I work evenings and get home well after dark. Because no one is at home to greet me upon my return, I used to leave the carport light on, for many energy-wasting hours. In order to conserve electricity I designed the Carport Light Controller circuit. It is simply a 555 timer IC, operating in the one-shot mode, that is triggered by light striking photo-resistors.
The photo-resistors normally have a resistance of several meg-ohms but, in the presence of light, that resistance drops to several hundred ohms, permitting current from the six-volt source to flow in the circuit. The R-C combination shown gives an on-time of about two minutes. Photo-resistors PC3 and PC4 are mounted at headlight-height on the carport wall.
Now, when I pull into the carport at night, the headlights illuminate the photo resistors, and the timer starts. That activates the relay, in parallel with the carport light switch, and the lights are turned on long enough for me to get safely into the house. The lights are automatically turned off when the timer's two minutes are up.
I have also included a push-button switch, mounted inside the house, and when I go out at night, the button allow me to turn on the outside lights to see my way out to the car, knowing they'll turn themselves off after I leave.
The timer is not triggered during daylight due to PC1 and PC2 photo-resistors. These are mounted on the outside of the house where they are in the sun much of the time and keep the timer from triggering during daylight hours. Resistors R1 and R2 establish the thresholds for proper on/off control.
The unit has been in service for several years and has not given any problems. All the components used are stock items.
David S.
