How to detect very low voltage pulses fast?

Forum 2005-2010 (read only) Hardware Interfacing
1

Hi there all. I need to detect pulses going from 0v - 0.3v. The line is pulsing very fast so has to be done digitally.
I'm thinking there's a couple of ways to go. Either boost the high voltage somehow or use a Mosfet.
i'm thinking a Mosfet's the way to go, but need a part that will switch at 0.3v. Any ideas anyone?
Failing that, how can I boost the voltage up to a level the Arduino will recognise as high?
Thanks for any help,
The Cageybee

2

You can't use a mosfet or transistor as the thresholds are too small.
You need a voltage comparator like the LM339.

3

Thanks for your quick reply.
I've just been looking at the datasheet for the LM339, but can't quite figure out how it would all connect up.
Do I just connect the ground and poitive from the device I'm testing to one of the gates, then the output from that gate to the arduino digital pin?

Cheers,
The Cageybee

4

Surely the arduino can detect 0.3 volts as a HIGH if you input it as an analogue value and set an internal threshold to switch when voltage is at the level you desire.
jack

5

Problem is speed. What i'm trying to do is read the sync pulses from a pal rgb signal.
I need to read 5 62microsecond pulses, to get the start of a frame, then an 18microsecond pulse to signify the start of the line.
I haven't done the math yet, but think analogRead will be too slow.
was hoping to use pulseIn to get the sync timings.

6

but can't quite figure out how it would all connect up.

First give the chip power and ground. Then take the output and connect it to a digital input pin. It's open collector so enable the internal pull up resistors.

Now there are two inputs on each comparator. Connect one to the source of your pulse and the other to the voltage you want to be the threshold voltage, that is the voltage over which you want it to indicate a one and under a zero. Use a potential divider or pot to generate this voltage.
It doesn't mater which of the inputs you use for what, the only difference will be the output signal. One way it will invert the other way it won't.

You are quite right analogue input will be too slow.

7

Thanks again for you're quick reply. I got how gt works now.
Thou art truly a king amongst men!
The Cageybee