In article <4n7q30$>,
Dubaniewicz <> wrote:
> I need to manually switch a 5 v, 1-10 mA variable current signal on-off.  
> I would like to have a clean signal within 1 ms of throwing the switch.
> Board power supply is 6 V.  Any suggestions on how to do this would be 
> greatly appreciated.

The simplest way is to ensure you have a break-before-make switch,
ground the changeover contact and use the NC and NO contacts to
drive a S-R flip-flop.  There are lots of variations depending on
your requirements and preferences.  A simple way is like this.

                   (off)           |\
                  o----------------| >o------------------- Q-bar
     -------o-----      |          |/         |
    |             o     |                     |
   GND       (on) |      ----/\/\/\/-------------
                  |                           |  |
                  |      ----/\/\/\/----------   |
                  |     |                        |
                  |     |          |\            |
                   ----------------| >o------------------- Q

This circuit is a simple S-R latch made from two cross-connected
inverters.  The switch holds the latch in one or other state.
During switching, the switch goes open circuit to both contacts,
and the latch holds its state.  As soon as one of the contacts
closes, the latch flips to that state.  As long as you use a
switch that breaks before it makes (also called non-shorting),
you get a bounce-free output.

You can use CMOS gates (4049, 4069, 40106, 4584, 74HC04, 74HC14 etc)
with 10K or 100K resistors.  For LS or TTL you should use lower
resistor values e.g. 1K because they draw more current.  You can
even short the resistors out entirely, if you use open collector
TTL or LS parts (7405 or 74LS05 I think from memory), so you get
three debouncers per chip with no external components apart from
the switch.

Kris Heidenstrom                        Time flies like the wind                  Fruit flies like bananas
Wellington, New Zealand                   -- someone else's .sig

Date: 18 May 1996 19:01:49 +1200

Original Subject: Re: toggle switch debounce

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