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Topic: "Large" Seven-Segment/Scoreboard Controller Design (and Questions!) (Read 4140 times) previous topic - next topic


I need to build a couple of displays for a drag race setup; basically displaying alternately speed and time with a few extra options to display the winning lane or a fault. My plan is to get handy with a router and pop out the segments from a few boards and then wire in by hand up to four LEDs per segment. You can see by the layout of the board (pic attached) that the pad layout makes it straightforward for figuring out what wire goes. I've set the segments mirrored left-right so I can attach it to the back of the display without the need to stand on my head backwards and get confused with the mess of wires.

This design is based on the Texas Instruments TLC5925 chip (~$1.50); it's a current sink, shift register LED driver that handles up to 45ma per channel at 17V. The advantage here is that there's no need for wiring a whole lot of resistors as I can just put the LEDs in series and power the segments from a lead acid battery -- I've not seen any product anywhere that would be as cheap or more fitting than designing my own. Although I've left open the opportunity to talk to the shift registers directly I'd like to put an ATmega328 on so I can speak to it using serial, and that's where my questions all pile up:

1) I've read that the crystal (resonator) is needed when serial will be used; however it wouldn't be necessary at slow speeds (in the neighborhood of 2400 baud), correct? I've not seen anyone mention what kind of top speed should be expected without an external crystal.

2) If I'm not doing any analog input/output is the cap between GND and AREF required?

3) Did I screw up the wiring of the reset pin or miss anything required for the programming of the ATmega?

Any other comments/criticism is most welcome.


I'd put another decoupling cap on the '328 2nd VCC line, be sure to install them next to the pins.
Run a ground out with the Rx/Tx lines also.
Maybe even consider a MAX3232 chip & 4 1UF caps so you have an RS232 output. Then can use an inexpensive USB-RS232 cable to connect to a PC.
or add a USB-Serial adapter board & connect the 5 wires needed - Rx, Tx, Gnd, Vcc, DTR.
This one is good as you can cut the RST trace and wire DTR instead, and be all set for downloading sketches after programming Bootloader via the ISCP pins & AVP ISP programmer.  And then not have to deal with soldering the impossibly close & small SMD pins.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.


He has contacts to go off to multi-LED/segment display, maybe this kind of part

This one has 4 LED chips per segment.
Or a similar part with multiple banks of LEDs in series per segment so 12V/segment still but at higher current draw.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.


Yes, I was trying to describe a design akin to the Sparkfun design (albeit a bit more weatherproof). I'm trying to avoid paying obscene amounts of money for digits like KE7GKP's 12-incher ($50 for one digit!). I intend to use discrete 5mm LEDs, wirewrap the interconnections, and then ultimately solder the power leads to the board permanently.

The circuit board and components currently come somewhere around $30 total. I suppose in that respect it's a bit of proof of concept as to how economically such a scoreboard can be built.

TLC5925 @ 3 = $4.50
ATMega328 = $4.50
Circuit board = $2.50 (each, quantity 10 required from Seeedstudio)
5mm LEDs @ ~150 = $7.50
....plus wire/resistors/headers/common stuff = $10.00.

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