
Adapting the IV-18 IceTube clock design to use the 13 digit IV-27M VFD tube.
After successfully building several of the IV-18 revision 2.2 clocks, and having an abundance of spare time due to the COVID-19 pandemic, I decided that I wanted to build a project that utilized the 13 digit IV-27M VFD display tube.
My first thought was to build a calculator to utilize the added digits, until I received a request from my wife for a clock that displayed the day of the week constantly. That would definitely put to use the extra digits of the IV-27M tube, so I started to see what changes would need to be made to adapt the IceTube clock design.

The first issue was that the IV-27M tube needed drivers for 22 elements and grids to utilize the full capabilities of the tube, and the MAX6921 driver IC only has 20 outputs. Another issue is that the IV-27M tube has connections on both ends of the tube envelope, requiring some traces along the full length of the board. That was not a big problem, but was something to think about if I wanted to maintain a similar board width to the IV-18 design.
Luckily the MAX6921 has a serial interface data output, DOUT, allowing any number of devices to be cascaded on the same serial interface. So I started my design using two of the MAX6921 IC’s and placing them on opposite ends of the board near their associated tube connections.
The next issue was the the lower filament supply voltage of the IV-27M tube. The tube specifications call for 3.15 volts at 180 mA current, compared to the 5 volts at 85 mA current of the IV-18 tube.
In my first prototype design I used the existing LM317 voltage regulator for the filament supply with different setpoint resistors for 3.15 volts and added extra area on the board as a heat-sink to hopefully handle the extra current. I ended up adding an extra heat-sink to the top of the LM317 regulator in the prototype board as it was still running very hot.

After building the revision 0.3 board, I wasn’t happy with the amount of heat generated in the filament regulator, so I started looking at alternative designs for the circuit. The problem was that I was asking the LM317 to dissipate considerable wattage to deliver 3.15 volts from a 9 volt supply which is a 5.85 volt difference. At 180 mA of current for the filament, the power dissipated was (5.85 * 0.180 = 1.053 watts).
The easy way to solve this problem was to reduce the supply voltage for the regulator to reduce the voltage drop across it. I briefly looked at utilizing the regulated 5 volt supply, but quickly dismissed that. The solution that I came up with was to switch from a 9 volt main power source to 5.25 volts.
Why 5.25 instead of 5.0 volts? Mainly because I want the ATMEGA 328P microprocessor and the VFD boost converter to operate as close to 5 volts as possible, and there are several elements such as the fuse, blocking diode, and protection diode which can reduce the input voltage significantly.
Most of the Raspberry Pi power supplies have voltage outputs greater than 5.0 volts to make up for the voltage drop in the cables, connectors, fuses, and protection circuits in some of the models. I found a supply with a nominal 5.25 volt output that was more than capable of supplying the required current.
I then redesigned the clocks power circuitry so that everything was on the 5 volt bus. This involved increasing some trace widths and adding a MAX40203A “Ideal Diode” in the supply circuit to reduce the voltage drop from the previous Schottky diode that was preventing reverse current to the battery sense circuit. The MAX40203A has a voltage drop that is an order of magnitude smaller than that of Schottky diodes. In the 5 volt supply circuit at a current of 220 mA the Ideal Diode should have a forward voltage drop of 81 mV.
The HV supply for the anode-segment voltage was capable of generating the needed voltage even at 5 volts input with a change in the PWM signal width in firmware. I ended up with a voltage of 33 volts nominal at a medium output brightness.

I then changed the filament voltage regulator to a fixed 3.2 volt Ablic U.S.A. S‑13A1A32-U5T1U3 unit in a SOT-89–5 package with a expanded board heat-sink area for heat dissipation.
The design changes worked well with the filament regulator just noticeably warm to the touch after operating for several hours.
I also added a Maxim DS32KHZ TCXO crystal oscillator in a SO package, instead of the standard quartz crystal and load capacitors.
I had used a dip package version of the DS32KHZ on the version 1.31 IceTube clock, and was extremely impressed with the very low drift.

I am currently operating the filament in DC_FWD mode instead of one of the AC filament modes until I can troubleshoot some issues in firmware with the AC modes. It is working very well in DC mode with no noticeable variation in display intensity between digits.

The IV-27M tube required new sideboards for each end of the tube. One tube end has 15 pins and includes both elements and grids, while the other end has 11 pins and has only grids and the filament connections. I utilized a 16 and 12 pin right angle header for connection to the main board.

