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Build this really snazzy ••• EGG TIMER How many eggs have you seen that could keep time? This one can - it ticks away the seconds and then plays a musical chime. We must admit it looks a bit weird for an egg, with a switch at its pointy end and a knob on its side, but what do you eggspect? By DARREN YATES They say one of the best things in life is a perfect 3-minute egg; that nottoo-hard, not-too-soft, just right edible wonder that has had man searching the globe for centuries. However, unless you 've had the time to sit and watch the water boil as you count away the minutes, the results may have ended up anywhere between a sloppy, watery mess or something that resembles the solidity of your average golf ball! But man, in his usual adaptive style, invented the egg timer, that marvel of modern engineering, to save him from this time-consuming chore of creating the perfect egg! Here at SILICON CHIP, eggs for lunch have never been high on the menu, but we realised that the humble egg timer is well overdue for a facelift both on the inside and the outside. As you can see, what we've come up with resembles something like an egg although most eggs won't sit on their big end. This one does. That's all well and good but what if you hate eggs? Well, we've catered for other applications by providing an extended timing range up to 17 56 SILICON CHIP minutes. You could use it as a move timer for a game of chess, for example (and completely scramble his game at the same time)! The biggest bonus about this proj- The Egg Timer is built into an eggshaped case that normally contains a Pavlova mix. You can vary the time from 15 seconds to about 17 minutes. ect is that the egg-shaped case can be bought from just about any supermarket for just over $2 - and you get a free Pavlova mix thrown in! To be specific, it is a Pavlova Magic egg. Buy one today and have a pavlova on the weekend. Well, that's what the outside of the timer looks like. Now we'll delve inside and see how the circuit works. Circuit details If you look at the circuit diagram in Fig. l, it may appear at first to be a little complicated. But as we will soon show, it is fairly simple. The circuit uses three ICs: a 4093 quad Schmitt trigger NAND gate, an LMC555 CMOS timer, a 4001 quad NOR gate and not much else. Most circuits based on the 555 timer IC use two series resistors to charge and discharge a capacitor. This works well but when when long time delays are wanted (more than 10 minutes for example), the resistors and the capacitor have to be quite large in value. This gets to be a real problem when the capacitor value has to be around lO0µF or more because capacitor leakage becomes the limiting factor. You get to the point where the capacitor leakage is much the same as the charging current and so the timer circuit does not work. The technique we've used instead is to dump small discrete amounts of current into a charge pump circuit several times per second. Because it's no longer a continuous current flow, it takes a lot longer to charge up the capacitor. So if we make the number of r;urrent pulses per second quite D2 ,--_.,._._+9V IC2 LMC555 3.3k +9V 56k E 100 16VW ·~ ~+9V ~ S1a T 9V : ...L.. l 56k ... B EOc ... VIEWED FRDM BELDW TICKING EGG-TIMER Fig.1: ICla operates as a variablefrequency pulse generator & is used to charge the 6.8µF timing capacitor on pins 6 & 2 ofIC2. IC2 is a CMOS version of the 555 timer. When the voltage across the 6.8µF capacitor reaches 2/3 Vee, its output goes low & triggers the chime generator circuit (IC3a,b,c & IClc,d). The chime generator circuit then drives complementary output stage Ql & QZ. All the King's horses & all the King's men ... yes, it really does all fit inside the Pavlova Magic case. Power comes from a small 9V battery. small , we can also reduce the size of the capacitor needed and still produce a sizable time delay. The technique works because we can use a relatively small capacitor which has a much lower leakage. To make this a bit clearer, imagine you're filling a big bucket of water from a tap. If you leave the tap running continuously, it will take a certain amount of time to fill it. Now if you try to fill it again, but this time turning the tap on, off, on, off, so that for half the time it's on and for the other half it's off, it will take twice as long to fill it. In this circuit, the length of time that the tap is on is kept constant, say a second if you like. The time the tap is left off, though, is varied. So if the tap is on for one second in every 10, obviously it must take 10 times as long to fill the bucket than if the tap was left on continuously. This holds true whether it's a half a second in every five or a quarter of a second in every two and a half. The important thing is the ratio of how long the tap is on compared to it being off. This is called the "mark/space ratio". By varying this ratio, we can vary the time it takes to fill the bucket; or in our case, the time delay before the alarm sounds. This technique allows us to use a small low leakage tantalum or aluminium electrolytic capacitor and still generate a time de~ay of up to 17 minutes. NAND gate ICla is connected as a variable-frequency pulse generator. It produces pulses of fixed width, no matter