I have a simplex 4005 and an old simplex duct detector, but every time I connect it to the panel it goes straight into alarm. Any suggestions please?
By guess is that you may have accidentaly wired the panel to NC contacts on the detector
Hm, what’s the model of the duct detector you’re using, and what value EOL resistor do you have on it? Seems the 4005 is detecting a short or current-limited condition on that zone.