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No more morning din! Fit a kill switch to your smoke detector These days, with more and more homes being fitted with smoke detectors, a problem has arisen. The alarm goes off when you burn the toast! This little circuit copes with that problem. By RICK WALTERS You are in your normal early morning daze preparing breakfast, when the smoke alarm starts screaming. The toast has jammed in the pop-up toaster and smoke is wisping (billowing?) up into the alarm. Sure, you can turn the toaster off but how long does it then take to clear the smoke? In the meantime the alarm is sounding off, giving you and the rest of the household a high stress factor. The only solution is to climb up on a chair and disconnect the battery. Peace and quiet at last! Of course, you don’t have time to reconnect the battery now, as you have to get away, but you will do it tonight or as soon as you get round to it. Well, that is your plan anyway. How long will it be before you actually do it? Smoke detectors with the battery disconnected are useless. Problem solved! The problem is solved with this simple kill circuit which will cost about $2 and take five minutes to build. It won’t stop you from burning the toast but it will disable the alarm for 10 minutes to allow the smoke to clear. Only a few components are required, including a miniature pushbutton switch which is mounted on 8  Silicon Chip Fig.1: FET Q1 is normally turned on fully because the 330µF capacitor at its gate is charged to +9V. When kill switch S1 is pressed, the capacitor is discharged, Q1 turns off and the smoke alarm is disabled for about 10 minutes while the 330µF capacitor charges up again via the 4.7MΩ resistor. Fig. 2: this is how easy it is to build. Most of the components (three out of four) are mounted on the back of the 9V battery connector, while the 100Ω resistor is wired directly to the kill switch (S1). If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: www.avico.com.au the lid of the smoke detector – this is the “kill” switch. Now, when you burn the toast and the alarm goes off, climb on a chair or grab a broom handle and push the “kill” switch. The alarm will stop and will be disabled for 10 minutes. After that time, the smoke detector will be powered up again. If smoke is still present, it will immediately sound off again but if the smoke has cleared, the alarm will merely “chirp” to indicate that it is back in business. The smoke alarm we modified for this article was a Kambrook unit but most battery operated smoke detectors should work equally as well with this kill circuit. How it works Fig.1 shows the circuit. A low cost Be sure to disconnect the battery before soldering the parts to the connector. February 1996  9 Most smoke detectors will easily accommodate the extra parts on the battery snap connector. The switch mounts on the lid of the case. Left: be sure to sleeve the switch contacts and the 100Ω resistor with plastic sleeving, to prevent shorts when the case is assembled. N-channel Mosfet (Metal Oxide Silicon Field Effect Transistor) is used as a switch. It is connected between the battery and the smoke detector circuit. As you can see from the circuit (Fig.1), the gate is con­nected to the battery positive via a 4.7MΩ resistor. Normally, this will keep the 330µF capacitor charged to +9V and so the FET will be turned fully on and the smoke detector will work. If it detects smoke 10  Silicon Chip it will sound the alarm in the normal way and will keep sound­ing until the smoke has dispersed. However, if you’ve burnt the toast and pressed the kill switch, the capacitor will be dis­charg­ed, thereby turning the FET, and the alarm, off. The 4.7MΩ resistor will now take about 10 minutes to charge the 330µF capacitor to around +5V. At this point, the FET will start to turn on but will not be fully turned on until the gate reaches more than +6V. It is while the FET is turning on that the smoke detector will give an audible chirp or two, tell­ing you that it is back in business. The audible chirp is not a part of the kill circuit but is a feature of most battery-operated smoke alarms: when PARTS LIST 1 BS170 or BS170P Mosfet (Q1) 1 330µF 10VW PC electrolytic capacitor 1 4.7MΩ 0.25W, 5% resistor 1 100Ω 0.25W, 5% resistor 1 pushbutton momentary contact switch, DSE Cat P-7560 or equivalent (S1) the battery gets low, they chirp once every thirty seconds or so. Building it Because there are so few components, a PC board is not necessary. Instead, the parts are mounted on the top of the battery connector – see Fig.2. You will need to drill a 6mm hole in the lid of the smoke detector to accommodate the kill switch and run a couple of insulated wires to it. When you have installed the kill switch circuit, check the smoke detector for normal operation. Do this by pushing the test button and also by exposing it to smoke. If it responds by sound­ing the alarm, then all is well. It should then be mounted on the ceiling. Now try not to burn the toast. It SC makes such an awful smell!