At 05:11 1998-10-29 +1000, Paul Webster wrote:
>Dave Johnson wrote:
>
>> I need to make a push button "toggle": push goes high, next push goes
>> low, and so on.
>
>  Well, we will all tell you how to do it in software, so I might just
>mention the hardware solutions - use a single gate from a 74HC14 or
>two gates which do not then *need* to be Schmidt.
>
>         2M2 ohm                       2M2 ohm
>  *------/\/\/------*            *---/\/\/------*
>  |                 |            |              |
>  |  ___ PB    |\   |            |  ___ PB  |\  | |\
>  +--o o-----+-| o--+----O out   +--o o---+-| o-+-| o-+-O out
>  |          | |/                |        | |/    |/  |
>  |          | 1/6 74HC14        |        |   74HC14  |
> --- 0.47µF  |                  --- 0.47µF|           |
> ---         |                  ---       |           |
>  | 220k ohm |  220k ohm         |        | 100k ohm  |
>  +--/\/\/---+---/\/\/---Vcc     |        *--/\/\/----*
>  |                             gnd
> gnd
-snip-

Two soles sharing the same thought :)

I described the right one in my previous posting but was too lazy to make
ASCII art
-this is nice drawn  :)

For the left one: Yes i have had it working too, but it is sensible to
glitches as V/2 is close to V/3 and 2V/3.  Especially as the scmidt trigger
input is the wire that goes to pusbutton, and might catch much EMI.
Another problem is that the schmidt-trigger levels are not guaranteed very
preceisely at all by the manufacturers, and i wouild not be surprised if
some schmidt-trigger inverters in real life would have one level at around
V/2...
The design is neat, but I would not ship that design to customer.
Also, often the extra inverter in the right solution is cheaper than the
saved resistor, compared to the left solution.

Substitute the inverter in the left solution with a 555 timer ckt and the
trig levels will be guaranteed.  (Still it is more EMI sensitive than right
solution though)
in= 2&6; out = 3; 1=GND; Vdd=8; use pin 7 and resistor to drive a LED.  Pin
4 to system reset (low) or tie to Vdd.


The right one is the right one (what a language...)  to use  ;)

/Morgan
        Morgan Olsson                   ph  +46(0)414 70741
        MORGANS REGLERTEKNIK            fax +46(0)414 70331
        H€LLEKS           (in A-Z letters: "HALLEKAS")
        SE-277 35 KIVIK, SWEDEN               mrt@iname.com
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