Building an Off-Grid HHO System

A complete guide to building an HHO (oxyhydrogen) system for off-grid use — heating supplementation, generator fuel savings, or torch applications. From component selection through Lateralus-powered monitoring.

◉ What We're Building

An HHO system with these specifications:

• Output: 500-2000 L/hr (adjustable)
• Power: 12V or 24V DC (solar/battery compatible)
• Monitoring: RP2040-based with Lateralus firmware
• Safety: Flame arrestor, pressure relief, auto-shutoff

Total cost: $150-300 depending on size and components.

◉ Safety First

HHO is flammable. The hydrogen-oxygen mixture is more reactive than either gas alone. Essential safety measures:

• Flame arrestor: Sintered bronze (50μm or finer) between generator and output
• Bubbler: Gas passes through water to prevent flashback
• Pressure relief: 5 PSI max, vents to outside
• Ventilation: Never run in enclosed spaces
• No sparks: Keep away from electrical connections
• Kill switch: Emergency power cutoff accessible

Take this seriously. A flashback can destroy the cell and injure you.

◉ Components

Electrolyzer Cell

The heart of the system. Two designs:

Type	Pros	Cons
Dry Cell	Higher efficiency, cooler operation	More complex to build
Wet Cell	Simpler, cheaper	Lower efficiency, more heat

We recommend dry cells for off-grid use — efficiency matters when you're on solar.

Materials for 7-plate Dry Cell

• 7× 316L stainless steel plates, 6"×6"×1/16"
• 6× EPDM gaskets, 6"×6"×1/8"
• 8× M8 stainless bolts, nuts, washers
• 2× Acrylic endplates, 1/4" thick
• Hose barb fittings (gas outlet, water fill)

Reservoir

• 1-gallon HDPE container (food-grade)
• Float switch (water level sensor)
• Fill port with cap
• Return line from cell

Bubbler

• Quart-size jar with two-hole lid
• Input tube to bottom
• Output from top (above water level)
• Fill with water — gas bubbles through

Controller

• PWM board (30A minimum)
• RP2040 microcontroller
• Current sensor (ACS712 30A)
• Temperature sensors (DS18B20 × 2)
• Pressure sensor (MPX5010DP)
• Display (optional, 0.96" OLED)

◉ Assembly

Step 1: Plate Preparation

# Sand plates for better electrolysis
# Cross-hatch pattern at 45 degrees
# Use 120-grit sandpaper on both sides
# Clean with isopropyl alcohol
# Wear gloves — no oils on plates

Step 2: Cell Stack

1. Cut gaskets with proper holes (plate opening + 2 bolt holes)
2. Alternate: Plate → Gasket → Plate → Gasket...
3. End plates: Acrylic with bolt holes
4. Torque bolts evenly in star pattern
5. Test for leaks with water before first use

Step 3: Wiring

For 12V system (7 plates = 6 cells in series):

Configuration: +N-N-N-N-N-N-
  +     = Positive terminal plate
  N     = Neutral plates (no connection)
  -     = Negative terminal plate

Cell voltage: 12V / 6 = 2.0V per cell (optimal)

Step 4: Plumbing

Water Reservoir
     |
     v
[Electrolyzer Cell]
     |
     v
Flame Arrestor
     |
     v
Bubbler (water trap)
     |
     v
Output (to torch/appliance)

◉ Electrolyte

KOH (potassium hydroxide) is preferred:

• Concentration: 3-5% by weight
• Water: Distilled only — tap water contains minerals
• Handling: KOH is caustic — wear goggles and gloves

# Mixing (for 1 gallon):
# 3% = 115g KOH per gallon
# 4% = 150g KOH per gallon
# Start with 3%, increase if efficiency low

◉ Lateralus Firmware

The RP2040 runs monitoring firmware:

@freestanding
module hho.control

import hw.adc
import hw.pwm
import hw.gpio

const MAX_TEMP = 75.0      // °C
const MAX_PRESSURE = 4.5   // PSI
const MIN_WATER = 0.2      // Level sensor

fn main() {
    init_peripherals()

    loop {
        let sensors = read_sensors()

        // Safety checks
        if sensors.temp > MAX_TEMP {
            emergency_stop("Over temperature")
        }
        if sensors.pressure > MAX_PRESSURE {
            emergency_stop("Over pressure")
        }
        if sensors.water_level < MIN_WATER {
            emergency_stop("Low water")
        }

        // Adjust PWM for target current
        let duty = calculate_duty(sensors)
        pwm.set_duty(0, duty)

        // Update display
        display.update(sensors)

        sleep_ms(100)
    }
}

fn calculate_duty(s: Sensors) -> u16 {
    // Lower duty as temperature rises
    let temp_factor = 1.0 - ((s.temp - 40.0) / 40.0) * 0.3
    let target = TARGET_CURRENT * temp_factor

    // PID control for current
    pid_update(s.current, target)
}

◉ Performance Tuning

Parameter	Optimal Range	Effect
Cell voltage	1.8-2.2V/cell	Too low: slow, too high: heat
Temperature	50-65°C	Higher = faster but less efficient
KOH concentration	3-5%	Higher = more conductive
Plate spacing	2-3mm	Smaller = lower resistance

◉ Integration

Common applications:

• Wood stove: Inject into air intake for hotter, cleaner burn
• Generator: Mix with air intake (5-15% fuel savings)
• Torch: Direct use for brazing/soldering
• Propane heater: Supplement with HHO

◉ Troubleshooting

• Low output: Check electrolyte concentration, plate cleanliness
• Excessive heat: Reduce current, improve cooling
• Brown electrolyte: Contamination — drain and refill
• No bubbles: Check wiring polarity, connections

Full build documentation at the HHO page. CAD files at hho-genesis.