The linked diagram uses two opt-isolators. One that fires the
SCR's and one that provides a comparison. If the output
circuit conducts at a time when U2 is not conducting, then D5 the "error" light lights. Bleed-through on the firing
circuit (unloaded) will light D6, the output indicator.
Back in the 80's, I added my own output detector circuits to my dimmers, but it was a little simpler.
Diode, zener,
LED and
resistor across the filter
choke. If there was any flow through the
choke (meaning a load was attached) then the
LED would light. Even at idle set, enough waveform appeared across the
choke that the
LED would light if a load was attached. (There was enough noise at all settings to light the
LED.) The zener and
resistor limited
current and
voltage flow through the
LED.
Note, this was my own modification, and as such technically removed the UL
rating on the
unit. (Hey, it was the 80's!) The
circuit was so simple that I would be surprised if no one duplicated it in the last 30 years.