Using a 555, this configuration appears elsewhere on this site. This is from American documentation from the 90s. The indicator can be an LED or some device controlled by a relay.
Indicated by Texas Instruments, this 1968 circuit measures the duration of a light pulse captured...
We found this circuit in an old English documentation. The integrated circuit is of a specific type of...
We found this circuit in a 1975 publication. The circuit can be made with more modern equivalent...