PI GPIO run at 3.3v. SSREZ (And the rest of the DIY SSRs) are designed to work at 5v. The GPIO may work if the wire run is short enough, but then again it may not. For two reasons, you really should put in a buffer:
- Voltage shift will allow increased reliability and distance to SSR.
- Buffer protects the GPIO from being damaged by things happening on the wire. Once a GPIO output gets damaged, it can take out more outputs and you will need to replace the PI. Replace $3 buffer / replace $35 PI. I go for a buffer in a socket.
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