Monostable action can be an important factor in many projects. Monostable action is understood as the production of an output pulse of constant duration that is independent of the speed or extension of the detected object, that is, of the interruption time of the light beam. This action can be easily obtained with the help of a 555 integrated circuit in monostable configuration. The duration time of the output pulse depends on R and C in the circuit of figure 8. The time constant is given by:
T = 1.1 .11.C
The adjustment of the circuit trigger point depends on the intensity of the light source and can be done by a 100 kΩ trimpot on pin 2. For weak sources, this potentiometer can be increased to 1 MΩ. It is worth remembering that the 555 integrated circuit can directly excite a stage with a relay, or even stages of logic circuits with TTL or CMOS technology. The light sources depend on the desired sensitivity. The complete circuit of this detector is shown in the figure.
