470 1855
You've Gotta Getta Goldstar from:
A.C.T. John Pope Electrical 806576 N.S.W. D.G.E. Systems (049)691625e W.F.Dixon (049)
69 5177 • Macelec (042) 291455 • Newtek (042) 271620 • Novacastrian Electronic Supply
(049) 62 1358 e Obiat Pty Ltd 698 4776 e Digitel 709 6511 e Geoff Wood 427 1676
N. TERRITORY J Blackwood (089) 84 4255, 52 1788 e Thew & McCann (089) 84 4999
QUEENSLAND St Lucia Electronics 252 7466 • BAS. Audiotronics 844 7566 • The Electronics
Shop (075) 32 3632 • Solex (Townsville)(077) 72 4466 • Xanlhos Electrical (079) 27 8952
S.AUSTRALIA Force Electronics (08) 212 5505 VICTORIA G.B. Telespares 328 4301 e The
Electronic Components Shop 670 6474 • Factory Controls (052) 78 8222 • Mektronics 587
3888 • Truscott Electronics 723 3094 W AUSTRALIA Atkins Carlyle 481 1233 • Leda
Electronics 361 7821 PAPUA NEW GUINEA TE (P.N.G.) Pt Moresby 25 6322 Lae 42 6246
~Recommended prices only
Install the zener diode with a loop in one lead to provide stress relief as the
device warms up. Immediately to the left of this diode is the 0.47µF capacitor.
This device must be rated at 250V AC.
mounting tab to improve heat transfer.
Now mark out the locations for
the cord grip grommets. The inlet
(plug) lead is located on the solder
lug terminal end of the box. Mark
out the position for this cord grip
grommet in the centre of the box
and 15mm below the top edge.
The outlet (socket) lead is located
on the Triac end of the box but is
located slightly off centre away
from the Triac to allow access to
the Triac screw. Mark out the position for this cord grip grommet
15mm down from the top edge of
the case and 15mm apart from the
Triac mounting screw.
Drill out the holes required and
file out the cord grip grommet holes
using a round file. The cord grip
grommet must be able to securely
grip the mains cord, so do not make
the opening too large.
At this stage, the holes in the lid
can be marked out and drilled. The
Dynamark label can be used as a
guide for the hole positions. The six
hole positions at the end of the label
are for the piezo transducer. The
four inside holes are to allow the
sound to pass through the lid while
the two remaining holes are for
26
SILICO N CHIP
securing screws. These screws
secure the transducer to the underside of the lid on 6mm spacers.
Alternatively, you could use Superglue.
Note that the four sound exit
holes are all that are necessary for
this purpose. Do not drill an extra
hole in the centre since this could
allow someone to make contact
with the metallic disc of the
transducer with a metallic object
WARNING!
Under some circumstances
(eg, if the Active & Neutral wires
are transposed at the power
point or in an extension lead), the
low voltage portion of this circuit
will actually sit a 240V AC and
could therefore be extremely
dangerous.
For this reason, never attempt
to work on the Iron Timer while it
is powered from the mains. If you
do, you could get a fatal shock.
The only safe way to test this
circuit is to power it from a 1 2V
DC source as described in the
text.
such as a screwdriver - a very
dangerous situation.
The hole for the LED is made
large enough for the bezel, while
you will need to drill and file out a
square hole for the pushbutton
switch. Incidentally, the reason we
have specified this square switch
rather than a larger one with a
round mounting hole is to give adequate clearance to the circuit
board underneath.
Once the Dynamark label is
secured to the lid, the holes can be
cut out with a sharp knife to allow
the switch, piezo transducer and
LED to be mounted.
The PCB can now be installed in
the box. First, apply a smear of
heatsink compound to the lower
face of the Triac, then insert the
four corner screws for the PCB.
This done, install a 6mm spacer
over each screw, then lower the
PCB into position and do up the
nuts. The Triac tab can now be
secured with a screw and nut.
For each of the mains cords, strip
150mm of sheathing off each cable
end to allow termination to the PCB
and connection of the plug and
socket. The connections to the plug
and socket must be as shown in the
packaging information supplied
with them.
Secure each mains cable with the
cord grip grommet and connect the
leads as shown in the wiring
diagram. It's best to solder the two
earth leads to the solder lug before
securing it to the base of the case. If
you screw the lug to the case and
then attempt to solder it, you may
have difficulty since the case will
act as a good heatsink and possibly
cause a dry solder joint. Use a star
washer between the nut and solder
lug to prevent the nut from coming
loose.
