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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. 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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. For Features and Performance ... 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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