Schmitt Trigger = flip flop

[Matthew Sanford]

Hysteria regarding hysteresis! So.. flip flops are nice, just 5 caps, 6 resistors, 2 NPN, and one momentary. But learning on Schmitt Triggers and thinking can't we just use that? Well, really for switching other things to change the send from the momentary to be high once in a while too... so I was thinking, could we pull through the momentary from TR1's collector to it's base, so the inverted output to TR2 would change with each stomp? On the diagram I attached I realized I should probably remove the input and R7 so it is just collector->momentary->R1, so considering it that way would it work as a flip flopish type thing? I realize only one output off TR2 (unless we tap TR1 to go out as well as switchbase), still, only 6 resistors and no caps means less parts to flip.

Pretty please let me know what you think. Assume VCC = 9v, R3 = 10k, R6 = 4k7, and R2 = 33k, therefore VH trigger is 6.1v and VL is 2.1v.

[Rob Strand]

The basic idea works.

There's a few values missing:  R1 = 10k?,  R4=10k?,  R5=100k?,  R7=100k?

Some details:

The main beef with the BJT Schmitt-trigger circuit is the output doesn't swing down to 0V.   When you use these things for signal processing you can allow for that.   But as an on/off switch it's a bit of a pain.   You can put a PNP switch at the R6 output, but that's more parts.

The normal Schmitt-trigger circuit puts a cap across R4.  You can sometimes pull the cap.   The standard flip-flop has 2 caps between each C and B for a similar reason.   The caps ensure the device switches under all circumstances.

Another requirement for the circuit to work in the real world is to handle switch bounce.  As is your version won't handle switch bounce.  You need to add a cap from the R7/R1 junction to ground.   With R7=100k and R1=10k, a value of 47n to 100n will fend off most switch bounce.  If you look at the Boss or Ibanez flip-flops you can see there is an RC network across the switch to help stop switch bounce.

A similar circuit to yours is the CMOS inverted circuit used by DOD.  It floats the switch from ground like yours. It also has a cap to fend-off switch bounce.  It seems to work well.   IIRC it doesn't even use hysteresis.

[Rob Strand]

The basic idea works.

I don't think it works.

I didn't check it correctly.   It doesn't actually toggle.   Need to compare against DOD ckt.

[Matthew Sanford]

I may just go with the bosstech way, R.G.'s explanation is clear...I just have many momentaries.