LFO Taptempo Buddy

Small but powerful ATtiny402/412 based tap-tempo LFO module and firmware. Output is a waveform (Sin, Triangle, Pulse, Ramp up, Ramp down, Random) at the Speed set by the Speed Pot or by tapping. Multiple Random algorithms. Leslie like speed change on long tap (faster, or slower when already fast). Intended for tremolo, phaser, vibe and similar modulation effects.

This project is adaptation of Hydra Delay Taptempo Buddy.

Tip: You can use my Simple Serial UPDI programmer to program the microcontroller for this project.

• LFO Speed can be controlled by Speed Pot or tapped by Tap Button. Period range is 50 ms (~20 Hz) to 2000 ms (~0.5 Hz).
• LED always blinks at the LFO Speed with 50% duty cycle, locked to LFO phase (both pot and tap control).
• Tap Button does four jobs:
  • Two or more taps — Speed from tapping.
  • Hold from rest (over 500 ms) — Leslie.
  • Short tap, then hold (over 500 ms) — Random algorithm.
  • Hold and flip Shape — alt bank (Ramp up / Ramp down / Random).
• Tap Tap Button at least two times to switch from Speed Pot control to Tap Button control.
  • First tap aligns LFO and LED to the downbeat without changing Speed.
  • Second tap must follow within 2.5 s after the first one.
  • Subsequent tap times are averaged until tapping finishes.
  • Tapping finishes if the next tap is not performed for 3 periods of the current Speed, but never later than 2.5 s.
  • Very slow subsequent taps (gap between 2 s and 2.5 s) still count; only that interval is clamped to 2 s. Faster taps are left as measured.
• Move Speed Pot to take control from tap tempo or latched Leslie: the pot is ignored until its speed matches the current sounding Speed (catch-up), then control passes to the pot with no jump. In normal pot mode the LFO tracks the pot continuously.
• Shape on-off-on switch selects the LFO waveform:
  • Flip without holding Tap Button — normal bank: Sin, Triangle, Pulse.
  • Flip while holding Tap Button — alt bank: Ramp up, Ramp down, Random.
  • Hold-flip only changes the shape bank. Speed, Leslie, and Random algorithm stay as they are. The press still lines up LFO phase.
  • Move the switch without holding Tap to return to Sin / Triangle / Pulse.
  • With Leslie latched, wait for Speed to settle after release, then press Tap Button again before hold-flip for the alt bank.
• Random shape algorithm (Hybrid / S&H / Wander) is cycled with a short tap then one long press of the Tap Button (hold the second press over 500 ms). Works in any shape; the sound changes when Random is selected. The LED still blinks 1 / 2 / 3 on any shape so you can see it was stored.
  • LED blinks 1 / 2 / 3 times (Hybrid / S&H / Wander) to indicate the selected algorithm.
  • The same blink is shown on power-up when Random is selected. See Random shape algorithm.
• Leslie — long Tap Button press from rest (over 500 ms, no short tap before it):
  • While held: Speed glides toward 2× (half period, min 50 ms) when current speed is 300ms or slover, or toward 1/2x (double period, max 2000 ms) when faster. Speed Pot defines glide time (~0.3–8s). Long tap is occupied — no tap tempo, Random cycle, or hold-flip. Foot-down at the start of the hold does not phase-sync LFO.
  • Release at the Leslie target (2× or 1/2x) -> latch there, pre-Leslie Speed is remembered.
  • Release before target -> glide back to the origin speed without latching, press again to ramp toward Leslie.
  • While latched:
    • Short tap — downbeat align only, stay latched.
    • Tap session (two or more short taps) — new Speed, Leslie cancelled.
    • Short tap then long hold — Random cycle change, stay latched.
    • Speed Pot catch-up — Leslie cancelled.
    • Long hold from rest — unlatch, glide to remembered origin even if you release early, long tap again during that glide to head back toward Leslie.
• Current Speed Pot/Tap Button control state, tapped-in Speed, Random algorithm, and shape bank are preserved over power-off. Latched Leslie Speed is not stored — boot restores the last non-Leslie Speed.
• Trimmer or fixed resistor to set LED brightness.
• UPDI pins to re-program the soldered microcontroller.

• firmware.hex - firmware binary
• firmware/ - VSCode/PlatformIO project with firmware
• LFOBuddy.dch - schematics
• LFOBuddy-rev1_gerber.zip - Gerber file for PCB fabrication
• LFOBuddy.dip - PCB design file
• ExampleLedDriver.dch - schematics of the LED driver example

Schematics and PCB design file can be opened/edited by DipTrace.

