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@Stuart.d.d
The other issue here is one of Isolation. A relay gives you two electrically separate circuits. The relay coil, the low voltage (safe) side of the circuit and the contacts which can switch hi voltage hi current loads (depending on the relay spec).
An SCR can switch hi voltage hi current loads but the input (gate) would need to be connected to the low voltage side of the circuit. If this was switching mains or hi voltages it would mean that the Espruino itself would be at dangerous voltages.
The solution is to use an optical isolator. The Espruino drives a small LED and a Photo Transister provides the gate voltage to drice the SCR.
There are several packages that contain both optical isolator and SCR that can operate as Solid State Relays.
Search articals on SCR, Triac, Thyristors and Solid State Relays.
A big advantage of a relay is the configuration of the contacts, often multiple sets of isolated contacts with normally open and normally closed switches. An SCR is a single normally open switch.
The advantages of an SCR are the speed at which it can operate and that there so it can be uses for power control such as light dimmers and large motors.
Hope this helps:)
Stuart