Switched-On Switches
Switched-On Switches:
Switching Basics
Switches come in a variety shapes and sizes and their uses can be just as varied. At their core switches are beautifully simple yet extremely useful. Unlike most electronic devices, switches are one of the few components that have changed relatively little since their inception outside of the usual trend of growing more compact and affordable.
Poles and Throws
Switches are defined by their number of poles and throws. A pole in a switch is similar to a lane of traffic; it’s a set of contacts across which switching can happen. Throw refers to the number of positions or combinations that can be found on that set of contacts; if a switch’s pole represents a lane of traffic, the number of throws is equivalent to possible exits or destinations.
Single Pole, Single Throw
The simplest type of switch is the single pole, single throw switch (SPST). This type of switch is what we use to turn our lights on and off. A SPST switch has 2 contacts. In one position these 2 contacts become connected and in the other position they are disconnected.
In the world of synthesis, a SPST switch can be useful as a mute or on/off control for any type of signal. For example, if you had an LFO patched into the Cutoff CV input of a filter, putting a SPST switch in series with that connection, you could turn the modulation on and off without having to unplug any patch cables or adjust the index of modulation. Alternatively, if you run your VCA’s output through a SPST switch before going to your output mixer, amplifier, or interface, the switch would allow you to turn off the signal without needing to change the output level. This can be helpful for bringing parts in and out abruptly without affecting the relative level balance within your mix; it would effectively be a mute switch.
Single Pole, Double Throw
By incorporating one additional contact, the single pole, double throw switch (SPDT) provides twice as many switching opportunities. In a SPDT switch, the center contact is switched between the two outer contacts.
If we were to number these contacts 1, 2, and 3, with 2 being in the middle, one position has pins 1 and 2 connected and the other position has pins 2 and 3 connected.
If one had, for example, a VCF’s audio input patched to the center contact, one could patch two different VCO waveforms to the respective outer contacts. By flipping this switch. we change the waveform being fed into the VCF without needing to unplug anything. Another example, would be a case where an LFO is fed to the center contact, and the outer contacts are patched to a VCO frequency CV input and a VCF frequency cutoff input, respectively. This setup would allow the modulation to be switched from one source to two different destinations. In one position we modulate the VCO, but not the filter, and in the other position this is reversed.
It’s worth noting that a SPDT switch can function as a SPST switch simply by leaving the second outer contact unconnected. This allows for on/off switching of the signals at the other two contacts.
Double Pole, Double Throw
The double pole, double throw (DPDT) switch further expands upon the SPDT switch, by doubling the number of contacts. Each pole’s contacts are separate from the others’, but the switched position is shared.
If we were to label the pins as 1,2,3 and 4,5,6, we see that in one position pin 2 connects to pin 1 while pin 5 connects to pin 4 and in the other position pin 2 connects to pin 3 and pin 5 connects to pin 6.
This now allows multiple parameters to be changed with a single switch movement. For example, one pole could switch an oscillator’s frequency CV input between two different sequencer CV outputs, while the other pole could switch a filter’s cutoff frequency CV input between two different envelopes. This means that without repatching any cables, we can use our switch to vary between two different sequences and filter envelopes.
It should be noted that for SPDT and DPDT switches there are varieties available in which there is a “Center OFF” position. This allows a 3rd position in the center wherein no connections are made. Additionally, there are some DPDT switches that include a “Center ON” position where pins 1, 2 and 5, 6 remain connected when the switch’s shaft is in the center position.
Rotary Switches
While SPDT and DPDT are some of the more common types of switches in synth modules and guitar pedals due to their compact size, for situations requiring more than 2 positions, there are rotary switches. Again these are labeled based on the number of poles and the number of possible positions. So for example, a rotary switch may be 2 pole, 4 position or 1 pole, 12 position, etc.
In reference to our earlier traffic analogy, rotary switches are very similar to a rotary/traffic circle/roundabout on the road. Each car represents a pole and each exit from the rotary represents a position. Rotary switches have become less common in modern devices as they can be quite bulky and expensive and many situations that call for switching between more than 2 sources can be tackled with microcontrollers, logic circuitry, or relays.
Foot Switches
The concepts discussed up to this point all relate to foot switches as well. A foot switch generally has one or several poles and 2 possible throws. A very common foot switch type is the 3PDT switch. This is used in guitar pedals to create “true bypass.”
In this scenario one center lug connects to the pedal’s input, another center lug connects to the pedal’s output, and the third center lug attaches to ground. In the “On” position, the input is connected to the effect circuit’s input, the output is connected to the effect circuit’s output, and the ground lug attaches to the cathode of an LED to indicate that the pedal is on. In the “Off” position the alternate lugs of the input and output poles are connected together. The remaining lug (which will now connect to ground in this position) may be connected to the effect circuit’s input to remove any DC offset while the effect is bypassed.
While toggle switches offer a clear indication as to which position they’re in, foot switches generally have no indicator as to their state. However, foot switches (and pushbutton switches) that rely on being pressed or stomped on and which stay in that changed state are called “Latching.” Switches that do not stay in their switched state are called “Momentary.”
Momentary Switches
Unlike most toggle switches, momentary switches only make or break certain connections while they’re being pressed. Although many momentary switches are single pole, single throw, they can be found as SPDT or DPDT. In the cases of momentary switches with more than one throw, one position will be “normally open” and one will be “normally closed.” Normally open means that the connection is only made when the switch is pushed and normally closed means the connection is being made except when the switch is pushed.
Tap tempo pedals and doorbells are common examples of momentary switches in the wild.
Jack Switches and Normalling
The final switch type we will consider is that of the “switching jack.” Switching jacks are very common in synthesizer modules and occasionally are found in guitar pedals as well. The switch in a jack is an extra contact that makes a connection with the hot lead when nothing is plugged into it. When a plug is inserted into the jack, it breaks this connection.
In synthesizers this is quite common on CV inputs where the jack connects to an attenuator for a certain parameter. With nothing plugged in, the switching lug may be “Normalled” to +5 Volts, but this is bypassed in favor of the incoming signal when a patch cable is inserted. Our passive switch module normals some of the adjacent jacks together to cut down on necessary patch cables when creating a true bypass patch or when mult-ing the center lugs together.
In guitar pedals that contain batteries (ours don’t), a switching DC jack is used so that the battery makes contact with the power input connection until an external power supply is plugged in, at which point the battery is disconnected from the circuit.
Switching Off
There is almost no limit to the possibility of usage when considering switches. Switches are an important and often overlooked part of the world of sound creation and manipulation. They can turn things on and off, they can reroute signals, and they can help us input binary information. Switches can be a necessary utility, a valuable performance tool, or simply a way to adjust certain parameters. Happy switching!