• Plan module and controls (Tap Button, Shape on-off-on, Speed Pot, LED) placement. Use long enough wires.
• Power and output:
  • GND - ground
  • OUT - LFO voltage with 0..5V swing (or 3.3V if VCC is 3.3V only)
  • VCC - 7V+ or 5V/3.3V power, see note in Building module
  • You can use a connector to disconnect module easily for programming, spacing on PCB is 2.54mm for it.
• Speed Pot - connect pot's 1, 2 and 3 lugs to the module's P1, P2 and P3. Use B type pot, ideally B10k. Higher voltage is higher LFO Speed (shorter LFO period). The firmware uses a logarithmic map so more of the pot's travel sits in the ~200–800 ms range.
• Tap Button - connect momentary button to the module's TAP pads.
• LED - connect LED to the module's L+ and L-. Use TL trimmer to set LED's brightness. Used 5k value should be OK for the most LED types and 5V internal VCC, if too small for your LED, use higher trimmer value, or connect additional resistor in series. Alternatively use fixed value resistor RL if you figured out exact value and wanna to save some space.
• Shape switch - SPDT on-off-on. Common to SH. Throws to SHG and SHV. Left of / is the normal bank (no hold), right is the alt bank (hold Tap).

    SHV ---- throw     Pulse    /  Random
                |
    SH  ---- common    Triangle /  Ramp down
                |
    SHG ---- throw     Sin      /  Ramp up
  

Analog OUT is the LFO control voltage PWM:

• PCB contains simple filter to smooth it out. Cut frequency should be 100–200Hz, so eg. R=1kohm and C=1uF is good into high-Z load. So buffer it, or adjust filter for small-Z loads.
• Maximal amplitude is module VCC (5V or 3.3V), so scaling may be necessary for next circuitry
• You have to implement LFO Depth pot if you want it.

Exact circuitry consuming this module's output depends on how do you want to drive the rest of the pedal circuit - LDR LED/s, lamp, JFETs etc.

Example driver for optical harmonic tremolo/trmolo/vibe:

Notes:

• Expects normal OpAmps without rail-to-rail output, eg. TL07x
• VCC - pedal VCC (9V), not LFO taptempo module VCC (5V). OpAmps run on it also.
• LFO - signal from LFO taptempo module with 5V amplitude
• C1 - decouples LFO swing baseline (2.5V) from driver baseline (4.5V) created by U3, injected by R7 and R4.
• Scale trimmer - use it to set maximal unclamped LFO signal at U2 output
• D1, D2 - LDR LEDs for optical harmonic tremolo, only one (whichever) for normal tremolo or vibe
• R5, R6 - adjust for your LEDs if necessary to get correct brightness
• R3 - removes dead end of Depth pot, adjust if necessary

Module schematics:

Note: you can omit U2 and bridge its pins 1 to 3 if you have 5V or 3.3V from the main pedal board.

PCB BOM:

External components:

PCB:

Source code contains constants for the LFO period range and tap timeout. Change them and rebuild if you want a different Speed span.

const uint16_t c_lfo_max = 2000;   // slowest LFO period [ms] (~0.5 Hz) - Speed pot on minimum
const uint16_t c_lfo_min = 50;     // fastest LFO period [ms] (~20 Hz) - Speed pot on maximum
const uint16_t c_lfo_range = 1950; // MUST MATCH c_lfo_max - c_lfo_min [ms]!
const uint16_t c_tap_end_max = 2500; // max wait for the next tap [ms]; must be > c_lfo_max

Output PWM carrier is 20 kHz, range 0..c_pwm_max (999). The analog LFO waveform appears after the RC filter on OUT.

pot_to_period() uses a 64-step log LUT (period_log_lut in firmware/src/main.c) so more physical pot travel covers ~200–800 ms. Use a linear B pot, catch-up and tap/pot handoff use the same map.

const uint16_t c_pot_catchup_ms = 40; // minimum catch-up window [ms]; also 5% of sounding period

Long Tap Button hold (from rest, over 500 ms) glides LFO rate toward 2× or ½ while held, Speed Pot defines glide time. Constants are in firmware/src/main.c. See Leslie under Features for latch, unlatch, and footswitch behavior.

Speed multiplier — c_leslie_speed. Default 2 (integer only; use 3 for 3×, and so on).

const uint8_t c_leslie_speed = 2;

Why 2× by default? A real Leslie cabinet jumps roughly 8× between chorale (~40–48 RPM) and tremolo (~340–400 RPM). This feature borrows the idea (hold to shift speed, latch, glide back), not that ratio — it drives a single tremolo LFO, not separate treble/bass rotors with inertia. 2× (one octave of rate) is a practical default: clearly audible (e.g. 500 ms -> 250 ms) but predictable, without slamming the speed limits. Most of the “Leslie” feel here comes from speed glide, not the endpoint ratio. For a bolder shift try 3×, values near 8× usually hit the 50 ms floor from moderate base speeds and are a poor fit for this LFO.

