Light activated circuit

audioguru said:

Gophert, your timer will not timeout if something blocks the LED (a parked truck?) for time longer than the timeout.
The original circuit has the same problem.

Click to expand...

I now understand your concern, audioguru. The possibly slow high/low transistion of an LDR may make the capacitively coupled trigger recommended in the datasheet unreliable. Hopefully this simple resistor/diode combination can be used to pull the trigger high after a triggering event.

Note, it is only necessary if there is a chance that something keeps blocking the laser beam more than 5 minutes (truck parking in the beam). I haven't tried the circuit but I suppose it will work to pull the trigger pin high.

Your diode is too fast. The datasheet shows the "minimum pulse width required for triggering" is 3 times longer than the output risetime.

I got my circuit going by putting a 3 inch piece of pvc over the ldr.

Even bright sunlight does not interfere with the circuit.

Only if the laser beam is interrupted will it sound the buzzer.

thanks for all the help.