a more complex detector circuit which turns the relay on for an adjustable period of time. When the beam is broken, the LED goes on, your effect is powered, and capacitor C1 begins charging. As C1 is charging, the transistor will be kept on. Once C1 is charged, the CdS cell will be able to turn Q1 off again. Then C1 will be discharged through R5 by the relay. Opening switch S1 will disable the time period to allow you to adjust R1. Adjust the time period with R2 (Radio Shack #271-211 $1.50) which shortens the time period by charging C1 faster, or leave out R2 and simply change the size of C1. A 470uF capacitor (Radio Shack #272-1018 $1) can give up to 60 seconds, but that will vary depending on the transistor. The time period is sensitive to the adjustment of R1, so adjust R1 before adjusting the time period. R3 extends the time period by slowing the charging of C1. The time period can be maximized by adjusting R3. As R3's resistance goes up, the time period will lengthen to a maximum, and then shorten to zero. The maximum depends on the characteristics of the transistor. 100k ohms should work well with most transistors. If the detector misses brief beam breaks, then it's probably on the edge of its range. Larger lenses and/or a brighter light will increase the range. I achieved a range of 25 ft with a `super bright` LED and a 1` lens in the transmitter, and a 3` lens in the receiver. Possible enhancements: Send a modulated signal from the transmitter and detect only that signal to prevent interference from other light sources. Use an infrared LED and photo transistor pair to make the beam invisible.
The transmitter is simply a light behind a 1 to 3 inch diameter cheap plastic lens. I used a toy magnifying glass. See figure 1. Make a tube the size of your lens out of rolled up poster board or stiff paper. Cut one end at an angle to form a hood which will conceal the light from view. Paint the inside flat black. Glue the lens in the tube. Build a cylinder which will slide inside the tube by cutting out two circles of corrugated cardboard and gluing them into the ends of another tube. Mount your light in the center of one end of the slider, and run the wires out the other end. A pen light bulb (Radio Shack #272-1141 $1.19) or "super bright" LED (Radio Shack #276-087A $2.50) is sufficient. To focus the transmitter, put a white screen of some sort where the detector will be. Adjust the slider to get the brightest spot possible on the screen. Depending on your lens and the distance, the spot may be large. That's okay. Experiment with your lens beforehand and be sure to build the tube long enough to allow the beam to be focused.