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
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
