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Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →You can build a large scrolling display from surplus Soviet IV-25 vacuum fluorescent display tubes by grouping 12 tubes into an 84-element module, driving each module with three SN75518 chips, and repeating the module to add width. The key is to keep the low-voltage filament supply distinct from the higher-voltage anode drive, test each tube before installation, and prototype one module before scaling up.
What an IV-25 tube displays
The IV-25 is a seven-element vacuum fluorescent indicator arranged as a vertical column. The geometry makes it useful for a distinctive dot-matrix-style display rather than a conventional character screen: the elements in each tube can be switched to form pixel patterns, and multiple tubes placed side by side create a wider canvas.
In the documented large-display build, 12 tubes make one section with 84 independently controlled elements. The builder used 75 surplus tubes overall. That is a project example, not a required tube count: each additional 12-tube section adds another 84 elements and more display width.
Listings sometimes describe the IV-25 as a “seven-segment” indicator, while MIT’s Tube Electronics lab calls it a “seven-dot column display.” For planning a pixel display, the column-of-elements description is the more useful one. The exact appearance of a character depends on the font and patterns you program.
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- Original tubes
- Same Date
- Phosphor color - green
- Vintage handmade clock or any DIY projects
- IV-6 are Soviet 7-segment VFD tubes, showing digits 0...9 and a decimal point to the right
Plan the electrical rails before wiring
An IV-25 needs a low-voltage filament supply and a separate higher-voltage supply for the selected anodes. These are different parts of the tube; confusing their connections can destroy it. MIT’s lab gives less than 2.5 V for the filament and about 20–25 V for selected anodes, and cautions against raising filament voltage above 2 V in its teaching setup. Supplier specifications give 2.4 V and 35 mA for the filament, and 25–30 V for anode or segment drive. Those ranges are useful for planning, not a substitute for checking the exact tube’s original datasheet.
| Part of the circuit | Published planning value | How to use the value |
|---|---|---|
| Filament | MIT Tube Electronics lab: less than 2.5 V; its teaching setup says not to exceed 2 V. IP Electron supplier listing: 2.4 V, 35 mA. | Confirm the specific tube’s specification and begin with current limiting. Do not assume the supplier value overrides the lower limit in a particular teaching circuit. |
| Selected anode or segment | MIT Tube Electronics lab: about 20–25 V. Supplier listings: 25–30 V. | Use a regulated, current-limited high-voltage rail and validate the drive on a single tube before connecting a module. |
| Segment current | Amedia116 supplier listing: 4–10 mA total segment current. | The listing does not establish a universal per-element operating current for every tube and driver arrangement. Do not extrapolate it into a full-board supply rating. |
| Logic | The documented build uses an ESP32 with a 3.3-to-5 V logic converter. | Match the logic interface to the controller and driver board; do not apply the high-voltage display rail to logic inputs. |
Supplier figures are component specifications on undated pages accessed in 2026; actual surplus tubes may differ in condition, and the original tube datasheet should take precedence where available. Use a current-limited bench supply during testing. The glass body and flexible leads are also vulnerable to mechanical strain, so avoid bending leads to compensate for a poor bracket or board alignment.
Build a 12-tube driver section first
The documented architecture repeats a custom PCB section, with one board for each group of 12 tubes. Each board has three SN75518 driver ICs. An ESP32 sends control data through a 3.3-to-5 V logic-level converter; a 20 V boost converter supplies the display drive rail, and a 5 V buck converter supplies the lower-voltage electronics. This describes the published project design, not a complete schematic: verify the driver datasheet, board layout, connections, and rail requirements before assembling your own circuit.
- Sort and test the tubes. Inspect the glass and leads, then check each tube individually with a current-limited supply using the correct filament and anode connections. Set a tube aside if a lead is missing or it does not illuminate as expected.
- Assemble one driver board. Build a single 12-tube section with its three SN75518 chips and the required connectors. Confirm the board’s connections against the driver and tube documentation; do not infer pin assignments from the module description alone.
- Bring up the low-voltage side. Check the controller, logic converter, and 5 V buck output before enabling the display rail. Confirm that the logic levels presented to the driver board are compatible.
- Test the display rail and tube operation. Add the boosted rail only after checking polarity, current limiting, and the intended tube connections. Test a small number of tubes first, then exercise the complete section and watch for unexpected heating, flicker, or failed elements.