The next hurdle was modifying the firmware to utilize the extra digits of the IV-27M tube.
I was able to get with the original author of the Xmas Icetube clock, John Archie who pointed me in the right direction for making the changes.
Here are the changes I made in the display.c file to utilize the second MAX6921 driver chip:
red — original code / green — updated code
const uint8_t vfd_digit_pins[] PROGMEM = {
3, // digit 9 (dash & circle)
7, // digit 8 (leftmost digit)
8, // digit 7
9, // digit 6
6, // digit 5
10, // digit 4
5, // digit 3
12, // digit 2
4, // digit 1 (rightmost digit)
};
// vfd segment selection wires are on these MAX6921 pins
const uint8_t vfd_segment_pins[] PROGMEM = {
11, // segment H
16, // segment G
18, // segment F
15, // segment E
13, // segment D
14, // segment C
17, // segment B
19, // segment A
const uint8_t vfd_digit_pins[] PROGMEM = {
12, // digit 14 (v‑bar dash & circle IV-27)
13, // digit 13 (leftmost digit IV-27)
18, // digit 12
17, // digit 11
16, // digit 10
15, // digit 9 was IV-18 (dash & circle)
14, // digit 8 was IV-18 (leftmost digit)
23, // digit 7
26, // digit 6
25, // digit 5
28, // digit 4
27, // digit 3
21, // digit 2
20, // digit 1 (rightmost digit)
};
// vfd segment selection wires are on these MAX6921 pins
const uint8_t vfd_segment_pins[] PROGMEM = {
5, // segment P was (H on IV-18)
10, // segment G, M
7, // segment F
9, // segment E
11, // segment D
8, // segment C, S
6, // segment B
19, // segment A, W
// bits to send MAX6921 (vfd driver chip)
uint8_t bits[3] = {0, 0, 0};
uint8_t bits[5] = {0, 0, 0, 0, 0};
uint8_t bitflag = 0x08;
for(int8_t bitidx=2; bitidx >= 0; –bitidx) {
for(int8_t bitidx=4; bitidx >= 0; –bitidx) {
Next I made some changes in the mode.c file switch(time.timeformat_idx) section to change the time display formatting and adding the extra display information.

I am still waiting on a back-ordered switch for the alarm on/off circuit, but for the most part pleased with the final IV-27M clock assembly.
I still also need to design and laser cut an enclosure for it.
I have exhausted my supply of IV-27M tubes for now, but may build some more next year.
EagleCAD V7.7 board and schematic ZIP file for IceTube Clock Rev. 0.38
Firmware ZIP file for IV-27M IceTube clock including pre-compiled .HEX files
OSH Park circuit board fabrication Main board Revision 0.38
OSH Park circuit board fabrication 15 pin Sideboard Rev. 0.3
OSH Park circuit board fabrication 11 pin Sideboard Rev. 03
I’m having a hard time figuring out the firmware for hex, can’t you send in the Atmega firmware code? I use AVR ISP Shield. Thank you .
Here is a link to compiling and uploading HEX files from the original author of the project.
AVR Programming
Can you share please list of pcb components?
Hi Andrei,
I do not have a list (Bill Of Materials) of PCB components.
Most of the parts are well documented in the schematic. Check the part “attribute” and “properties” information in Eagle CAD.
Some of the parts I used were “new old stock” without part numbers. I currently do not plan on building any more
of these clocks, so I hope you can understand, that I do not want to spend the time to build a BOM and keep it up to date.
Greg (Barbouri)
Hi Barbouri,
Thanks for the reply.
This week I found the same tube from a local flea market and decided to try to make a clock from this.
Searched all the internet for details and strange thing that everybody used to make clocks from IV-18 and nobody from IV-27m.
Nobody did, except you and some Japan guy 🙂 Link Above.
http://ishi-13th.com/electronics/disp_tubes/iv27_clock/manufacture_memo/manufact_vfd_iv27.html
So question is: will other IV-18 projects will be compatible with this tube? Except that only few grids will not be available?
Is pretty hard to find MAX6921AQI, and I see that Japan guy used 3 TD62783AFG drivers which are more available and cheap.
Will your project be compatible with TD62783AFG?
Hi Andrei,
I doubt that any IV-18 projects will be directly compatible, as the tube pinouts are very different along with the operational voltages.
The TD62783AFG IC’s are only drivers, and the MAX6921 has an I2C interface along with the drivers. So not compatible.
Greg (Barbouri)
For those interested, here is a BOM I created to source the parts @ DigiKey.
https://www.dropbox.com/s/grgojj37zmmtc9q/IV-27-digi-bom.csv?dl=0