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VQB 71 GPS Clock

A tiny version of the GPS Wall Clock

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A small clock using vintage VQB 71 segment displays from East Germany. Time comes from space courtesy of a NEO-6M GPS module. This project is the little brother of GPS Wall Clock.

Details and copious photos in the project logs:

tiny_gps_vqb_71_rev1.zip

KiCAD project for Rev 1.0

x-zip-compressed - 63.32 kB - 03/29/2020 at 05:21

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tiny_gps_rev1_schematic.png

Rev 1.0 schematic image

Portable Network Graphics (PNG) - 952.01 kB - 03/29/2020 at 05:06

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  • Design and bring-up of a VQB 71 GPS clock

    Stephen Holdaway03/29/2020 at 05:15 2 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...

    Read more »

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