What does this do?

[ballooneater]

Sorry for the non-descriptive title, I have absolutely no idea what the function of this piece of circuit is.  I've seen it in a good number of schematics (with a few differences), so I figure it must have some importance.



If anyone could tell me, I'd really appreciate it.  It's been bugging me with every layout I make...

[George Giblet]

The part of the circuit R135, R136, R137, C121 are used with Sw1 to create a clean pulse when you press the switch.

The main part of the circuit around Q104 and Q103.  This circuit has two outputs the collector of Q103 and the collector of Q104.  The circuit to the left of Q104 (R138 etc) drives the LED.  What this circuit does is takes pulses from the switch and it alternates the state of Q103 and Q104 each time the switch is press.   So it converts the push button action of the switch to a toggling function with two alternate outputs.   The input to this circuit is where C119 and C120 join.  The circuit is called a two-transistor bistable flip-flop; bistable means it has two stable states ie.(Q103 on, Q104 off) and (Q103 off, Q104 on).

BTW: The CR devices are a resistor and capacitor in parallel in the one package (typically 100k and 100pF).

[ballooneater]

So it essentially turns a momentary switch into a latching one? (not physically, but functionally)

[George Giblet]

Yes,  but there's no switch contact you can connect anything to.  If you take the outputs and connect them to a JFET(s), as is done in the effects, then the JFETs become the switch contacts.

[ayayay!]

It pretty much looks like what's generically called flip-flop circuit.  Your example is close to a Boss style, but for some reason makes me think Ibanez.

[Artie]

So it essentially turns a momentary switch into a latching one? (not physically, but functionally)

It performs one other important function thats often overlooked. A standard DPDT footswitch will always power-up in its last state. The electronic switch can be powered-up in a known state. Typically, you'ld power-up, always "off". Its a nice side benefit.

[ballooneater]

Thanks for your help, everyone!

@ayayay!
It's an Ibanez.  Can't remember which one...

[earthtonesaudio]

There's a lot of flexibility in that circuit too.  You can use it to switch multiple effects in and out, and with some extra circuitry you could make yourself a switch with two throws and however many poles you need.  4PDT, 6PDT, no problem.  As long as you have enough current to drive the switches, you're golden.

[ayayay!]

Sorry to dig up an old one, but here's a great primer on flip-flops in easy to understand jargon:

http://www.bossarea.com/forum/topic.asp?TOPIC_ID=1036