3-way input selector using CMOS gates

[BradM]

Looks like there is lots for me to catch up on.  Thanks once again for all the great discussion.

With some help from Milan and another friend, here's the circuit I have come up with.  I built it and it works.  I chose to use a single 40175 quad D-type flip flop for simplicity.  The only quirk is that if you press and hold one switch and then press a second switch, and then release the two switches in just the right order, both outputs will go low.  But if you simply hit one of those switches again then the respective output will go high.  I've set up the circuit to default to Input A when the unit is powered on.  The circuit does remember it's state for a minute or so while the caps still have charge on them.  But after a few minutes it will default to Input A again upon power up.  

Brad

 

[Harpo]

Just turn the switches around and feed your +12V in series thru the switches, i.e. +12V -> SW1 pin2, SW1 pin1 -> SW2 pin2, SW2 pin1 -> SW3 pin2, switches pin3 connecting to R18,R14,R15. Last switch pin1 in this series string can be used as default connection, i.e. connecting to R14 for default Ch.B.(rubbish, he wants momentary switches.)
Just an (maybe silly) idea.

 

[JohnRoberts]

While this is a little rube goldberg, you could use the other selector switch poles (if really there) to alternately connect one end of three resistors to + supply or ground, when pressed. Connect the other end of the three resistors together and to the reset pin. If no switches are pressed the reset voltage is hi and happy. If only one switch is pressed the voltage is 2/3V so still logic hi and happy. If two or three switches are low at the same time, you get logic low and reset.

For more than three inputs, use pull down resistors only (floating when not pressed), working into a single pull up resistor, with values calculated such that one pull down divider is still logic hi, two or more pulling down is logic low.

JR

Note: many low cost switch solutions like dome switches used in keyboards, or circuit board patterns that are shorted by a conductive puck on the bottom of a moulded button, are SPST.

 

[BradM]

Thanks John.  I'll have to re-read your post when I get home and try to digest.  But yes, the 2nd poles are really there since I'm already using that same part elsewhere throughout the design.  I'll try to test out what you are suggesting tonight and report back.

I still need to catch up on this thread, but I did want to thank Milan for his lastest schematic contribution.  There's been some really great stuff in this thread that I think a lot of folks will find useful at some point in their DIY adventures.

Brad

 

[BradM]

Hi John,

I finally got a chance to build your "Rube Goldberg" modification to my circuit.  It didn't exactly work.  There were still combinations of switching that would make all the outputs go low.  Perhaps I hooked something up incorrectly.  I don't know.  One thing I did discover was that with actual switches connected to my original circuit, it was nearly impossible to create a switching1 combination that resulted in all outputs going low.  I had to do some crazy things hitting all three buttons at the same time, and even then it took me a minute of doing it for it to fail in just the right way.  So I'm going to claim victory.  Thank you all for your advice and help.  It's been a great learning experience.

Looks like I have some microcontrollers in my future. 

Brad

 

[JohnRoberts]

When it comes to free advice sometimes you get what you paid for it..  :  Sorry bout that..  Some time would need to be invested into looking at timing around the reset, and/or immediately after it's removed.

The nice thing about micro's is it's trivial to apply complex logical rules like if this do this, or if that do that... but if this and that then do something completely different

JR

 

[BradM]

No worries.  It was fun to try it out anyway.

Thanks John!

Brad

 

[AudioJohn]

The attached PDF shows a 3 button circuit that I have used in a variety of projects. It works very reliably and uses only 3 gates from a CMOS 4001 - leaving a spare to do something with!

Few circuits are this useful - and low cost, with just one IC, 6 resistors and a capacitor.

 

[PRR]

Ohhhh.... elegant!