Silent Digital Typewriter — HP45 Inkjet

A modern, silent typewriter that prints on plain paper using an HP45 inkjet cartridge.

Features

Instant boot (ESP32-S3 microcontroller)
USB keyboard input
USB flash drive support (save / load text files)
Plain text editor + print buffer
Real ink-on-paper, not thermal
Ultra-quiet motion (NEMA-14 steppers + TMC drivers)
Dedicated HP45 inkjet controller for microsecond firing pulses
Portable, battery powered
Serviceable and long-lived design philosophy

This project explores a practical "digital typewriter" that behaves like a real appliance — no operating system, no laptop required.

Motivation

The Problem: Traditional analog typewriters are too noisy and distracting to be used in a classroom. iPads and laptops are too distracting for both students and teachers/professors. Handwriting is sometimes not preferable.

The Solution: A silent, distraction-free, battery-powered typewriter with keyboard input for students who want to type in class or use a typewriter without the noise distraction.

Most modern "portable typing" is trapped behind screens. This device aims to be:

Quiet enough for classrooms and libraries
Lightweight and battery powered
Compatible with standard plain paper
Able to save work to a USB stick
Serviceable and long-lived
Not just a novelty — a real writing tool

Why This Project?

Existing Solutions & Limitations:

Freewrite Smart, Alpha Typewriter: Digital-only, expensive
Older thermal paper machines (Canon Typestar, Brother Ep43, etc.):
- Thermal paper is toxic and uncommon
- Old hardware is hard to find and unreliable
- No file export capability

This Project's Advantages:

✅ Real ink on paper (non-toxic, standard paper)
✅ Modern, open-source design
✅ File export capability
✅ Multiple modes and programs
✅ Interactive CLI interface
✅ Serviceable and long-lived design
✅ Potential Raspberry Pi version
✅ Educational tool and art piece
✅ All while maintaining: distraction-free, battery-operated operation

Where This Device Excels

Beyond the classroom, this typewriter excels in various scenarios:

Libraries and quiet spaces: Silent operation allows typing without disturbing others
Focus writing sessions: No internet, notifications, or apps to distract from the task
Journaling and personal writing: Physical output provides a tangible record
Field research and note-taking: Battery-powered portability for on-site documentation
Creative writing retreats: Distraction-free environment for deep work
Accessibility: Alternative input method for those who prefer typing over handwriting
Offline documentation: Create physical records without digital infrastructure
Artistic projects: More than just typewritten documents — create visual art, graphics, patterns, and mixed-media works as physical artifacts
Workshops and conferences: Quiet note-taking during presentations
Travel writing: Portable, battery-powered typing without laptop bulk

System Architecture

USB Keyboard → ESP32-S3 → Text Buffer → Rasterizer → HP45 Controller
                                                         ↓
                                               microSD / USB Flash (save files)

- Carriage Axis (NEMA-14 + TMC2209 + rail)
- Paper Feed Axis (NEMA-14 + TMC2209 + pinch rollers)

The ESP32-S3 provides instant-on firmware, a file system, and a small UI. The HP45 controller board manages all inkjet pulse timing and nozzle drive safely.

Hardware Overview

MCU: ESP32-S3 with USB-OTG (USB Host)
Inkjet: HP45 cartridge + dedicated controller (serial protocol)
Carriage: NEMA-14 stepper, GT2 belt, MGN7/MGN9 linear rail + end stop sensor (home position)
Paper Feed: NEMA-14 stepper + pinch rollers + AS5600 magnetic encoder (virtual detents)
Drivers: 2× TMC2209 (silent / microstepping)
Storage: USB flash + optional microSD
Power: 2S Li-ion → buck 5V → 3.3V LDO, plus 18–24 V boost for HP45
UI: Monochrome OLED 1.3" 128×32 (SSD1306/SH1106 via I²C), 2-line layout, buttons (PRINT/MODE/FEED/BKSP)

Firmware Features

Reads USB keyboards (HID)
Saves .txt files to USB flash
Simple line buffer editing (typewriter mode / line mode)
Prints lines as raster sweeps
Silent stepper motion (StealthChop + low jerk)
Autosave and document export
Real-time 2-line OLED display with keystroke echo (≤10 ms response)
Status indicators (battery %, USB, microSD, mode)
Virtual detent paper feed (AS5600 encoder, 6/8 LPI switching)
Free-roll lever with soft spring detent alignment
Carriage homing (end stop sensor for repeatable print alignment)
Character spacing: 10 or 12 CPI (characters per inch) selectable

