Single digit Nixie clock

(Can drive any other display from the BCD output)

PIC pins connections details

PIC16F628a (PIC16F648 for DCF / GPS options)

The timing source is from external 20.000 MHz crystal RA6/RA7

Controlled HV is obtained from PWM @ 40 KHz on RB3 and voltage feedback input on RB0 (connect to a voltage divider Vref ~1.25v)

The PWM Duty-cycle is dynamically updated according to RB0 input.

PWM is turned off in case no/low Vctrl feedback on RB0 for a too long time during initialization steps. The clock will not initialize.

If the HV supply is not to be used (or not generated from the PIC program), RB0 MUST connected to VCC

A single Nixie tube display is attached to the BCD output via 74141 driver (or 4028+transistors)

The microcontroller pins assignation is as bellow:

RB0 input: HV control feedback (Vctrl from Voltage divider (1Meg. & 6k8 to ground for approx 180V, a 4.7v Zener diode may be added to secure the PIC input)

RB1 (N/A) reserved DCF-GPS - driven high during initialization and start display sequence

RB2 (N/A) reserved DCF-GPS

RB3 output: PWM for HV generation (40kHz, dynamic duty, to Mosfet gate, a 0.47Ω may be added in between the gate, IRF640 do very fine)

RB4 74141 BCD 0

RB5 74141 BCD 1

RB6 74141 BCD 2

RB7 74141 BCD 3

RA0 output: Anode control (luminosity management, fad-in display, couple MPSA42/MPSA92 does a good anode driver)

RA1 output: backlight / auxiliary output DP (drive the backlight LED directly or via transistor or not, active high)

RA2 output: DP 1/2Hz

RA3 input: master switch (add 10K pull-up resistor, pull-down to GND by the switch)

RA4 output: alarm ON/OFF (Active low output, open collector) (10K pull up, can drive PNP like MPSA43, or low power active buzzer directly)

RA5 MCLR / master switch (pull-up as per PIC datasheet (~1K ) or voltage controlled reset by external component (i.e. TCM809)

RA6 crystal driver / output command for electromagnetic chime gong (crystal default)

RA7 input: crystal frequency

Grayed texts above are optional program options / features

The BCD (0-3) output are connected to one 74141

The tube anode is connected to high voltage through resistor (12K fit a IN-14 Russian tube)

A NO switch is connected from GND to RA3 through a 100ohm resistor. RA3 tied to a pull-up ~10K resistor.

Kanaugh algorithm is used to debounce the switch inputs.

Holding the switch down during power-on will reset the clock to factory settings (date and alarm are preserved)

Default configuration after reset is as bellow:

- H24 and HH:MN display format,

- Alarm is reset,

- Date is in French format,

- Luminosity is set to full.

The clock heart contains four registers:

- Clock time register HH:MN:SS (SS can set at minute O’clock),

- date register DD/MM/YY (French) or MM/DD/YY

- Alarm register (HH:MN)

- sleep register (one entry per hour)

NOTE: the alarm and sleep registers are saved to EEPROM.

Schematic here – Text description here

The schematic shows optional second slave mpu to provide RGB color washer effect. The slave mpu is driven by commands from the DP (RA2) output and from a user push button

The schematic differs slightly from the description, it is used to produce a versatile PCB with options for DCF / GPS /RGB / 74141 or 4028 + transistors tube driver, power-failure backup, etc…. If you don’t have the PCB, use the schematic just as partial reference. Also, options selection during initialization (BCD port connected to pull-up/pull-down resistor can not be used with the MPU stand alone delivery)

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Version 1 rev 0 - Patrick Mignot 2009