what the frequency, and we actually change the mark/space ratio by changing the frequency. Mark/space ratio In our circuit, diodes Dl and D2 and variable resistor VRl do the job of changing the mark/space ratio. If we assume that the output of ICla has just gone high, then a current flows through diode D1, through the 3.3kQ resistor, and charges up the O. lµF capacitor. The voltage across the capacitor increases until it reaches the threshold voltage. ICla now sees a high on its inputs , and so its output NOVEMBER 1990 57 Because the charge can take anywhere up to 17 minutes to develop across the capacitor, if we put too heavy a load on it, the voltage will drop and the timing will be out. The CMOS 555 has inputs with an impedance of 10 12 ohms (or one million megohms) and so does not upset the circuit. While the 6.8µF capacitor is charging up, the internal flipflop of ICZ is reset and the output at pin 3 is high. This output directly controls two low frequency oscillators formed by Schmitt NAND gate IC1b and IC1c and two NOR gates, IC3a and IC3b. Clicking oscillator While ICZ's output is high, IC1b is enabled and oscillates at a frequency of about 0.5Hz. This produces a "click" about once a second to indicate that the timer is running. When the 6.8µF capacitor voltage at pin 6 of ICZ is charged to 6 volts, the output at pin 3 goes low. This disables the ticking oscillator ICl b and allows the oscillator formed by NOR gates IC3a and IC3b to begin oscillation. This is another low-frequency oscillator which drives a 2tone alarm, indicating that the time delay has expired. Tone oscillators Here's how everything fits together inside the case. Use small pieces of foam rubber or plastic insulation to ensure that there are no shorts from the loudspeaker or battery to the PC board. The two halves of the case are simply clipped together & are held by matching grooves. goes low. The capacitor now discharges back through the 3.3kQ resistor, diode D2 and the 500kQ potentiometer, VRl. Depending on its setting, it takes a longer time for the capacitor to discharge than to charge, resulting in the output staying low for a longer period. Also connected to the output pin of ICla is diode D3, a lMQ resistor and a 6.8µF capacitor (for th.:- time being we'll ignore the 555) . This part of the circuit represents our "water bucket" analogy. Whenever the output of IC1a is high, a current flows through D3 and the lMQ resistor, charging the 6.8µF capacitor. When the output is low, 58 SILICON CHIP the diode no longer conducts and the capacitor stores the voltage across it. Next time the output goes high, the capacitor charges up a little more and so on. This part of the circuit is called a "charge pump" because we are pumping and storing a charge in the capacitor. The lMQ resistor and the 6.8µF capacitor set the base time constant of the circuit. Increasing the value of either component increases the base time delay. ICZ is a CMOS version of the 555 timer, connected here as a threshold detector. It also acts as a controlled time delay for repeating the alarm, which we'll talk about a little later. The output of IC3a is connected directly to NAND gate IClc and to gate IC1d via NOR gate IC3c. IC3c acts as a switched inverter. It is turned on via a low output from pin 3 of ICZ (the 555) and inverts the drive signal from the output of IC3a. This allows the two tone oscillators, IC1c and ICld, to operate alternately. NOR gate operation The reason that NOR gate IC3c is in the circuit is that both tone oscillators need to switch off at the same time, but operate alternately to produce the two tones. When the output of ICZ is high, the output of gate IC3a is held low and so is the output of IC3c. These hold one input of both IClc (pin 8) and ICld (pin 13) low, so both oscillators are off. However, when the output of the 555 goes low, IC3a is enabled and so is IC3c, which now acts as an inverter, providing the opposite phase enabling signal for ICld. The outputs of the three oscillators, IClb, IC1c and IC1d, are mixed PARTS LIST 1 Pavlova Magic shell case 1 PC board, code SC08110901 1 Dynamark clear label artwork 1 DPDT pushbutton switch 1 57mm 8Q loudspeaker 2 PC stakes 2 pieces of double-sided foam tape (see text) 1 9V battery (Eveready 216 or equivalent) 1 9V battery snap connector 1 500kQ log potentiometer 1 knob to suit pot Semiconductors 1 4093 quad Schmitt trigger NANO gate (IC1) 1LMC5~,~55CMOStim~ (IC2) 1 4001 quad NOR gate (IC3) 1 BC338 NPN transistor (01) 1 BC328 PNP transistor (02) 3 1N4148, 1N914 signal diodes (D1,D2) Fig.2: make sure that all parts are correctly oriented when installing them on the PC board & take care not to confuse Qt & Q2. The external wiring leads should be made long enough so that everything can be correctly positioned inside the case. together via three lOkQ resistors and coupled to the output stage. This stage is formed by transistors Ql and QZ which drive the speaker. Once the output of ICZ goes low, causing the 2-tone oscillator to sound, the 6.8µF timing capacitor is discharged via the 470kQ resistor, until it reaches 3 volts. This causes the output of ICZ to go h igh again , and so the alarm tone stops and the cycle starts again. The end result is that the alarm CAPACITOR CODES 0 0 0 0 Value IEC Code 6.8µF 6u8 100n 0.1µF .018µF 18n EIA Code 685 104 183 rings for approximately 3 seconds after the circuit reaches its preset time and then rings every half time setting after that; ie, if the time setting is 10 minutes, the alarm rings after 10 minutes and then every five minutes after that. Why is that? Because the 6.8µF capacitor takes twice as long to initially charge to 6 volts as it does to charge from 3 volts to 6 volts in the succeeding cycles. The circuit is reset by switching it off and then on again. Construction When you buy or make the printed circuit board, check that there are no shorts or breaks in any of the copper tracks, particularly around the pads of the ICs. If there are any, correct them now. You may also need to trim Capacitors 1 100µF 16VW PC electrolytic 1 6.8µF 25VW tantalum or low leakage electrolytic 1 2.2µF 63VW PC electrolytic 4 0.1 µF metallised polyester 2 .018µF metallised polyester Resistors (0.25W, 5%) 3 1MQ 2 56kQ 1 470kQ 3 10kQ 1 120kQ 1 3.3kQ 1 100kQ Miscellaneous Solder, washers, nuts , hookup wire, etc the board, so· that it will fit into the Pavlova Magic egg case . Once you're sure the board is OK, insert the PC pins and solder in the TABLE 1: RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 No . Value 4-Band Code (5%) 5-Band Code (1°/!') 3 1MQ 470kQ 120kQ 100kQ 56kQ 10kQ 3.3kQ brown black green gold yellow violet yellow gold brown red yellow gold brown black yellow gold brown blue orange gold brown black orange gold orange orange red gold brown black black yellow brown yellow violet black orange brown brown red black orange brown brown black black orange brown brown blue black red brown brown black black red brown orange orange black brown brown 2 3 NOVEMBER 1990 59 i:7 10 I~., E E 3 }IME (MINS)___ --- i EGG~ Figs.3 & 4: here are actual size artworks for the PC board & front panel. wire links and the resistors. Use a multimeter if the resistor band colours are not very clear. Next, install the polyester capacitors. Remember to keep their lead lengths as short as possible because of the tight fit. After that , solder in the electrolytics capacitors, making sure that each has correct polarity. The 6.8µF capacitor can be a tantalum or low leakage aluminium electrolytic. Now insert the semiconductors , again making sure of their correct polarity. Once you've completed the board, check over it again for any solder splashes or dry joints. Add one egg Got your pavlova egg? Had your pavlova? Good, now drill the egg shells. When drilling holes through the egg shell, do it slowly as the shell has a tendency to bend and may crack if you go too hard, too fast. If you prefer, use a small drill first and then finish the holes to size with a tapered reamer. The hole for the switch goes straight through the top. The holes for the pot and the speaker simply go on either side of the upper section and there are a couple of holes in the bottom half of the shell for the speaker as well. The 2-pole pushbutton switch is fairly easy to wire up as it doesn't matter which way round you use it, so long as you keep the lead orientation the same. The second pole is used to short out the capacitor when the timer is turned off, so that when it 60 SILICON CHIP is restarted, the capacitor has no voltage across it, and we get a correct time delay. If you find that 2-pole pushbutton switches are hard to obtain, then you can do away with the second pole and just use a single pole pushbutton switch. The 6.8µF timing capacitor will discharge through the circuit but it will take about half a minute to get close to zero volts. Filling the egg When putting the whole thing together, the magnet side of the speaker faces the component side of the board. To stop any shorts from occurring, we suggest you use a piece of double- sided foam tape to attach the speaker magnet directly to the board. The 9-volt battery goes on the other side of the board, again spaced by a piece of foam tape or something similarly non-conductive. Note that it's easier to wire up the pot and the switch before mounting them into the egg shell case. This done, take hold of the board, with the battery and the speaker on either side, and place them in the bottom section of the egg shell so that the copper side of the board faces the front panel area. In order to connect the two shell halves together, you'll need to tilt the board forward about 10°, to clear the base of the switch. If you do it correctly, the base of the switch should just fit over the top of the 555 IC and be perpendicular to the 0. lµF capacitor alongside it. The lid should then just squeeze together with the bottom section, making a good, tight fit. Operation Drill several holes in the back of the case to allow sound to escape from the loudspeaker. It's best to use a small drill first & then finish the holes to size with a tapered reamer. To use the Egg Timer, set the time control to the desired length and then press the button. You should now hear it ticking away merrily. Once the alarm has sounded and you need to use it again, turn it off, set the time, and turn it on again. If you strike problems, switch off immediately & check for wiring errors. Note that the accuracy of the calibrations will depend on your particular potentiometer. Now, all you need to do is start enjoying the eggs of your labour! ~