Review on Keyestudio 5V DCAC 4-Channel Relay Hat Shield Module Expansion Board for Raspberry Pi 4, A+, B+, Pi 2 Model B, Pi 3 Model B by Christine Castrillon

I needed a way to control some of the Raspberry Pi's heating elements and fans to circulate warm air around my 3D printer. This board was just a ticket. It's well thought out for the most part and has a lot of good design features. Pros: - 5V operation is convenient as it matches the Raspberry Pi itself. - The compact layout fits perfectly in the airspace above the Raspberry Pi. - Relay races are very powerful. Many competitors have much lower contact current ratings. - LED indicators are a nice touch. - Actually connects to the GPIO header instead of relying on jumpers like most competing products of the same performance. Cons: - The screw block doesn't work well with stranded wire. It is clearly intended for use with solid or stranded wire that has pins or eyelets on the end. I would be happy if the device were more independent of cables. There are parts like that. - Inconsistent and wrong online documentation about which IOs are connected to which relay. I just had to try them all to find out. Maybe there are multiple hardware revisions? - You do not have to worry about providing mounting material. The screws are of very poor quality (very soft) and a bit long. Nuts are an odd choice - it's better to use a body with threaded inserts. - I had to add flyback diodes to the control coils to prevent the Raspberry Pi from restarting when the relay was de-energized. - No good way (e.g. moving resistors, jumpers, DIP switches) to remap IO in case of collision. In the end I only disabled the first channel to avoid collision with GPIO 4. And I have a feature request. It would be great if there were additional diodes to turn channels on when other channels are on. I ended up adding a diode myself between the two channels to force the fan to turn on when the heater was turned on as a safety measure. I admit this is a bit esoteric and unlikely to be implemented in future versions. Overall I am very happy with this purchase. But if I needed another board to do something like this, I'd probably design it myself.