Low switching transistor question

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

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

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.

thank you

it will probably be easier to read the 0-5v and do something with it that way

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.

Hi Polymorph

100mV would probably be ok as a threshold

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.

Gadget999:
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.

If it’s an arduino project, and you have a free analog input available, your

it will probably be easier to read the 0-5v and do something with it that way

is definitely the easiest solution.