New monitors on old machines

Roger Merchberger wrote: You mean a two-way switch (single pole, double throw), and yes there is a way to have three or more, if you use DPDT switches or a crossover switch in the middle. Over here, the major manufacturers make "crossover" or "changeover" light switches for exactly this purpose: Conventional use of two two-way (SPDT) switches: o-------------o LIVE --------o--- ---o----------- BULB ------ NEUTRAL o-------------o If one switch is "up" and the other is "down", the light is off If both switches are "up", the light is on If both switches are "down", the light is on Actually it's usually wired with one switch the other way up, so the light is on if one is "up" and the other "down": o---. .------o LIVE --------o--- \/ ---o----------- BULB ------ NEUTRAL o____/\o______o Using three (or more) switches: o----------o|o----------o LIVE --------o--- --- ---o----- BULB ---- NEUTRAL o----------o|o----------o ^^^^^^ crossover switch connects upper left to upper right (and LL to LR) or lower left to upper right (and UL to LR) If one end switch is "up", and the other is "down", the crossover switch in the middle can switch current from the upper leg to the lower, and turn the light on. Another layout with a DPDT: o--------------.------ o----------o--- | LIVE --------o--- : o--------.------------ o------+ : | | | : o--------+ | +---o--- | o--------------+ ^^^^^^^^ ^^^^^^^^ a DPDT switch with wiring arranged to switch current from upper leg to lower leg ('o's are contacts, '.'s are connections, ':'s represent the mechanical linkage between the poles) You can extend this ad infinitum with additional crossover or DPDT switches. -- Pete Peter Turnbull Network Manager University of York