The passage of an object in front of the LDR, producing a shadow or even turning off the ambient light, causes the transistor to drive and the motor to start. Note that, unlike circuits with SCRs in which only one pulse of light is needed to obtain the activation that is maintained until a trip button is activated, this circuit has no lock. The motor remains activated only as long as there is a shadow over the LDR. Potentiometer P1 is used to adjust the trigger point. As in other circuits that use this component, the LDR can be mounted in an opaque tube with a lens to obtain greater sensitivity and directivity. The transistor must be equipped with a small heat radiator if the motor requires more than 500 mA of current. Any Darlington of NPN power with a collector current of 1 A or more can be used on this circuit.
