Flow sensor

[rubberlele]

Another question, Picture 1 is the schematics for a flow sensor(from manufacturer), Pic 2 is the info for the circuits, Pic 3 is what i intend to do...Please comment and do advise....thanks

 

[rubberlele]

Pic 3

 

[rubberlele]

why i cant attach pic 3?? hmmmm

 

[rubberlele]

pic 3

 

[Roff]

1. You need a current-limiting resistor between the 3V battery and the LED.
2. Provide a link to the datasheet for the flow sensor.

 

[rubberlele]

here's the link, is there any possibility that i burst out the diode if i connected it without current limiting diode?

http://www.rsmalaysia.com/cgi-bin/b...o=0257133&prmstocknum=0257133&prodoid=0242288

 

[JimB]

is there any possibility that i burst out the diode if i connected it without current limiting diode?

Yes, If the batteries are fresh, I think that there is a high probability that the LED is burned out.

JimB

 

[rubberlele]

hmmm should 100ohm resistor be enuff? Its because I=V/R should be 3V/100, so its like 30mA right?

 

[mstechca]

what is block "A"?

 

[rubberlele]

A - Voltage Regulator
B - Photodiode
C - Amplifier
D - Schmitt Trigger

All these is already inside the flow sensor when i bought them
So is it okay if i pu 100ohm resistor? And how can i know whether the LED have been burnt?

 

[mstechca]

So is it okay if i pu 100ohm resistor?

follow ohms law and don't exceed the maximum current sink specifications of the opto-coupler. Find the maximum current it can accept and use this equation for the lowest value resistor required: (resistor in ohms) = 3 / (maximum current in amps).

And how can i know whether the LED have been burnt?

If the output of the opto-coupler does nothing no matter what happens at the input, and the output is connected correctly, then the diode is burnt.

 

[Roff]

Rubberlele, your link doesn't work. How about a part number?
Mstecha, you forgot to account for the LED voltage drop.

R=(3V-Vled)/Iled

For example, if you want to run the LED at 20ma, and the forward voltage of the LED is 2V@20ma, then R=(3-2)/0.02=50 ohms. You have to look at the datasheet to find out what the LED forward voltage is.

 

[rubberlele]

yeah Ron, the manufacturer i think is RS,

http://www.rsmalaysia.com/cgi-bin/bv/home/Home.jsp?cacheID=myie

and the product code is 257 - 133, if insert a search for flow sensor that is...

 

[Roff]

Unfortunately, the datasheet does not list the LED's I-V characteristics. They just recommend 30ma. It would be better if you could use a higher battery voltage. With a 3V battery, getting 30ma will require some experimenting using a multimeter. Also, the current will drop precipitously as the battery discharges.
I suspect you were taken in by the "Reverse voltage at 25º C free air temperature=3.0V" specification. To determine LED current, you need to know forward voltage. This spec is just a breakdown voltage, which could occur if you connected your battery backwards.
EDIT:
I found an infrared LED datasheet that shows forward voltage~1.25V@30ma. Some of the guys here probably already knew that. :roll:

 

[rubberlele]

hoho too bad...hmm so what do you suggest Ron? Im totally at lost here...

 

[Roff]

hoho too bad...hmm so what do you suggest Ron? Im totally at lost here...

See edit above.

 

[rubberlele]

so it'll be like nearly 60ohm yeah? Thanks mate! now im testing whether the LED is still okay or not, mstechca said to check the output of the opto coupler, that is the output of E, this means that i need to connect all the devices(minus the oscilloscope) and run water through it right Ron?