Pages: 1 [2] 3   Go Down
Author Topic: Infrared Sensor  (Read 2972 times)
0 Members and 1 Guest are viewing this topic.
Manchester (England England)
Offline Offline
Brattain Member
*****
Karma: 596
Posts: 33265
Solder is electric glue
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
But why such a high low voltage?
Because the output transistor is either damaged or it is oscillating.

Quote
but it worked, sort of, with the LED
That could be the problem the absolute maximum rating of that output transistor is only 5mA. How much current was the LED drawing in that test? That could have fried it.
Logged

Connecticut
Offline Offline
Edison Member
*
Karma: 17
Posts: 1216
RTFD (Datasheet in our case)
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

It still works with the LED, though.
And it was always dim. I was using a 10k resistor, and it didn't turn completely off.
Logged

Manchester (England England)
Offline Offline
Brattain Member
*****
Karma: 596
Posts: 33265
Solder is electric glue
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

With 10k it should be fine. The tutorial said 200 to 1000R. Try enabling the internal pull up resistor and seeing if you can sense anything.
Logged

Connecticut
Offline Offline
Edison Member
*
Karma: 17
Posts: 1216
RTFD (Datasheet in our case)
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Should I pull it high or low?

And why, why, Grumpy Mike, would the LED change its brightness if there was only a change of about 100mV!?
Logged

Norway
Offline Offline
Jr. Member
**
Karma: 1
Posts: 71
Does it work?
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Should I pull it high or low?

And why, why, Grumpy Mike, would the LED change its brightness if there was only a change of about 100mV!?

You set it in high state to activate pullup.

regards
Logged

Connecticut
Offline Offline
Edison Member
*
Karma: 17
Posts: 1216
RTFD (Datasheet in our case)
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

thanks
Logged

Manchester (England England)
Offline Offline
Brattain Member
*****
Karma: 596
Posts: 33265
Solder is electric glue
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
would the LED change its brightness if there was only a change of about 100mV!?

We could be dealing with a bust transistor here so things don't behave normally. It could be there is now a non linear gain and the increased current will produce and increase gain. Is the 100mV change measured on the pin of the device with the resistor and LED as a load or is it with nothing on the output pin of the sensor? There could be something wrong with the pull up resistor in the chip which is why I suggested using the internal pull up.
With no LED resistor (I know you have a 10K one) a change of 100mV would show up as a big brightness change due to the non linear voltage current characteristics of the LED.
Logged

Connecticut
Offline Offline
Edison Member
*
Karma: 17
Posts: 1216
RTFD (Datasheet in our case)
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

The 100mV change was with no load. But why would the transistor be shot right out of the box? The first time I touched it it was doing the same thing it's doing now.
Logged

Manchester (England England)
Offline Offline
Brattain Member
*****
Karma: 596
Posts: 33265
Solder is electric glue
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
But why would the transistor be shot right out of the box?
1) It could happen, it's not unknown.
2) You said you might have shorted the output to ground with a meter, that would not damage it but a short to +5v would.
Logged

Connecticut
Offline Offline
Edison Member
*
Karma: 17
Posts: 1216
RTFD (Datasheet in our case)
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Hmm... When I was testing it, the LED was connected... and it blew up. I had to throw it away. But that may have been a short over the LED, as I had a resistor in place.
Logged

Manchester (England England)
Offline Offline
Brattain Member
*****
Karma: 596
Posts: 33265
Solder is electric glue
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

If the resistor was in place then you could have shorted out the LED and there would be no harm because the current would still be limited by the resistor.
Logged

Connecticut
Offline Offline
Edison Member
*
Karma: 17
Posts: 1216
RTFD (Datasheet in our case)
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

But the LED burnt up.
Logged

Manchester (England England)
Offline Offline
Brattain Member
*****
Karma: 596
Posts: 33265
Solder is electric glue
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
But the LED burnt up
No not by just shorting it out.
Logged

Connecticut
Offline Offline
Edison Member
*
Karma: 17
Posts: 1216
RTFD (Datasheet in our case)
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

How then?

On a side note (sorry smiley ) :

I know that voltage is pushed; hooking an LED up to a 1kV supply will blow it up.

But current is pulled. So hooking a 20mA LED up to 5Amp supply won't hurt it, right? (assuming 3V)
Logged

Manchester (England England)
Offline Offline
Brattain Member
*****
Karma: 596
Posts: 33265
Solder is electric glue
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

The supply is only capable of giving 5A if the load put upon it draws that much. That is the load has to have a resistance low enough to pull 5A out of the supply. You c an tell what resistance it has to be by using Ohm's law. For a 3V supply this is:-
3 / 5 = 0.6 ohm
Now a 20mA LED will not pull 20mA, the maximum current you must put through it is 20mA. It is a non linear load, that is the voltage and current relationship is not a straight line. See http://www.thebox.myzen.co.uk/Tutorial/LEDs.html
So you have a resistor to limit the current. When 20mA is flowing through it it will drop 1.2V (or what ever the data sheet says) across it. So if you connect it to a supply any greater that this without a series resistor you will exceed the maximum current and blow it up.
If you short an LED that means you are connecting the two wires of the LED together, this stops any current flowing through it and so it is perfectly protected from any harm. What is not protected however is the thing trying to drive the LED. If this is a resistor then it will take extra current when the LED is shorted out.
Shorting one end of the LED to the supply while the other end is at ground will blow it because you are allowing unrestricted current to flow. Similarly if one end of the LED is connected to the supply shorting the other end to ground will kill it for the same reason. But shorting the two pins together will keep the LED safe.
Logged

Pages: 1 [2] 3   Go Up
Jump to: