As u can tell, i whipped this up in paint, so excuse my poor graphics. Neway, i always like to go for the simpler circuit. I think that this will work quite nicely, could someone tell me if it won't though, i dont wanna mislead poor dexel.
The light sensing circuit will let current flow through the transistor when there is no light on the sensor. This will then activate the SCR which will stay on as long as current flows through it. That's the point of the normally closed pushbutton switch. When you hit this, it reset the buzzer. Neway, thats the way i think that it should work. It probably needs a resistor to protect the SCR, but I don't really know.
You could add a delay for birds flying through the beam (but what are the chances of that? :roll: ) like on chemelec's.
This here's about as simple as it gets