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[on:]

Hello all, I trying to control 16 LED's using SFE PCF8575c.  I was able to test the chip using this example    in which the LEDs are turned HIGH/LOW in sequence from LSB to MSB. I wired the 8575c breakout board as shown in the example however the LEDs are all very very dim. I can see them flickering so I know the code and wiring works. I believe the issue is the chip

PCF8575 in source mode can't provide enough current.

and my wiring is in

"sink" mode (5v -> LED -> PCF8575 pin)

. What can I do to provide more current to the LEDs. I read somewhere perhaps a PNP transistor. I also tried putting a 330ohm resistor on each LED pin (i.e 8575 i/o pins) but that did little to help. Any ideas?

[Reply #1 on:]

http://www.ti.com/lit/ds/symlink/pcf8575.pdf
From the datasheet, looks like the part really only sinks current, not sources it.
I would say you'd want to send a 0 to the pin that you want to turn on, with resistor from 5V to anode, and cathode to the pin.

[Reply #2 on:]

Thanks CrossRoads, I'll give that a try. Just to be clear, The part I'm using http://www.sparkfun.com/products/8130 is the PCF8575c http://www.ti.com/lit/ds/symlink/pcf8575c.pdf which as I understand is slightly different from the 8575 used in the demo and the datasheet you mentioned.

- Quasi output: upper transistor is on for half
a clock cycle and then held up by a weak
current source (25-mA sink)

Quasi Output (Weak Current Source)

PCF8575/C 16-bit with Interrupt

[Reply #3 on:]

Spec'ed the same:

P00, P-port input/output. Open-drain design structure. Connect to VCC through a pullup resistor.

So Vcc to Resistor to LED Anode, Cathode to Pxx.  Low = LED on.