Repository Structure (planned)

firmware/
├── esp32-s3/
│   ├── keyboard host
│   ├── filesystem (FatFs)
│   ├── rasterizer
│   ├── motion control
│   ├── HP45 controller protocol
│   ├── display driver (SSD1306/SH1106)
│   ├── text UI (2-line layout)
│   ├── encoder driver (AS5600)
│   ├── virtual detent controller
│   └── end stop sensor (carriage homing)

hardware/
├── main PCB (ESP32-S3, TMC drivers, power, I²C headers, GPIO inputs)
├── carriage module (CAD + BOM, end stop sensor mount)
├── feed module (CAD + BOM, AS5600 encoder)
├── OLED display module (I²C, 4-pin header)
└── free-roll lever mechanism

docs/
├── theory of operation
├── serial protocol to HP45 controller
├── wiring diagrams
├── display specifications
├── encoder & virtual detent system
├── end stop sensor specifications
└── maintenance notes

Status

✅ Idea and system architecture defined
🔄 PCB and firmware development in progress
🔄 CAD in progress
📋 Mechanical prototype upcoming

Contributions, ideas, and experiments are welcome — pull requests encouraged!

Goals (v1)

Typewritten text printed cleanly on plain paper
USB keyboard and flash drive working end-to-end
Basic editor + print buffer
Instant boot and safe power-off
Battery powered, quiet operation

Future Extensions

BLE keyboard support
Basic paragraph editor
Additional font options (10/12 CPI already implemented)
Simple plotting / vector graphics
Wireless export
Encrypted notes mode
Templating pages
Raspberry Pi version

Research Questions

User studies to understand preferences and effectiveness:

I like physical/digital documents over [opposite] ones
I can focus easily/get distracted on my computer when I'm typing a document
I prefer to type/handwrite my drafts instead of [opposite] them
I find digital/physical notes more effective than [opposite] notes

License

TBD — MIT recommended for hardware + firmware openness.

Contact / Discussion

Open an issue, start a discussion, or fork and experiment.

Display Specifications

Screen Technology

Type: Monochrome OLED
Size: 1.3″ 128×32 pixels
Controller: SSD1306/SH1106
Interface: I²C (preferred), SPI optional
Response Time: ~1–2 ms (no ghosting)
Power Consumption: ~20–25 mA at full brightness (typical much lower)

Display Layout

Lines: 2-line layout (typewriter aesthetic, fast edits)
Line 1: Live edit buffer (monospace 6×10 or 8×12 font)
- ~16–21 characters visible depending on font/margins
Line 2: Context/status display
- Shows: Mode, USB status, Battery %, microSD status, Quiet/Draft icon
- Alternative: Previous line in Line-Buffer mode

Font Options

Default: 6×10 monospace (~21 chars per line)
High-visibility: 8×12 monospace (~16 chars per line)

User Interface

Soft Keys (mapped to physical buttons):

PRINT - Print current buffer
MODE - Switch between Typewriter/Line-Buffer modes
FEED - Feed paper line
BKSP - Backspace/delete

Status Indicators:

Battery percentage
USB present (✓)
microSD present
Quiet/Draft mode icon

Hardware Implementation

I²C Configuration: SCL/SDA with 2.2–4.7 kΩ pull-ups to 3.3 V
Connector: 4-pin header (VCC, GND, SCL, SDA) + optional RST
Power: 3.3 V supply, ~20–25 mA budget for bright pages
Future: SPI pads reserved if faster refresh needed

Firmware Requirements

Display driver abstraction (SSD1306/SH1106 via I²C)
UI rendering functions:
- ui_render_line(text) - Update line 1 (edit buffer)
- ui_render_status(mode, usb, batt, sd) - Update line 2 (status)
Keystroke echo at ≤10 ms end-to-end
Screen invalidation (dirty regions) to minimize I²C traffic
Font tables: 6×10 (default), 8×12 (hi-vis)

Configuration:

DISPLAY_LINES=2
FONT=6x10 (or 8x12)
MARGIN_COLS=1

Rationale

Two lines maintain minimalism while improving editing and status visibility vs. single line
OLED chosen over LCD for instant response and legibility of small monospace fonts
I²C preferred for simplicity; SPI reserved if bus gets crowded
Fast response (≤10 ms) ensures real-time typing feedback

Paper Feed Encoder & Virtual Detent System

Encoder Choice

AS5600 (selected)

Interface: I²C
Resolution: 12-bit (4096 counts/revolution)
Wiring: Simple (2 wires + power)
Rationale: Plenty of resolution for virtual detents, simplest wiring, excellent for tactile feedback

AS5048A (alternative)

Interface: SPI
Resolution: 14-bit (16384 counts/revolution)
Wiring: More complex (4 wires + power)
Use case: Ultra-fine detent feel if needed

Mechanics (Paper Feed)

Roller Circumference (C): 50.8 mm (2.000″) recommended (clean LPI math)
Line Height (H):
- 6 LPI → 4.233 mm
- 8 LPI → 3.175 mm
Detents per Revolution:
- 6 LPI: 12 detents
- 8 LPI: 16 detents

Counts per Detent (Virtual Notch)

For AS5600 (4096 CPR):

6 LPI: ~341 counts/detent (4096 × 4.233 / 50.8)
8 LPI: ~256 counts/detent (4096 × 3.175 / 50.8) — power-of-two-friendly

For AS5048A (16384 CPR): multiply by ~4

6 LPI: ~1364 counts/detent
8 LPI: ~1024 counts/detent

Hardware Implementation

Electrical (ESP32-S3 + AS5600):

Power: 3.3 V to AS5600 VDD
I²C: SDA → GPIO (e.g., 8), SCL → GPIO (e.g., 9)
Pull-ups: 2.2–4.7 kΩ to 3.3 V (shared with OLED)
ADDR: Leave default unless multiple encoders
Magnet: 6 mm diameter diametric magnet on feed shaft end
- Centered on shaft
- 1–2 mm gap from encoder IC
- N52 recommended for strongest signal

Mechanical:

Silicone/urethane roller (≈16 mm Ø for 2.000″ circumference with jacket)
Free-roll lever switch (detects lever position)
Magnet mounting on feed shaft end

Firmware Model (Virtual Detents)

State Machine:

FREE_ROLL (lever up)
- User can spin paper freely
- Motor applies soft virtual spring to nearest detent
- Low hold current (80–120 mA)
- Provides tactile feedback without resistance
DETENT_LOCKED (within ±window of detent center)
- Motor stiffens slightly to give click/hold feel
- Medium hold current (180–240 mA)
- Holds paper at exact line position
PRINT_ALIGN (lever down)
- Nudges to exact detent center
- Normal line-advance stepping
- Print current (300–500 mA RMS) for traction
- Open-loop motion during printing; encoder used for alignment only

Core Math:

counts_per_detent = CPR × (H / C)
detent_index = round(angle_counts / counts_per_detent)
detent_center = detent_index × counts_per_detent
d = angle_counts - detent_center  (signed error)

Control Loop (~1 kHz update rate):

Outside snap window: Set low spring → target = angle - k_soft × d; low hold current
Inside detent window: Lock → target = detent_center; medium hold current
On resume print: Step tiny correction to detent_center; set print current

Suggested Gains & Thresholds:

snap_window: ~±(0.35 × counts_per_detent)
k_soft: 0.15–0.30 (dimensionless; tune by feel)
Currents (NEMA-14 via TMC2209):
- low_hold_current: 80–120 mA
- med_hold_current: 180–240 mA
- print_current: 300–500 mA RMS (tune for traction)

Stepper Integration

Feed Axis:

TMC2209 in StealthChop mode
Hold current varies by state (FREE_ROLL / DETENT_LOCKED / PRINT_ALIGN)
During printing: open-loop motion (encoder ignored)
After each line advance: recenter to detent if residual error appears

Carriage Axis:

Remains open-loop stepper (no encoder)
TMC2209 StealthChop
End stop sensor for homing (mechanical microswitch or optical)
Home on startup (seeks end stop, establishes reference position)
Constant-velocity sweeps for HP45 raster printing

Spacing Selection (CPI & LPI)

