Interactive Halloween props are always fun and they are surprisingly easy to make. This year I designed a simple system that rotates a skull to face you and follows you movements as you walk by.
To accomplish this I used light sensors to detect a person's shadow. An Arduino microcontroller then calculates where they are standing and activates a servo that turns the skull to face them. When they move, the skull turns to follows them.
Step 1: Select a Halloween Prop to Use
Step 2: The Light Sensors
When using photoresistors, you need to be aware that CdS photoresistors can vary quite a lot in their output characteristics. Even if you purchase a set of photoresistors that are the same type from the same manufacturer, the output of one photoresistor can still be very different from the next. There are several ways that you can compensate for this. You could change the other circuit components to match the photoresistors in the desired lighting. Another option is to change the Arduino code to compensate for different values of the photoresistors. You can add in a simple adjustment factor to any inputs, or you can write code that calibrates itself by referencing its average value. If all else fails you can purchase more photoresistors than you need in the hopes that you can get a group of them with approximately the same output characteristics.