Nixie Tube Clock PCB — IN-14 Tube Design

Project Overview

A six-digit Nixie tube clock using Soviet-era IN-14 tubes and matching K155ID1 BCD-to-decimal driver ICs. The design runs on a single ATmega328P with a DS3231 RTC for accurate timekeeping. The trickiest part of any Nixie clock PCB: routing 170V anode supply traces with proper creepage and clearance on a standard 2-layer board.

Board Specifications

Parameter	Value
Board Dimensions	120 × 80 mm
Layer Count	2 layers
Microcontroller	ATmega328P-PU (DIP-28, through-hole)
RTC Module	DS3231SN — ±2ppm accuracy, battery backup (CR2032)
Nixie Tubes	IN-14 × 6 (hand-selected, tested)
Tube Drivers	K155ID1 × 6 (BCD-to-decimal, HV compatible)
HV Supply	MC34063 boost converter: 12V → 170V (tested at 165-180V)
Copper Weight	1 oz (both layers)
Board Thickness	1.6mm FR-4
Surface Finish	HAL Lead-free
Design Tool	KiCad 8.0

Design Highlights

High Voltage

170V HV Trace Routing

Nixie tubes require 170V DC anode supply. On a 2-layer board, this means the HV traces share layers with low-voltage digital signals. Key layout decisions: minimum 1.5mm clearance between HV and LV traces (vs standard 0.2mm), separate HV trace zone on the right side of the board, and no copper pours under the HV section to prevent capacitive coupling.

Display

Multiplexed Tube Display

Instead of 6 × K155ID1 driving each tube independently (which would need 24 GPIO pins), the design multiplexes: one K155ID1 handles all cathodes (0-9), while transistors switch individual tube anodes in sequence at 1kHz. This reduces MCU pins from 24 to 10 and lowers power consumption by 60%.

K155ID1 Footprint — DIP-16 through-hole, routed with short traces to minimize HV exposure on the board surface
Boost Converter Layout — MC34063 switching section isolated with a ground cutout, no copper pour beneath the inductor to reduce EMI
Tube Socket Mounting — Custom footprints for IN-14 sockets (13-pin circular), with alignment holes for mechanical stability
Time Setting Buttons — Two tactile switches on the front edge, debounced in firmware
Clock Fallback — DS3231 battery backup maintains time for years during power-off

Design Considerations

⚠️ Creepage and Clearance for 170V

At 170V, standard PCB clearances are not optional. The IPC-2221B standard requires minimum 0.5mm clearance per 100V for bare board, but for reliability and safety:

Trace-to-trace clearance: ≥1.5mm between any HV trace and adjacent LV trace
Trace-to-edge clearance: ≥2mm from board edge (for handling safety)
No copper pour under the HV section — eliminates parasitic capacitance that could couple HV transients into the digital section
Slot routing: A 1mm slot is cut between the HV boost converter section and the rest of the board, increasing effective creepage distance to 4mm+

⚡ High-Voltage Power Supply Design

The MC34063 boost converter is a proven topology for Nixie supplies. Key design choices:

Inductor: 100µH shielded, placed directly adjacent to the MC34063 with minimal loop area
Feedback divider: Precision resistors (1% tolerance) set output to 170V ±5V
Output capacitor: 10µF / 200V rated — oversized for longevity at elevated voltage
Current limiting: 33Ω sense resistor limits inductor peak current to 350mA
Soft start: RC network on the MC34063 Ipk pin prevents inrush damage to cold Nixie cathodes

🔧 Multiplexing Timing

Each of the 6 tubes is illuminated for ~167µs in sequence (1kHz refresh / 6 digits). At 170V with ~2.5mA per digit, the average current draw is only ~15mA — well within the MC34063's capability. The firmware uses Timer1 CTC mode for precise multiplexing without MCU intervention.

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Nixie clocks, tube amplifiers, or any project with 100V+ traces. We'll review your creepage clearances and HV routing for free.

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🔴 Nixie Tube Clock PCB