Electronic Toggle Switches

Electronic Switching

Part 1: Electronic toggles

(The mechanical footswitch may be broken up into two elements: there is the mechanical toggling element that provides a controlling action, and then there are the metal contacts that are controlled, through which the audio signal passes. To create an electronic analog of a mechanical footswitch, we must tackle each of the two elements separately. Part 1 focuses on the control signal analog, while Part 2 focuses on the metal contact analog.)

Breaking down the mechanical "push-push" toggle

The mechanical footswitch works as a "push-push" switch. This means that you apply the same mechanical force each time you want to toggle the switch, as opposed to a "push-pull" type switch which requires opposite forces to toggle the switch. A common "push-pull" type switch is your standard wall switch for a light. An upward force turns the light on. Further upward force does nothing. A downward force shuts the light off. Further downward force does nothing. A ceiling fan pull cord is a common example of a "push-push," or perhaps in this case a "pull-pull," type switch. Pulling on the cord turns the fan on. Pushing upward on the cord does nothing. Pulling on the cord again turns the fan off. "Push-push" type switches are quite neat. Disassemble a ceiling fan type switch sometime and check it out. (You can purchase them for not too much from a hardware store.) A ramp or sawtooth mechanism is one way to realize the mechanical "push-push." Pushing the actuator (the thing you touch) will drive a contact up and over a ramp or sawtooth shape. Once the contact passes the top of the ramp, it falls into the new state. Pulling on the actuator does nothing because the contact merely pushes against the vertical wall of the ramp/sawtooth. Pushing again causes the contact to slide up the new ramp, and it can once again fall into another state. If we have two ramps connected in a circular fashion, then our "push-push" mechanism will give us a two state, on/off type function. Adding more ramp pieces can add more states (like for a 3 speed ceiling fan).

An actual footswitch is mechanically more like a sudden changing see-saw. The see-saw resists changing states until a critical amount of force is reached. See the Slow Push Test for testing a suspect mechanical switch by seeing how close you can get to this "critical force" area.

The T-Type Flip-Flop Analog

Creating the electronic analog of a mechanical "push-push" switch requires a flip-flop circuit. This circuit can be purchased as a ready made IC (CD4013 is perhaps the most common) or you can construct a discrete version using two transistors, usually BJTs. A toggling flip-flop circuit is known as a t-type flip-flop. Occasionally you may come across an old t-type flip-flop IC, but these are rare (see MFC4040, or the Maestro OB-1 schematic [IC1]).

The CD4013 is not a specific t-type, but is arranged to be a t-type by tying the not-Q and data pins together.

Discrete t-types can be copied/studied from Boss and Ibanez schematics. The CD4007 FET array is another possibility (see old TC Electronics schematics, the MXR 2000 series, Pearl, Nobels).

Instead of a physical ramp/sawtooth shape with a physical "trip" point (where the contact jumped over the edge), we will have two transistor circuits tied together so that only one can be active at a time. Adding a small voltage pulse to the input of our flip-flop will hit the "trip" point and the active device will turn off and the cutoff device will turn on. The sudden see-saw mechanism is perhaps even a closer analogy to an actual transistor flip-flop. The build of voltage is an analog to the pressure on the switch actuator. Once a critical voltage is reached, the system flips suddenly, like the see-saw.

Now we can have simple and reliable momentary stomp switches!

Once you have an electronic toggle, it only requires a brief pulse from a SPST momentary switch to work. As mentioned above, this can drive a relay and/or LEDs to maintain a mechanical, full bypass switching scheme. It can also drive electronic gates, which brings us to... PART 2.

Back to Bypass Schemes