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README.md

README.md

#425 The Boldport 3x7

Building the Boldport 3x7, Project #29, with custom flip-top layout so you can see inside this discrete 3-digit, 7-segment display.

Build

▶️ return to the LEAP Catalog

Notes

A lovely PCB and classic Boldport instructions:

This project is a challenge to construct

The Boldport 3x7 is essentially a 3-digit common-cathode 7-segment display, rendered with discrete components.

Since it's a chance to look inside the workings of a 7-segment display unit, I decided to modify my build somewhat to flip the lid, so the internal wiring is open for inspection.

Parts

Qty Item
x14 2x5mm lens red LEDs, Valcon VRLED-52-R
x7 2x5mm lens yellow LEDs, Valcon VRLED-52-Y
x1 1x12 right angle header, Multicomp MC34757
x4 N-channel MOSFET transistor , 2N7000_D26Z
x18 1KΩ resistors, MF25 1K
x7 470Ω resistors, MF25 470R
x1 Lovely PCB panel

kit_parts

kit_pcb

LED Current Limiting Resistors

The standard kit uses 1kΩ current-limiting resistors for red LEDs and 470Ω for yellow LEDs. I put these on a breadboard to see the effect, and after a few comparisons decided that 680Ω for red LEDs appeared to provided more even illumination. It is not particularly noticable in my pictures, but IRL the result seemed a bit better:

build_01_led_resistor_selection

Circuit Design

Here's the schematic from Boldport:

Schematic

And here's a quick redraw, focusing just on a single segment control across 3 digits. The key things to note:

  • segment and digit selection inputs are active high logic (common cathode)
  • an example of traditional grid multiplexing: enable to segment, and enable the digit to light
  • yellow LEDs have lower resistor value to ensure red and yellow LEDs run at roughly the same perceived brightness

The schematic fragment shows a single segment control across 3 digits. Note: Ra1, Ra2 are 1kΩ in original kit, but swapped for 680Ω to equalise apparent brightness of red and yellow LEDs.

Breadboard

Schematic

Construction

I actually super-glued the LEDs to the face plate in preparation for assembly. It seemd the best way to ensure alignment, and also provide maximal illumination for each segment.

build_03_boards_ready

Test, test and test again during construction! Once assembeld, I knew that any issues would be a major exercise to rectify. And since the kit is already sold out on Boldport - no chance to just buy a replacement!

build_02_led_test

Fully assembled, with "333" powered directly from a 5V source:

build_04_test_top_side

build_04_test_rear

build_04_test_rear_side

build_04_test_right_side

Next...

So far so good! I have a few projects in mind to follow.

The LED segments illuminate very nicely. I think super-gluing the LEDs to the face plate was a good decision. But I also notice quite a bit of bleed from a lit segment to unlit segments. In retrospect, I think it would have been a good idea to put baffles between the LEDs or paint their sides with an opaque coating to ensure all illumination only radiates via the face plate.

Build

Credits and References