Review on 💡 SainSmart 4-Channel Relay Module: Versatile Control for Your Electrical Devices by Jeremy May

Works well, although the relays make a fairly loud noise when activated. For those interested in logic low and what happens when a device loses power, no power is not the same as logic low. When there is no current, the circuit is floating and the NO (normally open) side is effectively open (disabled). Going low closes NO and completes the circuit (the NC side is now also open, breaking that part of the circuit). You can easily control this from your Raspberry Pi with the Pigpio package, just write 0 to the GPIO the relay is connected to and it will switch from the NC side to the NO side. When your Raspberry loses power, the NO side opens and breaks the circuit. If you turn on the Raspberry backup and run your software and write 0 to the GPIO, the NO side will be repaired again. Write 1 to the same GPIO to open it. Since this is a mechanical relay you need to make sure you don't try to switch it too quickly as there is a physical switch that moves back and forth. I've used it to control a 24V circuit in my home's HVAC system and it's great for the purpose (although the Makeatronics board is smaller and quieter, it's twice the price and takes longer).