Driving LEDs with USB-6501

[TedWagner]

I have a device/product designed (in the early 90s) around 16-bit DAQ cards (Advantech PCL-720s) plugged into old DOS computers. The devices each feature six pushbuttons, a 40x4 LCD, and a 2-color LED.

I'm in the process of replacing the old DAQ with USB-6501's. Everything is going well so far, except the brightness of the LEDs -- I can barely see them when turned on!

The LEDs have 320 ohm resistors between the USB-6501 line and ground.

Should there be no resistor, or perhaps something a lot less?

These LEDs were bright before, I need to get that back 🙂

-ted

[K C]

If you use the DAQ output as a sink output to drive the led then it should be enough to drive a modern LED,

However if you are using old LED that require 20mA you wil see them glowing.

The DAQ output can sink upto 8.5mA. So with the 320 Ohm resistor you are well over the limit.

Tell me how you connect the LED and if you are using a old LED.

Kees

[TedWagner]

Kees,

Thank you for the help!

The LEDs in my devices are old (no longer in the catalog) Radio Shack 2-color LEDs. I'm pretty sure they're 20mA LEDs.

I have a bag of assorted LEDs also from Radio Shack but they're dim too, and the bag doesn't say how much current they draw... probably also 20mAs... browsing the RS web site, they all seem to be 20 or 30 mA LEDs.

Are LEDs available that will work with my current device, if I remove the resistor?

[K C]

Hi Ted,

How did you connect the LED?

[TedWagner]

K C,

The LEDs are (old) Radio Shack 2-color models, green and red (3 pins). The cathode of the LED goes to a 320 ohm resistor, then to GND. The two anodes go to P1.6 and P1.7 on my USB-6501.

-ted

[K C]

Hi Ted,

In this way the output has to drive the LED. It also 8.5mA max with a voltage of 3.5V max.

If you want to use the LEDs you have than you must add a transistor (standard PNP) and a (1 or 2) resistors for each LED.

Finding a bi-color LED for this output will be difficult.

Do you want a schematic for this transistor solution ?

[TedWagner]

K C,

I would appreciate a schematic very much, if I can use my existing LEDs -- I have no way of replacing them, but I can change their wiring on the breadboard.

(I'm a software engineer by trade, but I can follow simple schematics.)

-ted

[K C]

OK, it will be simple.

With the 320Ohm resistor the maximum current is about 15mA. Beside that you can only drive one LED at the time because the resistor is between the cathode and 0V.

Now I have to find the time to make you the schematic. Busy with my Logic Analyzer to replace the broken CRT by a LCD, have to help making diner,...well maybe this evening.

Kees

edit

Is there a way that you can remove the resistor ?

[TedWagner]

Yes KC I can remove the resistor.

[K C]

Hi Ted,

I made a drawing for you. Maybe the PNP transistor will get too hot. In that case you should use a TIP30A

The part with the FET is the most simple one to make.

The two with the transistor(s) are current drivers. As it should be for LEDs

The one with the led will always drive the same (maximum) current. If all LEDs are the same (LEDs never are) than it will do.

Did not test anything but it should work (I think )

Succes, Kees