Comimark 3Pcs PCF8574 PCF8574T I2C 8 Bit IO GPIO Expander Module for Arduino & Raspberry Pi

These expanders work perfectly once you figured out the address. On Arduino, the address is 0x20 by default not as 0x40 as documented by the some datasheets. This is because the datasheet counts the R/W bit as the first bit and Arduino does not count this bit and thus the address is divided by 2. Thus if you change the jumper on A0 to VCC, the address is 0x21 in Arduino and not 0x42 like in the datasheets.

I didn't do my research so shame on me. Warning to others to avoid wasting time/money if this doesn't work for your needs like me.I give it 3 stars because it might work for a limited solution. You probably can use this for detecting a switch pulling a pin low but that is about as useful as this gets. Maybe an ok input expander. HOWEVER; if you want to use it as an output expander to do anything like drive a SS relay or something that needs any kind of current forget it. This will NOT work. Tried many different pull-up resistor arrangements and just could not get a 3.3v (ESP32 based setup) to work with any of my SS relays. I don't know if it would work better in a 5v setup but in my situation I'd have to add additional transistor circuitry to drive any kind of SS relay. Not what I wanted to do with better solutions available. Fair warning to anyone considering this.Search for details on the 8574 chip and you'll see. IMO something like a MCP23017 based module is a much more versatile solution. That's what I ended up using.

Bought 6, tried 3 so far, all worked perfectly. They appear on I2c starting at address 0x20 and on Arduino each one requires a separate declaration. These are very handy when you need a bunch of associated inputs: I needed 24 to read a hall effect sensor array for a wind vane sensor.The 3144 hall effect sensors are open collector but I did not use pull up resistors while prototyping though I was prepared to. They have no problem registering hi/lo inputs, at least well enough for my purposes. I’m polling three of these every 15ms and as I run the pin of a grounded Dupont wire down the length of these input pins I get clean, one at a time state transitions on each pin in sequence. Don’t ground the interrupt pin, odd things happen. It may be used to clock data out when you use it as an output port - I didn’t check.The interrupt pin is open collector so you could tie several together to interrupt the processor, except the interrupt appears to stay active as long as the input stays active: if a second input goes active that event can’t generate another interrupt. Normally the interrupt would clear when the register is read but I didn’t see that.