Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!husc6!seismo!cmcl2!phri!roy From: roy@phri.UUCP Newsgroups: sci.electronics Subject: Re: Question about multi-switched lights Message-ID: <2616@phri.UUCP> Date: Wed, 25-Mar-87 21:56:53 EST Article-I.D.: phri.2616 Posted: Wed Mar 25 21:56:53 1987 Date-Received: Fri, 27-Mar-87 06:20:21 EST References: <3972@fritz.UUCP> <18002@ucbvax.BERKELEY.EDU> Reply-To: roy@phri.UUCP (Roy Smith) Organization: Public Health Research Inst. (NY, NY) Lines: 49 In <18002@ucbvax.BERKELEY.EDU> albert@ucbarpa (Anthony Albert) asks about 2 or more switches controlling a light, and/or a phone. The 2 switches controlling a light work like this: *------------------* / \ Hot -------* A B *------------} } <--- this is supposed *------------------* +---} to be a light bulb | Neutral ---------------------------------------+ I've drawn switches A and B such that the lamp is on. If you reverse either A or B, the lamp goes off. If you reverse *both* A and B, the lamp is on again, using the other conductor for the hot leg. If you want to have more than 2 switches controlling a light, you put any number of the following in between A and B: ----------* *------------ -----------*---*----------- \ / \ vs. / \ ----------* *------------- -----------*---*------------ The artwork isn't very good, but imagine a switch with 4 contacts. In one position, the contacts are cross connected as shown in the left diagram, in the other position the contacts are connected straight through as shown on the right. If you think about it the right way, each switch is an exclusive-or gate. If Y = X1 xor X2 xor X3 ... xor XN, by changing the state of an odd number of the X's, you change the state of Y. If you change the state of an even number of X's, the state of Y remains the same. If you want, you can think of the lamp as the parity check bit for the switches. This all works fine for electric lights because one side of the line is hot and one side is neutral (or at least, it should be!). Phones don't have a neutral side, so I don't know what would happen if you try it with your telephone. Note: if you put the switches on the neutral (instead of the hot) leg, it *will* work, but will violate all known electric codes and be a serious safety hazard as well. If you don't know what you're doing, get a qualified electrician to do it for you. -- Roy Smith, {allegra,cmcl2,philabs}!phri!roy System Administrator, Public Health Research Institute 455 First Avenue, New York, NY 10016 "you can't spell deoxyribonucleic without unix!"