SN74AHC125Q driving LEDs directly

Andreas Brunner

Other Parts Discussed in Thread: ADS1299, SN74AHC125Q

Hey guys,

I want to use the GPIO pins of the ADS1299 for driving a led. Obviously I can not attach them directly to the GPIO ports. So I thought about using a SN74AHC125Q as a LED driver.

The LED will be a 1.8V and I want to drive it with about 2mA, so it should be no problem for the buffer gate. I assumed that this SN74AHC125Q is a open-collector gate. Will it work this way?

greetings,

Andreas

over 10 years ago

0 Andreas Brunner over 10 years ago

Intellectual 360 points

Nobody?

This is the current schematic with the SN74AHC125Q:

0 Vojin Ilic over 10 years ago in reply to Andreas Brunner

Intellectual 435 points

Hi Andreas,

what you dont understand about driving LED with GPIO? Your schematic is OK. You must only use GPIO register to set GPIO pins as output and to write data to the GPIO pins. In your schematic logical zero turns LED-on, and logical one turns LED-off.

Regards.

Vojin

0 Andreas Brunner over 10 years ago in reply to Vojin Ilic

Intellectual 360 points

Thanks Vojin. No I just wanted to make sure if this works. I thought it might be unconventional to use Bus Buffer Gates for this purpose. But I guess there is no easier solution, right?

Regards,
Andreas

0 Vojin Ilic over 10 years ago in reply to Andreas Brunner

Intellectual 435 points

Hi Andreas,

you can use Bus buffers as current amplifiers. Instead of that you can also use inverters. They have a pA input current and output current is about 10-20mA. Everithing that you say is correct.

Best regards,

Vojin

0 Andreas Brunner over 10 years ago in reply to Vojin Ilic

Intellectual 360 points

I haven't thought of inverters, that's a good idea. One thing I don't know is the type of output the SN74AHC125Q has. Is it open-drain? Or do all bus buffers have a certain type of output in common? I wonder how much voltage drop I get when it's switched on, since I need to know it for calculating the resistor value.

0 Vojin Ilic over 10 years ago in reply to Andreas Brunner

Intellectual 435 points

Hi Andreas,

all bus drivers have totem-pole output with tri-state capability. Output of the specified IC depends of the its mark. You must read data sheet of the IC to see type of the output. For LED driving you can also use small signal N-MOSFET, because it turns on with voltage on gate. When choosing mosfet you must choose logic level type and to see in the pdf what is the minimal turn on voltage Vgs. If this voltage is smaller than Voutmin of the GPIO of ADS1299 then you can use that mosfet for driving LED.

Regards,

Vojin

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SN74AHC125Q driving LEDs directly