Electrical – ZCD with LM358 not working

If you really have to design it yourself (and not recommended), one approach you can use is relays, that would be safer and easier to get right. But relays will take a little time to switch so the output of the device when switching over will drop, now if the mains devices connected to it are things like laptops where they have power supplies with a fair amount of capacitance or batteries involved, these will tolerate a momentary loss of mains power for perhaps 100 miliseconds. If your output devices can't tolerate a momentary loss of power then don't go down this route.

You need a sensor (or two) to detect the loss of mains input power, you can do this with a 240 volt relay (coil energised by 240 volts) and the relay contacts triggering a little circuit to give a logic level input into a microcontroller. So as the mains power fails you get a change in logic level to the microcontroller which you can detect in software and trigger the relays to switch power to the other mains input.

Triacs: will give a faster switch over time and better in applications where the devices being powered can't tolerate a momentary loss of power, but triacs need more sophisticated circuitry to drive them: you have to apply pulses to the gate connection. And preferably those pulses need to be synchronised to the mains waveform when the signal crosses through zero (called zero voltage switching) so as to reduce the amount of radio frequency interference generated. It's all getting a bit messy.

SSR - think you mean SCR - silicon controlled rectifier. These devices are only unidirectional, so not suitable in your application.

If you can buy something off-the-shelf, I'd recommend that.

A simple test is to use a multimeter to measure the voltage on the output. Since it should be PWM you should see something between 5V and 0V. If you see 5V or 0V, then it's not working, but if you see something between the two, it's probably working.

Another option is to connect an LED with an appropriate resistor between 5V and the output, then sweep your eyes across the LED. 100Hz is very low frequency, and it should appear that instead of a solid light streak in your vision there should be a dotted line of light pulses if the zero crossing is working correctly.

Another option is to connect a small, low power speaker and series resistor between 5V and the output. You can then hear the 100Hz zero crossings if it's working correctly.

If you have an arduino, though, it's easy enough to count the number of pulses per second and verify correct operation.