[sdiy] Multiple remote points control on/off switch

[tpaddock at seanet.com]

http://www.ednmag.com/ednmag/reg/2000/06222000/designideas.htm#13di5

Multiple remote points control on/off switch, 
by Tom Hornak, Portola Valley, CA

This circuit in the June 22 2000 EDN mag struck me 
as being very simple and clever. A flip-flop, 2 resistors, 
2 caps, and however many single pole momentary switches.

It looks like the cmos inputs are all at the rails when 
it's just sitting there, so I'm guessing the current 
draw would be very low. 

Just thought it was interesting,
 - -- -  Toby Paddock

>From the article:
Sometimes, you need to remotely turn on or off a two-state system, 
such as a light, from multiple points. You could connect simple 
pushbutton switches in parallel to a single-line bus. However, if the 
bus simply controls a toggle flip-flop, the system must know its 
current state to positively ensure the new, desired state. 
For example, if you want to make sure the light is on, 
you must have visual or electrical feedback via a second line 
before deciding whether to toggle the flip-flop. 
Also, bounce-free pushbutton switches are necessary.

The circuit in Figure 1 achieves positive state control according to the 
length of time you activate any pushbutton switch. 
An activation of a few milliseconds, for example, guarantees that the 
system is set to one state. An activation of 1 sec, for example, 
guarantees that the system is set to the other state. 
No visual or electronic feedback is necessary.

The circuit is a NAND R/S latch comprising two simple CMOS Schmitt 
trigger NAND gates. You can use half of a CD4093 or a 74HC132.