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I want to use a transistor as a sort of Analogue to Digital convertor

i have a signal that is 0-5v - i want to switch an earth if i see a voltage that is greater than 0v  sometimes the voltages are very weak

is there a low switching transistor that is normally open that i can use

or should i be doing this a different way ?

i have some transistors on the shelf -  http://www.micropik.com/PDF/bc635.pdf

they look way too powerful to me

any advice appreciated
« Last Edit: July 18, 2013, 06:12:30 am by Gadget999 » Logged

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here is the spec of some on maplins web site

are any of them any good ?

 VCEO    IC    PTOT    hFE    FT                  
                V    mA     mW       min/max MHz                  
2N3904    40    200    350    100/300    300                  
BC547C    45    100    500    420/800    100                  
BC550C    45    100    500    420/800    100                  
BC549C    45    200    200    420/800    100                  
BC635    45    1000    830    63/250    150                  
2N5551    160    600    500    80/250    100      



http://www.maplin.co.uk/low-power-lf-npn-transistors-to92-case-32952
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These silicium npn bipolar transistors don't really differ in this aspect.
It's rather about which load they can switch.

They all need a VBEvoltage of about 0.8 V to switch on.
If this is beyond your "very weak voltage", you better look for an opamp or comparator.
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 thank you

it will probably be easier to read the 0-5v and do something with it that way
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Lacey, Washington, USA
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You must choose a voltage. Do you really mean 0V? What is a real acceptable threshold? 1mV? 100mV?

There are comparators that can go down to the 0V rail or below. But if you want it to switch on "greater than 0V", you need to choose a threshhold.
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Steve Greenfield AE7HD
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Hi Polymorph

100mV would probably be ok as a threshold
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Quote
100mV would probably be ok as a threshold
Then you can't use a transistor, you need a voltage comparator or you need to read the voltage on the arduino's analogue input.
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Hi Polymorph

100mV would probably be ok as a threshold

This can pretty easily be done with a comparator or Op Amp that can work down to ground. I am making an assumption that this is a single supply? No negative power supply line?

An LM339 is a quad comparator with open collector output. IE, it only pulls low.

You just need the non-inverting input connected to a resistive voltage divider so that it has 100mV on it. Then when the inverting input rises above 100mV, the output will be pulled to ground.
 
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If it's an arduino project, and you have a free analog input available, your
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it will probably be easier to read the 0-5v and do something with it that way
is definitely the easiest solution.
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