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Design and bring-up of a VQB 71 GPS clock

A project log for VQB 71 GPS Clock

A tiny version of the GPS Wall Clock

Stephen HoldawayStephen Holdaway 03/29/2020 at 05:152 Comments

A year ago I picked up some new-old-stock "VQB 71" 7-segment displays — not for any particular project, but simply because they looked fantastic. These gorgeous, vintage common-anode digits have distinctive segments with six tiny LEDs each, internally connected with bond-wires and encapsulated in transparent red epoxy/resin:

Macro photo of a VQB 71 digit, powered off
Microscope photo of an unpowered VQB 71, lit to reveal its internal features. Hundreds of tiny bubbles can be seen in the red epoxy under this lighting, which aren't visible to the naked eye. The digit height within the display is 7mm.

This batch was manufactured in 1989* by WF Berlin (Werk für Fernsehelektronik), a division of the East German state-owned company VEB Kombinat Mikroelektronik Erfurt. The packaging is titled "lichtemitteranzeige" which is literally "light emitting display": an odd name by modern standards according to a German colleague of mine, who said this would simply be labelled "sieben segmentanzeige" today.

Tray of 20 RFT Lichtemitteranzeige VQB 71 7-segment LED displays made by WF Berlin in June 1989
VQB 71 displays in packaging
* The marking "X06" indicates a manufacture date of June 1989, assuming Wikipedia is correct about the manufacturer using IEC 60062 letter and digit codes. WF Berlin appears to have phased out most of its manufacturing in 1990 (source), so a mid-1989 date makes sense for new-old-stock parts.

A few partial scans of datasheets for the VQB 71 are available online, with the one below from tu-chemnitz.de being the most complete copy I've found. A datasheet isn't strictly necessary here, but it's useful to have dimensions provided for the footprint, and I wanted to know the maximum current per segment specified by the manufacturer. After all, These parts haven't been made for nearly 30 years, so it would be a shame to destroy one accidentally.

VQB 71 Datasheet
VQB 71 datasheet scan. The contents of this image is transcribed below.

It's easy enough to figure out the specs without knowing any German, but Google Translate makes quick work of the dry/technical language. Here's the English translation of this datasheet:

VQB 71
Not for new designs
Light emitting display made of red-beam GaAsP diodes in a segment design to display the digits 0...9 and a decimal point (DP). The wavelength of their max emission is 630...690 nm. The half width is 40 nm.


Limits  
Forward direct current / segment or DP at θa -25...25 °CIFmax 15 mA
DC blocking voltage at θa -25...+70 °CURmax 4 V
Operating temperature θa -25...+70 °C
Storage temperature for storage up to 30 days θstg -50...+70 °C
Characteristic values at θa = 25°C  
Luminous intensity / segment at IF = 10 mAIVtyp 150 μcd
↳ Decimal pointIVtyp 100 μcd
Forward DC voltage / segment at IF = 10 mAUFmax 3.6 V
↳ Decimal pointUFmax 1.8 V

Note that unlike modern LED datasheets, this doesn't specify maximum pulsed currents. Pulsed currents with modern LEDs are typically 3-5x the max constant current rating (dependent on duty cycle and pulse duration), which can give some head-room for driving them harder in multiplexed configurations.

Driving these segments with a constant 10mA as the datasheet suggests gives a high-contrast image in vivid red. The VQB 71's are so intensely red that it's proved difficult to capture an accurate representation of one on camera. Modern LEDs like those in the GPS Wall Clock look almost orange by comparison.

VQB 71 with one segment powered by a constant 10mA source
Segment powered with a constant 10mA source. Note that the segment is completely over-exposed here. With no light cast on anything, it's difficult to convey the true brightness.
VQB 71 exposed for segment at 10mA constant current
Same as above, but exposed for the segment - a reduction of 7 whole stops.

Driving all segments at this power increases the surface temperature of the digit by around 10°C, but who needs efficiency when you have style, right?

I hear we're building another clock

There are only so many applications for a 7-segment display these days, and I'm a firm believer that one person can never have enough clocks. A miniature version of the GPS Wall Clock for a desk seemed like a nice way to put these vintage digits on display.

This time around there are a few changes:

Bigger processor

It was fun squeezing the original implementation into 1KB of code space on an intentionally small part. This time I'm more interested in expanding functionality: better display accuracy, configuration of the GPS module, more buttons, less pin-sharing, etc. Initially I'd picked the ATMega88PA as I already have a tray of them, but after a first pass on the schematic it was obvious most of the pins would go to waste. I traded it for the physically smaller but otherwise comparable 8-bit STM8S003F3P6.

Due to a shipping error a few years ago, I have more than 100 of these STM8S chips, so they're an easy choice. While not necessarily a popular chip, these are half the price of similar AVR chips new, can be programmed with the dirt-cheap STLinkV2 dongles available from the usual suspects using a SWIM interface, and can be used with an open-source toolchain. The downside is that it's just you and the reference manual when things aren't working, but hey, I enjoy that challenge.

The die of an STM8F003F3P6 (2.35mm wide), sitting on a VQB 71. Decapped semi-successfully without chemicals using the method in this video from CuriousMarc

GPS module on-board

Like the original, a GPS receiver will provide time information *from space*, but this time the GPS module will be integrated into the layout instead of wiring up a separate breakout board. New GPS modules from distributors start at about $15 NZD each, so I went the cheap way and grabbed some grey-market u-blox NEO-6M modules for about $5 NZD each. The units I received appear to be leftovers from a production run that have been sitting in a factory for years collecting dust, but they do work correctly.

