Review on 🧊 Comimark PCF8574 PCF8574T: I2C 8 Bit IO GPIO Expander Module for Arduino & Raspberry Pi (3Pcs) by Brent Cole

Bought 6, tried 3 so far, everything works fine. They appear in I2c starting at address 0x20, and each requires a separate declaration in Arduino. This is very handy when you need multiple inputs connected: I need 24 to read the array of hall sensors for the weathervane sensor. The 3144 reverb pickups are open collector, but I didn't use pull-up resistors in prototyping, although I was prepared, they have no problem registering Hi/Lo inputs, at least good enough for my purposes . I poll three of them every 15ms, and if I run a Dupont ground wire pin along the length of these input pins, I get clean, one state transition on each pin in turn. Don't ground the interrupt pin, strange things will happen. It can be used for data sync if you use it as an output port - I haven't tested it. The interrupt pin is open collector, so you can wire multiple together to interrupt the processor, except the interrupt stays active as long as the input stays active: if the second input goes active, that event can't generate another interrupt. Normally the interrupt would reset when the register was read, but I didn't see that.