Need help with circuit to indicate last momentary switch pushed

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

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

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

--
The solution is going to depend on what your membrane switch(es)
look like.  Are you planning a rectangular matrix or a bunch of
individual switches or ???

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

This is one I've used before....

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

Luhan