Single
digit Nixie clock
(can
drive any other display from the BCD output)
Circuit
details
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)
·
RB0 to connected to a voltage
divider formed from a 1Meg & a 6.8K resistor
·
The 1Meg resistor is between HV and
RB0, 6k8 connected between RB0 and GND
·
A 4.7v Zener diode can be connected
in between RBO and GND to ensure PIC input pin protection
RB1 (N/A) reserved
DCF-GPS - driven high during initialization and start display
sequence
·
Can be used to drive a LED, turned
high also at each start of display sequence
RB2 (N/A) reserved
DCF-GPS
·
Not in used with the
PIC16F628 version
RB3 output: PWM for HV generation (40kHz, dynamic
duty, to Mosfet gate, a 0.47ohm may be added in between the gate, IRF640 / 740A
/ 840 will do very fine)
·
RB3 is connected to the Gate of a
IRF640 Mosfet (a low value resistor can be inserted in between to minimize RC
Mosfet Gate effect)
·
The Source of the Mosfet is
pull-down to GND
·
The Drain of the Mosfet is tied to
an Inductor (100uH to 330uH the higher value give the lower HV power (a 120uH
will do fine for most tubes even large ones)
·
The Inductor other side is tied to
the positive power input through a current limiting resistor (0.01 to 0.5 ohm)
(to the logic 5V side or the 9 to 12V input)
·
A low ESR capacitor between GND and
input voltage placed close to the input of the current limiting resistor
·
A ultra-fast diode is connected to
the Drain of the Mosfet (Anode side) (diode chosen with a recovery time of 75ns
or less / 300V or more)
·
The Cathode side of the diode is
connected to a low ESR high voltage capacitor(+) (about 250V or more – from
0.470 to 4uF)
·
A lower value non polarized
capacitor can be added between HV and GND
·
The connection between RB3 & mosfet
Gate, Mosfet Drain & Diode/inductor, HV capacitor & Diode are to be
kept as short as possible
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)
·
RA0 is connected to the MPSA42 base
through a 33K resistor
·
MPSA42 emitter is connect to GND
·
MPSA42 collector is connected to the
MPSA92 base through a 470K resistor
·
MPSA92 base is connected to HV through
a 100K resistor
·
MPSA92 collector is connected to
Anode Nixie through the current limiting resistor
·
MPSA92 emitter is connected to HV
RA1 output: backlight / auxiliary
output DP (drive the backlight LED directly or via a transistor, active
high)
·
Drive a LED Anode directly or a LED
cathode through an NPN transistor
·
If transistor is used, any NPN will
do, RA1 is connected to transistor base through a 1 to 10K resistor
·
Transistor emitter is connected to
GND
·
Transistor collector to the LED
Cathode
RA2 output: DP 1/2Hz
·
As per RA1, use HV transistor if
driving a neon bulb cathode
RA3 input: master switch (add 10K pull-up resistor,
pull-down to GND by the switch via a 100 to 270ohm
resistor)
·
RA3 is connected to a 10K pull-up
resistor
·
RA3 is connected to one side of a NO
switch
·
Other side of the switch is
connected to GNG directly or through a 100 to 270ohm resistor
RA4 output: alarm ON/OFF (Active low output, open
collector) (10K pull up may be needed, can drive PNP like MPSA43, or low power
active buzzer directly)
·
RA4 is connected to the base of a
PNP MPSA43 transistor through a 10K resistor
·
MPSA43 emitter is connected to VCC
·
MPSA43 collector is the alarm output
(!no frequency output, just on / off)
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
Version 1 rev 0 - Patrick
Mignot 2009