Low-Power Relay True Bypass

Modern pedals, especially the “boutique” ones, often employ relay-based true bypass circuits to replace 3PDTs footswitches with momentary SPSTs. SPSTs are usually cheaper and more durable, due to their non-latching nature.

If powered by a microcontroller, relay bypasses also allow for more intelligent switching and input – like a tap tempo control or the EQD Flexi Switch.

Motivation

Say you want to combine a simple fuzz circuit with a relay bypass. The LED and effects circuit should draw less than 10 mA in total. A low-power switching circuit allows running the pedal off a battery for a very long time.

There’s a variety of schematics, but also kits that come either prebuilt or include all the parts needed to build a relay bypass. None of the ones I found so far mention power consumption, although some use a latching relay.

Non-latching Relays

Non-latching relays are cheaper and more easily available than their latching counterparts, but almost always a bad choice for relay bypass circuits.

The non-latching EC2-5NU has a single coil that, when energized via pins 1-12, toggles the internal switch, thus connecting pins 4-5 and 8-9. The power needed to energize the coil enough to flip the switch is listed as Nominal Operating Power in the datasheet.

As soon as the coil is de-energized – which is the “normal” state of a relay sitting on the shelf – the internal switch resets back to its initial position, again connecting pins 3-4 and 9-10.

Nominal Operating Power

Their major drawback is the power required to maintain contact closure. The EC2-5NU, TQ2-5V, FTR-B3/4 and (N)A-5W-K list a Nominal Operating Power of 140 mW. At a coil voltage of 5 V they draw 28 mA, which is more than three times the current drawn by the fuzz circuit.

Latching Relays

The most effective way to reduce power consumption is to use a latching relay. The term “latching” refers to the ability to maintain contact closure after the control power has been removed. Only a pulse of coil power is needed to move contacts in a given direction.

Single-Coil Latching Relays

The EC2-5SNU is a latching relay with only a single coil. It connects pins 4-5 and 8-9 when power is applied to pins 1-12 with polarity S, and reset the switch when the polarity is reversed.

Double-Coil Latching Relays

An EC2-5TNU double-coil latching relay has a set coil 1-12 and a reset coil 6-7. The internal switch connects pins 4-5 and 8-9 when the set coil is energized. The reset coil brings the switch back to its initial position.

Single or Double-Coil?

There are no obvious benefits to either variant, they should be selected mostly by availability and the nature of the control circuit. In a circuit built around a microcontroller, the single-coil relay occupies only one of its pins instead of two. Their power consumption over time will be similar.

Low-I_Q Voltage Regulators

Low-power relay control circuits can be built out of discrete parts or CMOS logic gates such as the CD4069, CD4049, CD40106/7, etc. Logic ICs have a very low quiescent device current of typically less than 2 µA.

Microcontrollers like the ATtiny are much more flexible but require a voltage regulator to operate. Common supply voltages are 3.3 V and 5 V.

The 78L05 is a cheap and easily available voltage regulator, with Vout = 5 V matching the relay’s coil voltage. It supports up to 100 mA output current, supplying a stable voltage with two appropriately-sized capacitors.

Quiescent Current Draw

A voltage regulator’s quiescent current IQ is an important parameter in a relay control circuit that is inactive most of the time. It’s defined as the amount of current drawn in a no-load condition.

The 78L05 unfortunately has a relatively high quiescent current of IQ(MAX) = 5 mA. A good replacement is the MCP1702, a low quiescent current voltage regulator. Its IQ(MAX) = 5 µA is three orders of magnitudes lower, without being much more expensive.

Microcontrollers

Microcontrollers such as the ATtiny13 or ATtiny84 draw up to ICC = 5 mA or ICC = 10 mA respectively, when running at their highest frequency in active mode. Their maximum output current per I/O pin is 40 mA, which suffices to energize coils of all relays mentioned earlier.

Interrupted Sleep

Generally, microcontroller power consumption can be lowered by reducing clock frequency and entering Power-Down Mode as early and often as possible. The core should sleep most of the time and only wake up to handle interrupts, triggered by user input via the SPST footswitch.

The main loop should not be constantly checking the input pin’s voltage level, but rather do it only for a few cycles after the interrupt triggers (to de-bounce). And then put the device in sleep mode again.

Implemented properly, this brings the ATtiny’s ICC down to 2 µA while it’s waiting for the next stomp on the footswitch.