View Full Version : 7-Segment LED array

03-17-2010, 04:47 AM
I am using the BASIC Stamp Professional Development board and I created a project on it using three 7-Segment LEDs. The LEDs display as expected with uniform brightness across all segments and all digits. When it was time to transfer the project to a PCB, I reconstructed the project using the actual red LEDs I am intending to use. The LED segments are now very dim.

I understand the reasons behind this (power consumption vs. max mA output of a Stamp PIN) and I am aware of various solutions (using transistors on each digit, etc.), but my question is: Why does the project work on the Professional Development Board and not using my 3rd party LEDs? From the schematic of the PDB, I see nothing special on the wiring of the LEDs (no transistors, no special connections). What am I missing? Are the blue LEDs used on the PDB special low-consumption LEDs?


03-17-2010, 06:27 AM
Probably. Check out your current consumption against a smd BLUE LED.

- Stephen

03-18-2010, 10:15 AM
I'm using the same board and have not come across this problem when going to my own PCB, I too am interested to hear what others have to say. I believe the Blue LEDs are 270ohm or something like that, what value of resistors are you working with off the board? Are you using a battery? Got a schematic? Could be a high resistor value somewhere else?

On one of my circuits I’m pulling power for my LEDs off a rail, not trying to power off the pins of the BS2. Fingers crossed I don’t run into the same issue ;)


03-19-2010, 06:54 AM
In my simple experiment, I have 8 STAMP pins (0-7) driving the LEDs via 220 Ohm resistors. Two more pins (8 and 9) control which digit is displayed. The 2-segment LEDs I use are the Fairchild FND507.


No particular preference in these. Just happened to have them. I ordered some blue ones to see if there is any difference. And I am using a programmable power supply, so no limitations there either.

03-19-2010, 07:12 AM
Franklin said...
Probably. Check out your current consumption against a smd BLUE LED.

The specs for the LEDs I use say 20mA per segment. Trying to display an 8, requires 8 x 20 = 160 mA. My measurement shows that it draws about 135 mA. The STAMP allows for a maximum of 50-60mA across 8 I/O pins (if I am reading this correctly). It looks like I have the "wrong" LEDs.

PJ Allen
03-19-2010, 07:46 PM
Use less current, they should be nearly as bright·at 10 mA each.

03-19-2010, 10:19 PM
Unfortunately, it's much dimmer. I tried the green LED that Parallax sells (item# 350-00027) with exactly the same setup and the current requirements are 35 mA for all 7 segments. That's a significant difference. I guess I should stick to green LEDs.

PJ Allen
03-19-2010, 10:57 PM
It's a cognitive disconnect.