Relay True Bypass Switching Part 1: Relay Basics and Stereo Guitar Pedals

Relay switching in guitar pedals is an alternate method of achieving true bypass instead of the standard 3PDT (or 4PDT) mechanical footswitch. There are quite a few benefits that can be achieved with relay switching, most notably:

  • Stereo effects with one footswitch
  • Complex switching layouts (press once to activate effect A, press twice to activate effect B, press three times to activate effects A and B, etc.)
  • Complex switching functionality (such as holding down the switch for temporary effect)
  • Smart loop switching functionality, such as preset effect groups
  • A larger range of options for actuators, from soft-click footswitches to arcade buttons and more

Though there are many relays that can be used for guitar pedals, we will be using the Panasonic TQ2-L series of relays in these articles. They are small, breadboard-friendly, and similar in footprint to DIP integrated circuits. If using a different relay, please check the manufacturer’s datasheet to make sure that the pinout and nominal voltage are compatible.

If you are unfamiliar with switch poles and throws or terms like "DPDT", you may want to read our article "Switch Poles and Throws" before proceeding.

Relay Basics

Relays function very similarly to mechanical DPDT switches; however, the active connections are toggled via electrical signals rather than mechanically. In figure 1, the solid green lines on the mechanical DPDT switch drawing indicate the connections that are made with the DPDT switch in one state, and the dashed yellow lines indicate the connections that are made when the switch is toggled and it is in the other state. Similarly, the green and yellow lines indicate the two possible states of a Panasonic TQ2-L series relay. The middle six pins on the TQ2-L can be used and wired in the same way as the six pins of a DPDT switch.

Figure 1: TQ2-L relay and mechanical DPDT switch similarities

Figure 1: TQ2-L relay and mechanical DPDT switch similarities

Pedal Wiring

3PDT footswitches are common in guitar pedals because two poles are needed to control the audio routing while the third pole toggles the LED. If an LED is not needed, DPDT switches can be used for guitar pedal true bypass. Since the middle 6 pins of the TQ2-L relays operate similarly to a mechanical DPDT switch, those pins can be wired the same way as a mechanical DPDT switch would be to control the audio signal in a guitar pedal.

Figure 2: TQ2-L relay and mechanical DPDT wiring

Figure 2: TQ2-L relay and mechanical DPDT wiring

Note that there are multiple ways that a DPDT footswitch can be wired. The wiring in Figure 2 will ground the effect’s input when bypassed, which can help prevent clicks in the audio signal when the effect is engaged. Though relays such as the TQ2-L series are similar in functionality to DPDT switches, unlike a mechanical DPDT switch, using a relay does not mean that the indicator LED needs to be left out of the pedal. We will address the LED later, but first let's look closer at how a relay works.

Toggling Relays

While a mechanical DPDT switch's position is toggled via various physical actuators such as toggles, sliders, or in most pedal cases, plungers, a relay is toggled electronically rather than mechanically by connecting certain voltages to its pins. In the case of the TQ2-L, the pins that are used to control the switch position are pins 1 and 10 (See figure 3). Pins 5 and 6 do not have any function in the TQ2-L series.

Relays are available with various nominal voltages for toggling pins. Let’s consider TQ2-L-9V for a moment. TQ2-L-9V has a nominal voltage of 9V, which means that it needs signals of 9V across pins 1 and 10 in order to set the switch position (the actual value can be slightly more or less than 9V, though the spec sheet recommends it be within ±5%). If you plan to use any voltage besides the nominal value, please check the datasheet. The position of TQ2-L-9V can be set like this:

Latching Relay
Pin 1Pin 10Switch Position
GNDGNDNo change
GND9VPosition A (Green connection)
9VGNDPosition B (Yellow connection)
9V9VNo change
Figure 3: TQ2-L-9V state toggling

Figure 3: TQ2-L-9V state toggling

TQ2-L-9V is a latching relay, which means that if there is not 9V across pins 1 and 10 in either direction, the switch stays in the last position it was set to. Non-latching relays are also available, such as the Panasonic TQ2-9V (note the lack of the letter "L" in the part number). For the TQ2-9V non-latching relay, the switch does not retain its previous position. For the values where a latching relay would stay in its previous state, the non-latching relay switches back to the default position.

Non-latching Relay
Pin 1Pin 10Switch Position
GNDGNDPosition A (Green connection)
GND9VPosition A (Green connection)
9VGNDPosition B (Yellow connection)
9V9VPosition A (Green connection)
Figure 4: TQ2-9V state toggling

Figure 4: TQ2-9V state toggling

We will be focusing on the latching relay TQ2-L series in this tutorial, but the principles here can easily be adapted to non-latching relays as well.