CPI Switching (10/12):

10/12 CPI toggle in UI changes:
- steps_per_character calculation
- Carriage motion timing during print sweeps
- Display shows "CPI: 10" or "CPI: 12" on second OLED line

LPI Switching (6/8):

6/8 LPI toggle in UI changes:
- counts_per_detent calculation
- Firmware's steps-per-line during motor-driven advance
- Display shows "LPI: 6" or "LPI: 8" on second OLED line

Settings:

CPI and LPI settings are independent (can be combined in any way)
Both settings stored in NVS and persist across reboots
Display format: "CPI: 10 | LPI: 6" (abbreviated as space allows)

Calibration Routine (~1 minute)

Enter Cal mode in UI
Prompt: "Install magnet; rotate 1 rev"
Read AS5600 raw from 0→4095; verify clean wrap (no dropouts)
Measure actual roller circumference C_meas:
- Mark paper/roller
- Roll one revolution against ruler
- Store in NVS (Non-Volatile Storage)
Compute counts_per_detent from measured C_meas
Test: Show live detent_index; rotate by hand and verify predictable "clicks"

Parts Shortlist

AS5600 board (I²C breakout)
6 mm diametric magnet (N52 recommended)
NEMA-14 stepper (feed axis)
TMC2209 driver (feed axis)
Silicone/urethane roller (≈16 mm Ø for 2.000″ circumference)
Lever switch (free-roll detect)
ESP32-S3 DevKit (USB-OTG)

Benefits

Natural feel: Virtual detents provide tactile feedback like mechanical typewriters
Precise alignment: Encoder ensures perfect line spacing
Flexible: Easy LPI switching without mechanical changes
Quiet: Soft spring in free-roll mode; no mechanical clicks
Reliable: Magnetic encoder has no mechanical wear
Calibratable: Software calibration compensates for manufacturing tolerances

Carriage End Stop Sensor

Purpose

The carriage end stop sensor establishes a repeatable home (reference) position for the carriage axis. Since the carriage uses open-loop stepper control (no encoder), the end stop provides the absolute position reference needed for accurate print alignment.

Sensor Options

A3144 Hall Effect Sensor (recommended)

Type: Unipolar digital Hall effect sensor (Allegro A3144)
Operating Voltage: 4.5–24 V (5 V recommended)
Output: Open-collector, active LOW when magnet detected
Sensitivity: Switches at ~75–150 G (Gauss)
Response Time: < 5 μs
Advantages: Non-contact, reliable, no mechanical wear, long life, immune to dust/contamination
Mounting: Fixed to chassis at carriage home position
Actuator: Small diametric magnet (e.g., 3×3 mm, N42/N52) on carriage
Operating Distance: 1–5 mm from magnet (tune for reliable triggering)

Mechanical Microswitch (alternative)

Type: SPDT or SPST microswitch (e.g., Omron D2F-01F, Cherry D44X)
Actuation Force: 0.25–0.5 N
Travel: 0.5–1.0 mm
Advantages: Simple, low cost, passive (no power)
Mounting: Fixed to chassis at carriage home position
Actuator: Cam or flag on carriage that depresses switch at home
Considerations: Mechanical wear over time, contact bounce

Optical End Stop (alternative)

Type: Phototransistor/photodiode pair or reflective sensor (e.g., TCST2103)
Response Time: < 1 ms
Advantages: Non-contact, high precision, long life
Mounting: Fixed to chassis with reflector/flag on carriage
Considerations: Requires LED current (~5–20 mA), sensitive to ambient light

Hardware Implementation

A3144 Hall Effect Sensor (Recommended):

Electrical:

Power: 5 V (VCC) from ESP32-S3 5 V rail or LDO
Ground: GND connection
Signal: GPIO input (e.g., GPIO 10) with pull-up resistor
Connection:
- VCC → 5 V
- GND → GND
- OUT → GPIO (with 10 kΩ pull-up to 3.3 V or 5 V)
Logic: Active LOW when magnet detected (carriage at home)
Current Draw: ~5–10 mA typical
Note: A3144 is 5 V device; use level shifter or pull-up to 3.3 V with 10 kΩ resistor

Mechanical:

