Hi All,

I need some help on a circuit i am playing with... if i can get it working i can move on with a project i am working on.. i have setup the circuit described here (just knocked it up with visio) The part of the circuit in the box is one single unit with an ir diode and a photo sensitive transistor (i believe) facing each other, presumably when i put something between them it should open or close the circuit, i do not believe i have nay short circuits however when i power it up nothing happens, i tried turning up the voltage to 12v and the red diode went on but i could not get it to flash as i moved things passed the ir diode

does anyone have any idea what is going wrong here?

 

Principle appears OK
but the container used is likely to reflect the signal and nullify the effect of the obstruction created by you.. Imagine what happens in an Optical wheel mouse. the interruptions are created by a slotted wheel, and it is converted as signal. there the distance between the Tx and Tx is small. if you can manage in such a fashion, and if the obstruction is opaque to IR lighting emitted by the diode, it works.

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Regards,
Sarma.

 

You must look at the datasheet for the optical device to see its pins and its minimum current transfer ratio.
You have only about 2.2mA in the IR LED but want 2.5mA in the red LED.
Maybe you have the photo-transistor's pins connected backwards.
Maybe the current transfer ratio is only 20% then the current in the IR LED must be increased to at least 11mA.

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Uncle $crooge

 

why not modulate the ir signal. and use TSOP34838 at the recieving end!

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Khizer Naeem Mughal
IIUI

 

Here are the stats for that little optical unit.. can someone please steer me in the right direction i need to get this to work or let me know the equations i should be using.. i assume i should be changing the resistors..?

(i do not know what any of these mean.. i am just reading off the info sheet that came with it)

Maximums
INPUT
Forward dc current 60 mA (If)
Reverse voltage 6v (Vr)


OUTPUT
collector emitter voltage 30V
Emitter collector voltage 6V
Power Dissapation 100mW

INPUT
Forward voltage 1.2V typ 1.5V max Conditions If = 30mA
Reverse leakage current max 10 uA Conditions Vr = 4V

OUTPUT

Dark current 100nA max Condition VCE=10V If = 0

COUPLED
Output current 0.5mA Minimum Condition If = 20mA VCE = 5V
Rise time typical 5 uS Condition Ic = 2ma VCE = 5V
Fall time typical 5uS Condition RL = 100 ohm

 

Based on the above, for If=20mA and 4.5V Vcc and Vfw=1.2V you need 165 ohms instead of 1k5. And that will only give you 0.5mA of output, barely to light-up the output LED.

Put an uA meter instead of LED and find out what gain you might need when using If=2.2mA (with your 1k5).

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www.dipmicro.com

 

You have the collector-emitter of the opto-transistor backwards. The LED lighted when the supply was 12V because it exceeded the max allowed emitter to collector voltage of 6V.

The minimum coupled current in the transistor is only 2.5% so if it has the minimum spec then the current at the output will be very low which would make the LED very dim. Then adding a transistor at the output will boost the current.

I corrected your circuit and used 100 ohms to feed 33mA to the IR emitter. Then the transistor's collector current is a minimum of 0.825mA which would make the red LED very dim. If the coupling is more than 2.5% then the 220 ohm resistor will limit the current to about 11mA.

It uses a lot of current. Kiss your battery goodbye.

Attached Images

optocoupler.PNG (19.9 KB, 18 views)

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Uncle $crooge

 

Thanks a lot for your help there guys, i will head down to the electronics store on the weekend and let you know how it goes

 

Thank you very much audioguru

i plugged it in the way you suggested and it works perfectly.. i can move on with my project now..

Thanks to everyone else who posted also