Direction threshold — c_leslie_slowdown_ms. Default 300 (~3.3 Hz). Chooses speed threshold where hold speeds up or slows down:

const uint16_t c_leslie_slowdown_ms = 300;

• Period at or above the threshold → hold speeds up: period / c_leslie_speed, clamped at c_lfo_min (50 ms).
• Period below the threshold → hold slows down: period * c_leslie_speed, clamped at c_lfo_max (2000 ms).

Latch — c_leslie_latch. Default 1. When 1, release at the Leslie target stays there (see Leslie in Features). When 0, release at target also immediatelly glides back to the origin, without latching.

const uint8_t c_leslie_latch = 1;  // 1 = latch at target on release; 0 = always return to origin

Glide time — c_leslie_ramp_min_ms / c_leslie_ramp_max_ms. Total time for a full 2×/½ leg (constant rate over the distance):

const uint16_t c_leslie_ramp_min_ms = 300;  // Speed pot max — ~0.3s full glide
const uint16_t c_leslie_ramp_max_ms = 8000; // Speed pot min — ~8s full glide

The map is quadratic on the slow end of the pot so fine control sits in the longer ramps. While Leslie is held, the pot sets ramp time only (re-sampled live during the hold). Approximate full 2× leg (e.g. 500 ms → 250 ms) vs. pot rotation — 0% = slowest LFO end, 100% = fastest:

Why up to 8s? A real Leslie slews in about 1–2s; mid pot (~2s) is closest to that. 8s is for slow, ambient rate morphs, not a classic chorale->tremolo kick. Multi-second ramps also need a matching foot hold to reach the target and latch. For a tighter Leslie feel, lower c_leslie_ramp_max_ms (e.g. 3000–4000).

Random still follows the tapped or pot Speed, with one new random level per LFO cycle. What it does with that level is the algorithm:

• Hybrid — S&H when period is 400 ms or shorter, Wander when LFO is slower (default).
• S&H — jumps to a random level and sits there until the next cycle (classic stepped random).
• Wander — glides smoothly from the last level to the next over the cycle (no stairs; nicer when the LFO is slow).

How to switch: tap once (short), then press and hold (longer than 500 ms). That is not a new Speed, and it is not Leslie. You can do this tap pattern with any shape selected. The sound only changes when Random is selected; the LED still blinks the new algorithm on any shape.

The LED then blinks the new algorithm. The choice is stored in EEPROM, so it survives power-off:

• 1 blink — Hybrid
• 2 blinks — S&H
• 3 blinks — Wander

The same blinks happen at power-up, but only if Random shape is already selected (so other shapes do not look like an algorithm change). To always blink on power-up, set ANNOUNCE_RANDOM_ON_BOOT_ALWAYS to 1 in firmware/src/main.c.

A blank chip starts in Hybrid. To change that default, edit LFO_RANDOM_MODE in the same file.

The 400 ms Hybrid algorithm split is defined in c_random_hybrid_ms (400 ms and faster -> S&H in Hybrid).

After the 1/2/3 announcement blinks there is a trailing dark so they do not blend into the LFO tempo LED:

#define C_RANDOM_ANNOUNCE_DARK_MS 700     // dark after algorithm blinks so they don't blend into tempo LED

I'm using VS Code with Platform IO extension. You have to have Atmel megaAVR Platform installed in the Platform IO.

platformio.ini is committed in the firmware/ folder with all the basic settings, including pymcuprog related settings for my Simple Serial UPDI programmer.

Open the firmware/ folder in VS Code and it should work.

Easiest way is to set the microcontroller up before soldering it to the PCB.

If you want to change firmware later, you can use UPDI pins on the module, where GND is middle, UPDI left, VCC right.

Be careful, never program it with 5V VCC when the module is connected to a 3.3V chip like FV-1 DCP - you can damage the DSP or other chips.

I recommend to always disconnect it for programming, using a connector on the GND / OUT / VCC connection.

Revision string of each version is baked into firmware binary.

• v1.0 - rev_1 - work in progress
  • initial release

Functional changes (also marked by MOD: in the source):

• runs at 5V power supply by default, not 3.3V
• PWM is an LFO waveform, not a DC delay-time / Speed voltage.
• Hydra's digital "Tempo Division Switch" (DIV) changed to 3 state analog LFO Shape on pads SH / SHG / SHV.
• LED always blinks, locked to LFO phase (including pot-control mode).
• First tap aligns LFO/LED to the downbeat without changing Speed.
• Tap-session timeout capped at 2.5 s (first-tap window too). Hydra used uncapped 3× tempo and a 1.5 s first-tap window.
• LFO period range 50-2000 ms instead of Hydra delay 150-920 ms.
• Long-press is a Leslie function, not Ramp.
• Speed pot log map and catch-up takeover from tap or latched Leslie.
• Short tap then long press cycles Random algorithm (Hybrid / S&H / Wander).

© 2026 ElvisAlive Tone. This work is openly licensed via CC BY-NC-SA