Sensor Mounting: Fixed to chassis at leftmost position (home)
- Sensor face should be perpendicular to magnet travel
- Mount with small adjustment capability for fine-tuning
Magnet Mounting: Small diametric magnet on carriage
- 3×3 mm or 4×4 mm neodymium magnet (N42 or N52 grade)
- Mounted with pole axis perpendicular to sensor
- Distance: 2–4 mm from sensor face when at home
- Use epoxy or mechanical retention
Sensing Distance: 1–5 mm (adjust magnet distance for reliable triggering)
Repeatability: ±0.05 mm typical with proper alignment
Hysteresis: A3144 has built-in hysteresis (~50 G) to prevent oscillation

PCB Requirements:

5 V power supply (from main 5 V rail)
GPIO input pin with 10 kΩ pull-up resistor (to 3.3 V or 5 V)
Optional: 100 nF bypass capacitor near sensor VCC
Optional: LED indicator for visual feedback during homing
Optional: Level shifter if using 3.3 V GPIO with 5 V sensor

Alternative Wiring (3.3 V GPIO):

If GPIO is 3.3 V logic: Pull-up A3144 OUT to 3.3 V (10 kΩ)
A3144 open-collector output is safe with 3.3 V pull-up
No level shifter needed if pull-up is to 3.3 V

Firmware Implementation

Homing Sequence:

Startup Home (on boot):

- Move carriage slowly toward home (negative direction)
- Monitor end stop GPIO (active LOW when magnet detected)
- When triggered: stop immediately
- Back off slightly (~0.5–1 mm) to exit hysteresis zone
- Set position = 0 (home established)

Homing Speed:
- Slow approach: ~10–20 mm/s (Hall sensor responds quickly, less overshoot risk)
- Fast retry: If sensor not found in expected range, move faster to find it
- Back-off: ~5 mm/s reverse to exit hysteresis zone
Position Tracking:
- After homing: Track position in steps/mm
- Steps per mm = (motor_steps × microsteps) / (belt_pitch × pulley_teeth)
- Example: 200 steps/rev × 16 microsteps / (2 mm × 20 teeth) = 80 steps/mm
Safety Limits:
- Maximum travel: Hard limit (software) prevents over-travel
- Emergency stop: If end stop not found within limits, halt and error

Configuration:

#define CARRIAGE_HOME_GPIO       10
#define CARRIAGE_HOME_POLARITY   ACTIVE_LOW  // A3144 outputs LOW when magnet detected
#define CARRIAGE_HOMING_SPEED    15.0  // mm/s (Hall sensor allows faster)
#define CARRIAGE_BACKOFF_DISTANCE 0.5  // mm (smaller due to hysteresis)
#define CARRIAGE_MAX_TRAVEL      200.0 // mm (example)
#define CARRIAGE_DEBOUNCE_MS     2     // Minimal debounce needed (Hall is fast)

Benefits

Repeatable Alignment: Ensures print always starts from same position
Absolute Reference: Provides known position without encoder
Non-Contact: No mechanical wear, long life (millions of cycles)
Fast Response: < 5 μs response time allows faster homing speeds
Reliable: Immune to dust, contamination, and mechanical failure
Hysteresis: Built-in hysteresis prevents oscillation at trigger point
Low Cost: A3144 sensor < $1, small magnet < $0.50
Simple Integration: Open-collector output easy to interface with GPIO

Calibration

Initial Setup:

Install A3144 sensor at desired home position (fixed to chassis)
Install small magnet on carriage (diametric, pole perpendicular to sensor)
Adjust magnet distance (2–4 mm typical) for reliable triggering:
- Too close: Sensor may always be triggered
- Too far: May not trigger reliably
- Test by moving carriage manually and observing GPIO
Verify homing sequence works reliably:
- Carriage approaches, sensor triggers, backs off slightly
- Position resets to 0
Test print alignment repeatability (should be ±0.1 mm or better)

Magnet Orientation:

Use diametric magnet (magnetized across diameter)
Mount with pole axis perpendicular to sensor face
If not triggering: Rotate magnet 90° (may need opposite pole)
Test both poles to find which triggers reliably

Troubleshooting:

Sensor not triggering: Move magnet closer, check polarity
Sensor always triggered: Move magnet farther, check for stray fields
Intermittent operation: Check wiring, verify magnet is secure
False triggers: Shield sensor from other magnetic sources (motors, speakers)