NEO-6M from AliExpress
Seemingly legit and unused NEO-6M modules, with bonus corrosion and dust. These work fine, though I don't want to know where they've been sitting for the 5+ years between their 2013 manufacture and me buying them in 2019....

Improved accuracy

The GPS module has a TIMEPULSE pin that generates a rising edge at the top of each second by default. I was aware of this in the original project, but didn't have any spare pins or program space to handle it. The original build updates its display each time a NMEA RMC sentence is received over UART, which works fine, but does make the clock run almost a tenth of a second slow:

NEO-6M GPS PPS to RMC sentence delay

This is already accurate enough for practical human purposes, but we can go deeper. Offsets can be configured in the GPS module to account for electrical and processing delays, so in theory it's possible to have nanosecond accuracy. Since this is only keeping time for humans, I'd be very happy with ±1ms accuracy.

Schematic and layout

NEO-6M GPS, STM8 and VQB-71 schematic for a small clock
In the spirit of vintage parts, I've printed and scanned the new schematic several times to enhance its character. (Click for the original colour schematic).

Only a few things of note here:

This project started as a two-layer board just big enough for six VQB 71 digits, but it quickly became apparent that I was either going to need to make the board larger or go to four layers. I went with the latter as it wasn't prohibitively expensive to jump from two to four layers. It's possible this could've been routed with two layers, but I suspect it would've been quite time consuming. Space around the GPS module is particularly tight, but almost everything I wanted fit:

KiCAD 4-layer PCB layout stack-up

I'd originally tried to fit a CR2032 battery holder into the layout, but that was taking up 30% of the back and was cut in the end. To save space, the battery is exposed as a header and will reside in the eventual enclosure. The top layer is likely going to double as a front-panel, so I avoided using the space for general layout.

While the circuit is relatively straight forward and uses parts I've worked with before, the GPS module was something I wanted to be extra careful with. I've done zero RF work before, but the NEO-6M's hardware integration manual provides plenty of guidance around antenna layout. This manual in combination with study of existing NEO-6M breakout boards gave me a small amount of confidence that this wouldn't be a reception disaster. The digit pins being so close to the GPS module is an unknown, but there's not a lot that can be done without making the board a few centimetres longer (something I played with early on, but didn't like the look of).

The board went to fab mid-February, was manufactured over the course of 10 days, and took a further 10 days to get to my bench in New Zealand. I went with red solder mask to match the digit casings. It's still hard to beat fresh, shiny new PCBs:

PCBs from JLCPCB for the GPS clock

Assembly in pictures

A little fiddly to align all of the digit pins, but they fit well. I'd previously test fitted digits on a print-out of the layout, but it's nice to see the spacing work on the real board.
All aboard (still test-fitting)
3.3V regulation and the STM8 microcontroller soldered and tested. The polarity protection MOSFET ended up saving me later when wiring a cheap USB cable that has black-positive and red-negative wires!
Things didn't go quite as cleanly getting the NEO-6M module installed. I went through a couple of I-PEX connectors getting my iron temperature right for working on the large ground plane around the GPS module. I also found that the module's sticker had almost no resistance to the isopropyl alcohol I use to clean off flux.
Close-up of the damaged solder mask where reworking ground connections was challenging.
Tiny GPS clock PCB assembled
Fully assembled board. The MAX7221 chip went on and was tested before the GPS module, but I forgot to take photos of that.

Firmware

To get the clock up and running quickly, I ported the code from the original AVR project to run on the STM8. This only required swapping out a few registers and pin mappings, replacing the SPI, UART and ADC implementations, and adding mapping to set BCD values on the common-anode wiring of the MAX72XX.

When I went to display actual data on the display and turned off the driver's test pattern, I discovered I'd wired the digits in the order {0, 4, 3, 1, 5, 2} instead of the expected {0, 1, 2, 3, 4, 5}. I'd tested with a MAX7219 to find the correct output for digit 0, but I either didn't test the other digits or the MAX7221 has a different output order to the MAX7219. This was a simple fix in software anyway, adding to the existing mapping code.

I'll work more on the firmware over the next few weeks, but for now there's a working clock:

VQB 71 clock PCB
Long exposure VQB 71 clock

Discussions

Marty wrote 07/01/2020 at 01:42 point

Very nice post also! Nice little clock! Is this project for sale? Where might I send to order a board or a kit? I have a bunch of the LED's for such a project. If not a kit can the kicad file be converted to Eagle so I may get a board made?

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Jan wrote 03/29/2020 at 11:52 point

What an awesome post! Thanks for going into so much detail and even translating the datasheet for the non Germans :)
Myself being German, I guess not using "Siebensegmentanzeige" (seven segment display) instead of "Lichtemmiteranzeige" (light emmiting display) was because LE(D) was quite new tech at that time plus the German love for "special" sounding and long words.

Looking at the VQB26 datasheet, they call those not "Siebensegmentanzeigen" either but "Lichtschachtbauelemente", which literally translates to "light well components/parts", which doesn't sound much better, right?

Again, thanks for posting! a LOVELY display/clock!

Edit: they're still in (new old) stock here for 2€/pc: https://www.mos-electronic-shop.de/segmentanzeige-p-483.html

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