Need help with circuit to indicate last momentary switch pushed

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I am building a membrane switch surface and need some tips.  I need a
way to turn on a led of the last momentary switch pushed and keep it on
until the next one is pushed. Any suggestions of where to look?
Thanks alot
Travis


Re: Need help with circuit to indicate last momentary switch pushed



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This is two questions.

The membrane switch is just a switch, presumably closed when pressed.

To hold the last state and change it on the next press, you'll
need a T flip-flop.

Which is it you need help building? The switch, or the circuit?

Good Luck!
Rich



Re: Need help with circuit to indicate last momentary switch pushed


On 24 Nov 2006 14:42:05 -0800, in sci.electronics.design
snipped-for-privacy@lividinstruments.com wrote:

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I saw a nice little circuit, about 20+ years ago where the button
switched on a low current SCR, the supply was current limited so the
other SCRs turned off, but left this one on, sorry, no URL, pre
internet


martin

Re: Need help with circuit to indicate last momentary switch pushed


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  I just posted a circuit that does exactly that on 10/12
on sci.electronics.basic. The subject is:
"Re: help with project with map & LED's"
Here's the circuit:

+12 -----+------------------+---------+---{{{-----+
          |                  |         |           |
          |                [LED]     [LED]       [LED]
          |                  |         |           |
          |                [1K]      [1K]        [1K]
          |                  |         |           |
          |   Pb1          [SCR1     [SCR2]      [SCR_n]
          |   ___           /|        /|          /|
          +---o o---[330]--+ +---------+---{{{-----+---+
          |     |                    |           |     |
          |     +--->|---------+     |           |     |
          |         D1         |     |           |     |
          |   Pb2              |     |           |     |
          |   ___              |     |           |     |
          +---o o---[330]------------+           |     |
          |     |              |                 |     |
          |     +--->|---------+                 |     |
          |         D2         |                 |     |
         {{{                  {{{               {{{   {{{
          |                    |                 |     |
          |   Pb_n             |                 |     |
          |   ___              |                 |     |
          +---o o---[330]------------------------+     o
                |              |  +                    |
                +--->|---------+---[C1]----+----+   .->| N/C
                    Dn         |           |    |   |
                               +---[3.3K]--+  [RY1] o
                                                |   |
Gnd -------------------------------------------+---+

How it works: pressing any pusbutton biases the respective
SCR gate to turn on, and energizes the relay briefly through
C1, a 100 uF capacitor, as C1 charges. This opens the ground
path to all the SCR's (via the normally closed contact of the
relay), turning off any previously on SCR. The still pressed
button turns on the SCR when the relay drops. The 3.3K resistor
discharges C1 between button presses. The LED stays lit until
the next press of a button.  Your membrane switches would have
to be able to handle the current needed by the relay. Not
shown is a reverse diode across the relay coil.

Refer to the original post/replies on the other group.
Someone suggested changing the relay circuit for a
transistor, and posted a modified schematic which
you way want to try. It looks good, but I haven't
tried it. If you do try it, change his 100 pF cap
to 100 nF.  The one above with the relay is in
use today and works well.

Ed

Re: Need help with circuit to indicate last momentary switch pushed


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Some further thought - replace the 330 ohm resistors
with 10K resistors, use sensitive gate SCR's and
use the transistor circuit rather than the relay
circuit.  That will keep the current through your
switches low.  Be sure to change the 100 pF in his
daigram to 100 nF.

Ed

Re: Need help with circuit to indicate last momentary switch pushed



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---
The solution is going to depend on what your membrane switch(es)
look like.  Are you planning a rectangular matrix or a bunch of
We've slightly trimmed the long signature. Click to see the full one.
Re: Need help with circuit to indicate last momentary switch pushed



snipped-for-privacy@lividinstruments.com wrote:
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This is one I've used before....

http://members.cox.net/berniekm/onoffswit.gif

The capacitor charges to the opposite voltage and spikes the inverters
into reversing when pushed.

Luhan


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