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 3 legged phototransistor? schnikey!
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By: Anonymous: Chris G () on Sunday, November 02 2008 @ 01:20 AM PST (Read 9962 times)  
Anonymous: Chris G

I am trying to build a circuit that uses a phototransistor to turn on a led when it gets dark. My power supply is a computer power supply, modified to output 12v DC neg, 12v dc pos, 5v DC and/or 3.5v DC. I have 2 types of LED's and their corresponding resistors to drop the current safely from the 12v DC down to its required voltage. The phototransistors are (MAL100A 30V 50mA TO-18L PHOTOTRANSISTOR). 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. 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.

By: Windell (offline) on Sunday, November 02 2008 @ 11:56 AM PST  

Well, I did a search for that part number and the news is not good: *your question* about it on our forum here was Google's 9th hit for the MAL100A. I don't see a data sheet anywhere, although I do see some of them for sale. Looks like you'll have to test the behavior of the device to see how it works. There are three pins: base, emitter, and collector. (I would *not* suggest testing it in the final circuit until you know which pin is which.)

Windell H. Oskay

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By: Anonymous: chris gielens () on Sunday, November 02 2008 @ 04:25 PM PST  
Anonymous: chris gielens

I know when I hook up the MAL100A 30V 50mA TO-18L PHOTOTRANSISTOR to 2 of the pins I get a similar action as hooking it up to a cadmium photodiode or led type looking photo-sensor. The led is dimly lit, when a light source is brought to close proximity the led brightens. Sorry for sounding kinda of dunce, it the transistor that reverses the action of the phototransistor. I took apart a garden solar light in attempt to gain some insight. There were 3 resistors, 1 transistor, 1 diode, an led type photo-receptor, the led for the lamps light source and of course 4 leads, 2 the solar cell and 2 to the rechargable battery. The various parts I'm afraid were covered in drops of enamel paint, so I couldnt get any qualifiers. I did dismantle the transistor and tried to assemble the works as per this site project on "minimilist dark detecting led circuit". I used the led type photosensor, when powered the led was at full brightness. Same response with the MAL100A 30V 50mA TO-18L PHOTOTRANSISTOR. Sure it must be the transistor used, as it writing was covered with the aformentioned enamal paint. Thanks for replying before, its pretty cool when someone responds so quickly! Should I try a different transistor? and advice would be great. Thanks Chris

By: chrisgielens (offline) on Monday, November 17 2008 @ 10:13 PM PST  

Hi again, trying to emulate your dark sensing circuit used in the pumpkin eyes project. As I said before, trying to make this circuit run off a 12v power source, used a 22k resistor in place of the the 5k and 470 ohm resistors for each of the 4 15000mcd blue LEDs. When I hook up the power, the LEDs light up, but with no affect from the phototransistor. I have tried running this circuit using different phototransistors ranging from CdS photoresistor "radioshack" to a mal100a "30v 50mA to -18L". Do you think it might be the phototransistor or my attempt at running this off 12v? Really wanting to run this off the 12v if possible. In addition I have many of the mal110a phototransistors. Figuring out which is collector and emitter was fairly simple for these. And, all of the phototransistors behaved the same when used outside of this circuit. If I run the led with its resistor and phototransistor - hooked up to power the led is dim, when its brought to a light source the led brightens. Yep, brain is gonna pop

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By: Anonymous: Ashar () on Wednesday, February 18 2009 @ 11:51 PM PST  
Anonymous: Ashar

Dude if u want to light the led in the dark you need to put in a comparator .... like l741 or sumthing like it,,,,,

you would also need a variable resistance ....
you will use the 741 chip as a comparator and give a refrence voltage to pin 3 in case of 741 that is the non inverting input ....

the phototransistors emitter will be attacher to a variable resistance and would go into the negative of the 741 that means the inverting input ....

what will happen now is that when it goes dark the comparator having a high on its inverting input will now get a low as the darkness inhibits the transistor to out put voltage so subsequently the comparator's pin 3 which is also the refrence voltage will be the higher of the voltages on the 2 pins ....

so then the 741's output pin 6 will go to the high end ..... like a positive edge ... like 12 volts ... the 12 volts are the volts that you applied to the 741 ... if you powered it with a vcc of 5 volts that it will jump to a high of 5 volts ...

so there you have a darkness sensing cct ! Smile


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