Stereo Relay Switching

Now that we know what’s required to toggle the TQ2-L relay’s state, let’s look at a simple use of relays to implement true bypass switching for a stereo in/stereo out guitar pedal with an LED indicator. Stereo effects with true bypass can be implemented using a mechanical 4PDT switch, but you have to leave out the LED in that case. In order to control an indicator LED, an additional pole is needed and 5PDT footswitches are either very difficult or impossible to come by. Relay switches can offer one solution to this problem.

This technique will not give us all the advantages that come with more advanced relay switching circuits (we will cover those later), but it is a useful stereo solution and demonstrates how relays can be put to use. It uses a standard latching footswitch, and to the pedal user it appears to function the same way a normal true bypass pedal does, only with stereo ins and outs.

Controlling Stereo Relays

For this functionality, we will use two TQ2-L-9V relays and assume that the pedal is powered by 9V. If the pedal is powered by a different voltage, a different relay should be used, such as TQ2-L-12V for 12V guitar pedals. We will also be using a standard 3PDT latching footswitch. The simple explanation of the circuit is that we are controlling two relays with two of the 3PDT footswitch’s poles, giving us five usable poles total. Four of the usable poles come from the relays, while the fifth comes from a leftover pole from the 3PDT footswitch which is not used to control the relays.

Figure 5: Converting 3PDT footswitch to 5PDT

Figure 5: Converting 3PDT footswitch to 5PDT

Two poles of the three pole footswitch (named ~S1A~ and ~S1B~) are used to control both relays, which have 2 poles each (4 total relay poles). Since relays can be wired like a DPDT guitar pedal footswitch, we use each relay to control one channel of the effect. Since we need a fifth pole to control the LED status, the LED is connected to the third pole (named ~S1C~) of the 3PDT footswitch. If we consider the two states of the 3PDT footswitch to be ~\text{state A}~ and ~\text{state B}~, the below table demonstrates how the relays are affected by toggling the footswitch:

3PDT StateRelay Pin 1Relay Pin 10Relay Pin 3Relay Pin 8Effect
AGND+9VConnected to pin 2Connected to pin 9Off
B+9VGNDConnected to pin 4Connected to pin 7On

Full Stereo Relay Circuit

The detailed schematic can be found below. ~S1A~ controls whether pin 1 of the relays are connected to +9V or GND. ~S1B~ does the same for pin 10, except with GND and +9V on opposite throws from ~S1A~. In other words, when ~S1A~ is connecting pin 1 to +9V, ~S1B~ is connecting pin 10 to GND, and when ~S1A~ is connecting pin 1 to GND, ~S1B~ is connecting pin 10 to +9V. This causes both relays to toggle along with the footswitch.

Figure 6: Detailed schematic for stereo relay switching with LED

Figure 6: Detailed schematic for stereo relay switching with LED

Note that the Right Input jack in the above schematic is a switched mono jack:

The switched mono jack is not strictly necessary, but its switching mechanism sends the Mono/Left Input through the right channel (in addition to the left channel) if nothing is plugged into the Right Input. This allows for either stereo input/stereo output or mono input/stereo output when there is no jack plugged into the right input. If desired, a single output can be used for mono output.

Battery-powered pedals often use a stereo input jack to disconnect power when nothing is plugged into the input jack. This can be achieved in the Figure 6 schematic by using a stereo jack for the Mono/Left Input jack and connecting it the same way as a battery-powered mono pedal. In that case, a mono cable (with a TS plug) inserted into the Mono/Left Input will complete the power connection and engage battery power. Note that using a relay in this way will add to the current draw of the pedal. A more advanced relay switching method that we will cover in part 3 of this series will minimize the added current draw.

Though it is not common to need more than 5 poles in guitar pedals, the schematic in figure 6 could be expanded to achieve more than 5 poles by simply adding more relays with similar connections. For example, adding one more TQ2-L-9V with pins 1 and 10 connected the same way as the other relays will create a 6th and 7th pole with pins 2, 3, 4 and pins 7, 8, 9. In later parts of this series, we will demonstrate how a circuit containing a relay can provide some of the more advanced benefits that we mentioned, such as true bypass with momentary soft-click footswitches. Momentary footswitches are perhaps the most recognizable feature of many relay-based switching circuits. Some players prefer the soft-click switching that certain momentary switches offer, and momentary switches are often more reliable and long-lasting than latching footswitches. The Lehle P-H-LEHLE-BTN momentary switch, for example, has a soft but very satisfying click to it and it is rated for 1 million presses, while latching footswitches usually tend to be rated for around 5,000-10,000 presses.

In Part 2 of our Relay True Bypass Switching articles we will see how momentary footswitches can be used to control relays, allowing for the ability to control the effect with soft switches, pushbuttons, keyboard switches, and any other momentary switches.

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