[sdiy] Finding the correct IC...

media.nai at rcn.com media.nai at rcn.com
Wed Jun 19 16:34:18 CEST 2002


>Anyway onto the problem:
>I am in the process of building a gate-sequencer, however I do not wish to
>use those switches with a pin sticking out of the board, I want to use a
>pushbutton-type switch. The problem I have is that I havent found a
>>suitable one that is on/off, the buttons I have found are those that is
>on >when pressed, off when not pressed.

Well, they do sell pushbutton switches that are not momentary.

>There is a circuit called Latch (D-type, S/R,...)

SR and D are two different types, although there are type D (delay)
flip-flops with set-reset capability.

>that according to a friend can help me with this problem, as it
>triggers on a pulse and stays High/low until it is Reset (Set/Reset).
>However all the S/R-Latches I have found need ONE pin for SET and ONE pin
>for RESET, which equals two... I want ONE pin to do BOTH... does anyone
>>know of a circuit that can do this for me? Maybe I read the circuit
>>diagrams wrong I don't know, I am kind of stuck at the moment though....

According to the rules (although Harry might have his own ideas :) you are
not supposed to connect both S and R to the same value at the same time.

It sounds what you essentially want is a T (trigger) flip-flop -- it
changes state with each pulse.  You can do this with a JK flip flop, which
is like an SR and T rolled into one.  You can tie the J and K inputs
together (turning it into a T), or tie one input high.  You can also use a
4013.

Since you are using a mechanical switch, you probably want to debounce it.
You have several options:  use a SPDT switch with an SR, a schmitt trigger,
a non-inverting gate (or two inverting gates in a row), a 555, or an IC
specifically intended to debounce switches.

I like the CD4000 family of CMOS because it is inexpensive and widely
available.  Either that or I'm cheap and don't get around much ;)  I
suggest that you check out the following chips on the net:

4009/4049/4069  hex inverter
4013            dual type D with SR
4017            hex type D
4027            dual master-slave JK with SR
40106           hex schmitt trigger

I'm sure you'll be able to figure out an efficient circuit that both
debounces and latches.







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