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Electronics => Beginners => Topic started by: psycho0815 on February 14, 2014, 08:04:38 am

Title: Driving numitrons with PCF8574?
Post by: psycho0815 on February 14, 2014, 08:04:38 am

Hi all,

Yesterday i got a nicely packed package from the ukraine containing some numitrons i got off of ebay.
So obviously i want to play with them a little. I don't have a proper driver chip though.
What i do have in my parts bin is a couple PCF8574 I²C port expanders. Now i've seen projects where people use those to drive 8 leds but a couple of things have me confused.

1) in the datasheet it says max Io is 25 mA which is fine since according to the IV-9 datasheet the numitrons only take about 20. But it also says Iss max is 100mA. Now doesn't that mean that while i can sink 20mA per i/o-pin i can only turn on max 5 at a time?

2) in some of the howtos i've seen, they hook the leds directly to the pins, claiming that the chip will limit the current, but isn't that a bit risky? also wouldn't that mean that the chip has to dissipate the excess power? What resistor value would you recommend given a 5V supply? I was thinking 100 ohms, and decrease that to a min of 56 Ohms if it's too dim.

Thanks in advance.

Title: Re: Driving numitrons with PCF8574?
Post by: sync on February 14, 2014, 12:35:36 pm

Have a look at this thread: https://www.eevblog.com/forum/projects/multiplexing-iv-9-numitrons/ (https://www.eevblog.com/forum/projects/multiplexing-iv-9-numitrons/)