Help with a 3 legged phototransistor - Electronic Circuits Projects Diagrams Free

chrisgielens · 2nd November 2008, 08:46 PM

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.

kchriste · 2nd November 2008, 10:58 PM

You need a second transistor, or preferably a FET, which gets turned OFF when the photo transistor turns on in response to light.
Can you give us a part number for the LEDs you will be using? We need to know how much current it's rated for, what it's Vf is and how many you want to switch with one phototransistor. Also, where do you buy your parts so we can recommend a suitable FET.

Willbe · 3rd November 2008, 03:56 AM

Quote:

Originally Posted by chrisgielens

when you cover the phototransistor, the led dims.
So it can work; you are getting transistor action.

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

The pt should go from +V to "the base" and the LEDs should be in "the emitter circuit."
Or, the pt should go from ground to "the base" and the LEDs should be in "the collector circuit".
The first is more stable with temp.

Right now you have one of the two "opposites" of that above and so it acts oppositely.

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2nd November 2008, 08:46 PM	#1
chrisgielens	Help with a 3 legged phototransistor 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.

2nd November 2008, 10:58 PM	#2
kchriste	You need a second transistor, or preferably a FET, which gets turned OFF when the photo transistor turns on in response to light. Can you give us a part number for the LEDs you will be using? We need to know how much current it's rated for, what it's Vf is and how many you want to switch with one phototransistor. Also, where do you buy your parts so we can recommend a suitable FET. __________________ Inside every little problem, is a big problem trying to get out. Last edited by kchriste; 2nd November 2008 at 11:02 PM.