The circuit below can switch up to 4 Amps of current with a peak voltage up to 20 V. It is fully isolated using two inexpensive red LEDs. One LED forms a red light source, which illuminates a second red LED. The circuit takes advantage of the 1.5 V produced by super bright red LEDs when illuminated with red light. The voltage produced by one red is routed to two n-channel FETs, configured in a bi-directional switch circuit. The two FETs selected have a low gate-source threshold voltage, which allows them to turn on hard from the 1.5 V from the red LED.
The bi-directional configuration allows the circuit to switch both AC and DC power. Using quality super bright red LEDs, the circuit will switch on fully with only 5 mA of LED current.