- Replicate only after the module works. Add sections one at a time. Recheck power distribution, connector strain, and firmware behavior as the display grows rather than assuming one working section proves a larger assembly is safe.
The project describes a 20 V boost converter even though the cited operating guidance for selected anodes reaches 25–30 V. Treat those as different pieces of evidence, not interchangeable setpoints: the reported system architecture alone does not establish the voltage at every tube pin or the circuit conditions that determine its operating point. Follow the actual driver-board design and the tube and driver specifications; do not raise the rail simply to match a supplier’s maximum figure.
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Each 12-tube section adds 84 elements, so module count is a straightforward way to increase width. More tubes can make text and patterns easier to distinguish, but every module adds wiring, power distribution, connectors, physical alignment work, and firmware data. The 75-tube project demonstrates one large build; it does not establish a universal ideal size, brightness, or supply rating.
- Pixel density: Keep the seven-element layout in mind when designing a font. Narrow strokes, small gaps, and intricate shapes may not translate clearly to the physical display.
- Power: Size rails from the real circuit and measured load, including the filament arrangement and the number of illuminated elements. The published account does not provide a total supply-current figure for a finished marquee.
- Thermal and wiring checks: As modules are added, inspect regulators, driver boards, connectors, and conductors under the intended display pattern. A static test with few illuminated elements may not represent the heaviest content.
- Firmware capacity: More tubes mean more pixel data to store and transmit. Keep the module boundaries explicit so added sections do not require rewriting the character-rendering logic.
Send text and pixel patterns from firmware
The Hackaday.io IV25Display project represents each tube with one byte and provides a print method for text plus a raw-byte method for direct patterns. Its interface uses latch, clock, and data signals in a shift-register-like arrangement. A practical firmware flow is to convert each character in a pixel font into the appropriate tube bytes, arrange those bytes in physical display order, and stream them to the driver modules.
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This approach supports scrolling text, clocks, and transition effects, but the software representation must match the actual wiring order and element mapping. Test one known pattern per tube position before creating a full font; a reversed module order or incorrect bit mapping can make valid data appear scrambled. The available project description does not specify a universal ESP32 pin assignment or complete driver protocol, so select pins and timing from the board design you build rather than copying an assumed mapping.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Make repeatable brackets and protect the glass
The documented builder designed tube brackets in Tinkercad, printed them on a Prusa i3 MK3, and painted them matte black to improve contrast. The important design goal is not the printer model but consistent spacing and support that holds each long glass tube without transferring stress to its leads.
Make a small bracket sample and fit several tubes before printing a full display set. Check that the tubes sit straight, their glowing elements align from one section to the next, and the leads can reach the board without being forced sideways. Matte-black surroundings can make the glow stand out, but they do not compensate for inconsistent tube spacing.
Buy surplus tubes with a yield buffer
Surplus lots may include nonworking tubes or parts with missing leads, so purchase spares and test each tube before soldering it into a finished board. Inspect the glass for damage and the leads for completeness and usable condition. One IP Electron listing describes an IV-25 as approximately 78 mm long, 7.2 mm in bulb diameter, with nine leads; use the actual tubes in hand to finalize bracket and PCB dimensions.
The same supplier listing states a 30,000-hour resource figure. That is a supplier-stated specification, not a guaranteed lifetime for an old surplus tube or a prediction for a particular display’s operating conditions. Check availability, condition, and documentation for the exact lot you are considering.
Quick Recap
Common build failures and how to avoid them
- Tube does not light: Verify the tube’s condition and lead continuity, then check filament and anode connections separately using current-limited supplies. Do not respond by raising voltage blindly.
- Tube fails after wiring: Recheck that filament and anode connections were not mixed up, and confirm the actual drive conditions against the tube documentation. MIT’s lab explicitly warns that confusing the supplies can destroy the tube.
- Some elements are missing: Test the tube apart from the matrix, then inspect its wiring and the relevant driver path. This separates a tube fault from a board or data-mapping fault.
- Text appears in the wrong order: Verify tube order, bit-to-element mapping, and module sequence with a simple raw-byte test before debugging font rendering.
- Leads or glass are under strain: Adjust the bracket or board position instead of forcing the tube into place. Mechanical stress can break fragile leads or damage the glass.
- Supply behavior changes as sections are added: Check voltage at the load and inspect regulators, wiring, and connectors under a representative illuminated pattern. Do not assume the original single-section behavior will hold at a larger scale.
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