Finally, install insulating sleeving
over the wires running from the
piezo transducer and the LED to the
PCB terminals. This will prevent
any tendency for insulation
breakdown due to the possibility of
high voltage between the wires and
case.
Testing
When the wiring is complete, the
unit is ready to be tested. Don't connect the circuit to the mains yet. In-
SEE IN THE DARK
with our
0
0
See EA MAY 1990: Plans for
new housing included.
0
IRON TIMER
RESET
0
I.!..
I!!.=-
POWER ON
(2400 WATTS MAXIMUM 10A FUSE INSIDE)
Fig.3: here is a full-size artwork for the front panel.
stead we recommend testing the
circuit with a low voltage power
supply set to 12V DC.
The leads from the power supply
are connected across the zener
diode, with the positive lead going
to the striped end. Do not set the
supply to 15V or above otherwise
the zener diode will be blown.
Check that + 12V is present at
pin 8 of ICl, pin 16 of IC2, pin 14 of
IC3 and the emitter of Ql. That being the case, LED 1 should be lit.
Now check the DC voltage at pin 3
of ICl - this should be at about
half supply voltage because it is actually oscillating with a square
wave output of about 15Hz. If
everything is OK wait for about 9
minutes and check that the alarm
sounds. When this happens, the
LED should go out after a further 16
0
I
a~111)
C.c"E::m-
INFRARED NIGHT VIEWER
0
* PCB and components kit (tripler included) $59.95
0
* Case Kit .
. ..... . ..... $27.00 (no lenses)
* New lnfrared Image Tubes at approx. $200-$400
* Other lower priced tubes may be available: P.O.A.
0
I BARGAIN PRICED LASERS! I
-:..!1
seconds. The LED should light again
if the reset switch is pressed.
If the circuit does not function,
check for short circuits between
tracks, wiring errors and missed
solder joints. You can also check
the operation of each IC by measuring pin voltages. The outputs of IC2
& IC3 should be either high or low
as set out in the circuit description.
If everything checks out thus far,
the circuit can be tested on the
mains. First, secure the lid of the
case, then plug the device into the
mains without a load at the socket
end. If the LED lights when powerd
up, the Iron Timer can be tested
with a load. Check that the iron (or
other appliance) is initially on and
that the alarm sounds after about 9
minutes. The unit should then
switch off the power.
~
Complete 240V Laser Kit
(EA July 1990).
With 0.5-0.BmW Tube
... $239.90
With 3-4mW Tube
...... $349.50
;~1·
CURRENTLY WE ARE OFFERING THIS KIT WITH A
1-1.5mW TUBE FOR THE INCREDIBLY LOW PRICE
OF ONLY
................ $269.90
TUBE ONLY: Simply subtract $110.00 from the
above prices.
KIT ONLY: $120.00
A DC power supply kit for HeNe laser tubes will be
available at a cost of under $100.00.
LASER DIODES
Low voltage operation: for
communications, security, simple
laser pointers, etc, etc. 5mW,
infrared diode with collimator
lens assembly.
780nm (barely visible)
$89 .00
Visible laser diodes .......... P.O.A. (expensive)
A combined power supply and digital switch kit for
laser diodes is available for under ........ $30.00
JUST SEND US AN SAE AND WE WILL SEND YOU A
COMPLETE LISTING ON OUR LASER TUBES, LASER
DIODES AND ACCESSORIES: AT INCREDIBLE PRICES.
CHECK OUT THESE VERY SPECIAL
PRICES ON SOME STOCK ANO LIMITED
STOCK ITEMS:
....... $19.00
PIN Diodes
Very Fast Detectors For Lasers Etc
Large IA Detector Diodes
..... . .. . ... $3.50
Lenses For Detector Diodes
. $4.30
Mirrors For Deflecting Laser Beams .
. $4 .20
X-Y Scanners For Laser Beams .
. . $200.00
Photomultiplier Tubes
. $199.00
OATLEY
ELECTRONICS
PO BOX 89, OATLEY, NSW 2223
Telephone: (02) 579 4985
Fax No: (02) 570 7910
Certified p&p $4 -$6 Aust: NZ add $2.
Distributors : slightly higher prices may apply.
Fig.4: you can use this pattern to etch your own PC board.
MELBOURNE:
Electronics World - (03) 723 3860
AUGUST 1990
27
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