Maintenance:

Check sensor operation periodically (unlikely to fail)
Verify magnet hasn't shifted or fallen off
Clean sensor face if contaminated (rarely needed)
Check for loose wiring connections

Parts Shortlist

A3144 Hall effect sensor (Allegro A3144, or compatible SS41)
3×3 mm or 4×4 mm diametric neodymium magnet (N42 or N52 grade)
10 kΩ pull-up resistor (for GPIO)
100 nF bypass capacitor (optional, for sensor VCC)
Mounting hardware (screws, standoffs, small bracket for sensor)
Magnet mounting adhesive (epoxy) or mechanical retention
Optional: LED indicator for visual feedback
Optional: Level shifter (only if needed for 3.3 V GPIO compatibility)

Integration Notes

End stop sensor is independent of encoder system (feed axis only)
Carriage uses open-loop control after homing
Position is tracked in firmware based on step count from home
Homing required on every boot for accurate positioning
Optional: Periodic re-homing during long print jobs to correct drift
Magnetic Interference: Keep A3144 sensor away from:
- Stepper motors (may require 50+ mm distance)
- Power transformers
- Other strong magnetic sources
Power Supply: A3144 requires 5 V; can share with other 5 V peripherals
Sensing Distance: Test and document magnet distance for reproducible setup

Print Specifications

Character Spacing (CPI)

10 CPI (recommended)

Character Width: 0.100 inch (2.54 mm) per character
Characters per Line: ~80 characters on 8.5" wide paper (with margins)
Typical Use: Standard typewriter spacing, similar to pica type
Advantages: More readable, easier to scan, traditional typewriter feel

12 CPI (alternative)

Character Width: 0.0833 inch (2.117 mm) per character
Characters per Line: ~96 characters on 8.5" wide paper (with margins)
Typical Use: Elite type spacing, more compact, higher information density
Advantages: More text per page, professional document appearance

Selection:

Toggle between 10 CPI and 12 CPI via UI (MODE button or menu)
Display shows "CPI: 10" or "CPI: 12" on second OLED line
Selection stored in NVS and persists across reboots

Line Spacing (LPI)

6 LPI (recommended)

Line Height: 0.1667 inch (4.233 mm) per line
Lines per Page: ~66 lines on 11" tall paper (with margins)
Typical Use: Double-spaced equivalent, comfortable reading
Advantages: More readable, easier to edit, professional appearance

8 LPI (alternative)

Line Height: 0.1250 inch (3.175 mm) per line
Lines per Page: ~88 lines on 11" tall paper (with margins)
Typical Use: Single-spaced equivalent, compact documents
Advantages: More text per page, efficient use of paper

Selection:

Toggle between 6 LPI and 8 LPI via UI (separate from CPI setting)
Display shows "LPI: 6" or "LPI: 8" on second OLED line
Selection stored in NVS and persists across reboots

Print Resolution

HP45 Cartridge Capabilities:

Nozzle Resolution: ~300 DPI (dots per inch) native
Nozzle Spacing: ~85 μm (0.0033 inch) between nozzles
Print Head Width: ~0.5 inch (12.7 mm) with 50 nozzles
Ink Drop Size: Variable (can adjust for different print qualities)

Character Rendering:

Monospace Font: Fixed-width characters (all characters same width)
Rasterization: Characters converted to bitmap patterns
Character Height: ~2.5–3.0 mm (depending on font design)
Character Width: Matches CPI setting (2.54 mm @ 10 CPI, 2.117 mm @ 12 CPI)

Carriage Motion (Character Spacing)

10 CPI Motion:

Steps per Character: Depends on belt pitch and pulley
Example Calculation:
- Belt pitch: 2 mm (GT2)
- Pulley: 20 teeth
- Steps/mm: 80 (200 steps/rev × 16 microsteps / 40 mm/rev)
- Steps per character @ 10 CPI: 80 steps/mm × 2.54 mm = ~203 steps
Carriage Speed: Constant velocity during print sweep
Acceleration: Smooth start/stop to prevent ink smearing

12 CPI Motion:

Steps per Character:
- Steps per character @ 12 CPI: 80 steps/mm × 2.117 mm = ~169 steps
Faster Printing: More characters per second at same carriage speed
Same mechanical setup: Only firmware timing changes

