ADDAC 307 Heart Sensing (2019–)

Your pulse becomes a synth parameter - and your music follows it.

Overview

It starts with a little red glow pressed to your fingertip, a faint flicker syncing to something deep and involuntary—your heartbeat. Plug that signal into the ADDAC 307 Heart Sensing module, and within seconds, your pulse is no longer just a biological rhythm; it’s a gate, an envelope, a living control voltage coursing through your Eurorack. This isn’t just another modulation source. It’s a bridge between body and machine, a way to make your synthesizer feel less like a tool and more like an extension of your nervous system. You don’t program it—you inhabit it. And when your heart rate climbs during a tense musical passage, the synth responds in kind, as if it’s feeling the same adrenaline.

The 307 is part of ADDAC's 300 Series of "Expressive Controls." It follows the 303 Muscle Sensing module as the second in a line built around the human body as an interface. Most modular gear asks you to turn knobs or patch cables. The 307 asks you to sit still and breathe. The module ships with a clip-on optical sensor, meant for a fingertip or earlobe, that detects blood flow changes with each pulse. That raw signal becomes two outputs: a 0 to +5V trigger and a 0 to +10V AD (attack-decay) envelope. The envelope has four front-panel controls - Attack, Decay, Amplitude and Offset - that shape how your heartbeat translates into voltage. Crank the attack for a sharp, snappy response. Dial in a longer decay for a slow-motion throb. Offset lifts the baseline of the envelope so it doesn't bottom out at zero, which matters when you modulate parameters that need a minimum voltage to stay active.

The 307 isn't a gimmick. Patch the trigger output to a sequencer clock input and your resting heart rate becomes a tempo, around 60-80 BPM for most people, give or take. Speed it up with exercise or stress and the sequence accelerates. Feed the envelope into a filter cutoff and your pulse becomes a rhythmic wah, swelling with each beat. One patch popular among users sends the envelope to a VCA, which makes a self-modulating gate that opens and closes in time with your heart. That suits ambient textures and drones. Dual LEDs monitor the trigger and envelope outputs in real time, so you can see your pulse on the panel even in a dark studio.

Specifications

Key Features

The Body as Interface

Unlike MIDI-based bio-sensors that need computers or complex mapping, the 307 outputs standard CV and gate signals, so it works in any Eurorack system. The optical sensor is passive, needs no batteries, and draws power from the module via the 3.5mm input. It's sensitive enough to detect a pulse on most people, though results vary with skin tone, circulation or ambient light. Some users report better reliability with the earlobe clip than the fingertip, especially in bright environments where stray light can interfere with the sensor. The module doesn't output heart rate as a numerical BPM. It's purely analog and turns each pulse into a voltage spike, so there's no digital processing lag.

Envelope Shaping with Biological Precision

The AD envelope generator is where the 307 gets useful. Most envelope generators need a trigger to start. Here the heartbeat provides it, so the module is a self-contained rhythm source. Attack and Decay smooth or sharpen the envelope's rise and fall, which effectively lets you "filter" your pulse. Turn Decay high and each heartbeat swells into a slow ramp, good for modulating LFO rate or reverb mix. Lower it and you get a tight, percussive burst for triggering drums or stabs. The Amplitude control scales the envelope's peak voltage, while Offset lifts its floor and keeps it from dipping below zero. That matters when modulating parameters that respond poorly to negative voltages, like oscillator pitch or filter resonance. An inverted envelope output adds more options, such as one voice opening while another closes, all in sync with your pulse.

DIY Accessibility and Customization

ADDAC offers the 307 as a DIY kit for €100 (excluding VAT), one of the more affordable entry points into bio-responsive modular synthesis. The kit is classified as "SMD-Kit-2," meaning it includes both surface-mount and through-hole components and is aimed at intermediate builders. The assembly guide is well documented, with clear diagrams and step-by-step photos, and the community around ADDAC kits is active and supportive. Custom front panels are available in red, green, blue, white, silver gray, yellowed silver and bronze, with print colors chosen to contrast the panel. Each custom panel is ordered individually and takes 4-6 weeks to arrive. The standard black aluminum panel has a clean, clinical look that suits the module's medical aesthetic.

Historical Context

The ADDAC 307 emerged in 2019, when Eurorack was expanding into experimental control and interface design. Companies like MakeNoise and Intellijel were pushing Buchla-inspired touch plates and pressure-sensitive interfaces. ADDAC took a different path and looked inward, literally. The 307 wasn't the first attempt to use biometrics in music. David Rosenboom's brainwave-controlled performances in the 1970s and the MIDI-based BioMuse systems of the 1990s came first. But it was among the first to bring the concept into the modular mainstream at an accessible price. It arrived alongside a wave of interest in embodied performance, where musicians sought more physical connections to their instruments beyond the keyboard.

It also reflected ADDAC's broader philosophy of hybrid design, blending analog circuits with digital concepts, often in unexpected ways. The 307 is entirely analog in its signal path, but its function relies on a digital sensor. The module doesn't try to "perfect" the heartbeat signal with smoothing or averaging. Skipped beats, arrhythmias and natural variations in pulse strength all come through.

Collectibility & Value

The ADDAC 307 isn't a rare module, but it's not common either. Since it's still in production, it doesn't carry the scarcity premium of discontinued gear. New units sell for around $229 from retailers like Perfect Circuit, while used ones typically go for $150-$180, depending on condition and whether the sensor is included. The DIY kit remains available directly from ADDAC, which keeps prices stable and prevents scalping. The module has no moving parts and minimal circuitry, so failure rates are low. The most common issue reported is sensor degradation. Over time the optical clip can lose sensitivity, especially if the LED dims or the photodiode gets dirty. Replacement sensors are not officially sold, but third-party pulse oximeter clips with compatible 3.5mm jacks can often be used as substitutes, though results vary.

For collectors, the 307 is less about investment value and more about conceptual significance. Custom panels sometimes turn up on the secondary market, where they occasionally sell for 20-30% above retail. The module is still supported, and it has no firmware, so updates aren't a concern and technical obsolescence is unlikely. Some musicians find the idea of using their heartbeat too intimate, too exposed. Others use the 307 in live performances where their pulse becomes part of the show. If you're considering a purchase, test it if possible. Some people don't get a strong enough signal, especially in cold environments or with certain skin types.

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