I am trying to build a circuit that uses a phototransistor to turn on a led when it gets dark. Using a modified computer Power supply I get 12v DC neg/pos, 5v DC neg/pos and 3.5v DC neg/pos. Led’s are 5mm 3w green super intensity 15000mcd, I have 1/4w metal film resistors 100ohm for DC 5w & 470ohm for DC 12w. The 3 legged phototransistors I have are (MAL100A 30V 50mA TO-18L PHOTOTRANSISTOR). I have 200 of these all ready some I am trying to use these rather then order new ones if possible. What else do I need to complete this circuit? Presently if I hook up the LED with the resistor, depending on which of the 3 legs on the phototransistor I hook onto I either get full voltage to the led, always on. Or, when you cover the phototransistor, the led dims. I am trying to get the reverse, where when the phototransistor is exposed to light the led is off, when you cover it lights up. I assume I need a transistor to take the incoming current from the phototransistor and basically flip its function. Once again I apologize for not knowing the correct terminology. This is for a Christmas project I'm working on for a friend, unfortunately I no very little of electronics, so my methods are a little crude. The basic concept is to have 4 to 5 leds being turned on by the phototransistor when someone moves there hands over it, covering any light source. Like a fool, Im attempting to make about 200 of these for a friends christmas tree. Thanks Chris, I will attempt to include a picture in this thread of the MAL100A 30V 50mA TO-18L PHOTOTRANSISTOR.