Paper Feed Motion (Line Spacing)

6 LPI Feed:

Steps per Line:
- Roller circumference: 50.8 mm (2.000")
- Steps/mm: 80 (example)
- Steps per line @ 6 LPI: 80 steps/mm × 4.233 mm = ~339 steps
Feed Speed: Smooth acceleration, controlled deceleration
Alignment: Virtual detent ensures perfect line spacing

8 LPI Feed:

Steps per Line:
- Steps per line @ 8 LPI: 80 steps/mm × 3.175 mm = ~254 steps
Faster Feed: Shorter distance per line
Same mechanical setup: Only firmware step count changes

Font Design

Monospace Character Set:

ASCII Printable: 95 characters (space through ~)
Character Bitmaps: Stored in ROM/Flash
Character Height: ~10–12 pixels (2.5–3.0 mm @ 300 DPI)
Character Width: Fixed per CPI setting
- 10 CPI: ~30 pixels wide (2.54 mm @ 300 DPI)
- 12 CPI: ~25 pixels wide (2.117 mm @ 300 DPI)

Font Rendering:

Rasterizer: Converts text buffer to bitmap rows
Print Sweep: Carriage moves at constant velocity
Nozzle Firing: Triggered at precise positions based on character bitmaps
Ink Drops: Fired as carriage passes over paper

Configuration

Firmware Settings:

// Character spacing (CPI)
#define CPI_10_STEPS_PER_CHAR  203  // Steps per character @ 10 CPI
#define CPI_12_STEPS_PER_CHAR  169  // Steps per character @ 12 CPI

// Line spacing (LPI)
#define LPI_6_STEPS_PER_LINE   339  // Steps per line @ 6 LPI
#define LPI_8_STEPS_PER_LINE   254  // Steps per line @ 8 LPI

// Character dimensions (mm)
#define CPI_10_CHAR_WIDTH_MM   2.54
#define CPI_12_CHAR_WIDTH_MM   2.117
#define LPI_6_LINE_HEIGHT_MM   4.233
#define LPI_8_LINE_HEIGHT_MM   3.175

User Settings:

CPI selection: 10 or 12 (stored in NVS)
LPI selection: 6 or 8 (stored in NVS)
Settings independent (can have 10 CPI + 6 LPI, or 12 CPI + 8 LPI, etc.)

Print Quality

Resolution: 300 DPI equivalent (HP45 native resolution) Character Clarity: Sharp, clear characters suitable for documents Ink: HP45 compatible ink cartridges Paper: Standard plain paper (20–24 lb bond recommended)

Benefits

Flexible Spacing: User-selectable CPI and LPI for different document needs
Professional Output: Clean, readable typewritten documents
Traditional Feel: Mimics classic typewriter spacing options
Efficient: More text per page with 12 CPI / 8 LPI settings
Readable: More spacing with 10 CPI / 6 LPI settings
Software Configurable: No mechanical changes needed to switch spacing

Last Updated: 2025-01-XX (Display specs + Encoder specs + End stop sensor + Print specs added)

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Silent Digital Typewriter — HP45 Inkjet

Features

Motivation

Why This Project?

Where This Device Excels

System Architecture

Hardware Overview

Firmware Features

Repository Structure (planned)

Status

Goals (v1)

Future Extensions

Research Questions

License

Contact / Discussion

Display Specifications

Screen Technology

Display Layout

Font Options

User Interface

Hardware Implementation

Firmware Requirements

Rationale

Paper Feed Encoder & Virtual Detent System

Encoder Choice

Mechanics (Paper Feed)

Counts per Detent (Virtual Notch)

Hardware Implementation

Firmware Model (Virtual Detents)

Stepper Integration

Spacing Selection (CPI & LPI)

Calibration Routine (~1 minute)

Parts Shortlist

Benefits

Carriage End Stop Sensor

Purpose

Sensor Options

Hardware Implementation

Firmware Implementation

Benefits

Calibration

Parts Shortlist

Integration Notes

Print Specifications

Character Spacing (CPI)

Line Spacing (LPI)

Print Resolution

Carriage Motion (Character Spacing)

Paper Feed Motion (Line Spacing)

Font Design

Configuration

Print Quality

Benefits

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