How to light an LED????

[labview1958]

I am trying to write a LV regarding frequency measurement. The file is attached. Any comments?

[Buechsenschuetz]

The signal you will get from your setup will be - or should be - digital. I.e. you will have a signal changing from on/off state (or logical 1/logical 0) on your daq input. However, you should add some pulse-shaping circutry to obtain clear TTL signals, since the output from the photodiode will NOT be compatible to TTL inputs. You will need a voltage source and a series resistor or a current source to bias the photodiode, an amplifier stage and a discriminator circuit, i.e. a comparator giving output pulses when the current through the photodiode has reached a certain limit. You should also add a TTL compatible driver.

Depending on whether you have a more or less constant rpm speed your signal will be more or less periodic - you will get one pulse per turn. The photodiode itself will not generate properly shaped pulses, you need to add some circuitry as stated above.

[labview1958]

Your advice.

"You will need a voltage source and a series resistor or a current source to bias the photodiode, an amplifier stage and a discriminator circuit, i.e. a comparator giving output pulses when the current through the photodiode has reached a certain limit. You should also add a TTL compatible driver"

Q: Where can I buy a such a circuit ready-made. I am not that good at electronics wizardry. If it is not sold, can you sent me a circuit diagram so that I can ask someone to make it.

[Buechsenschuetz]

If you are rather an electronics newbie (we all have been in that situation at the beginning of our career), I would strongly recommend using a ready-made unit for your application. There are quite a few slotted optical sensors available. RS, for example, has comprehensive application documents on their slotted sensors. I cannot give you a link since I store these documents locally, but here is the title:

'Reflective and slotted opto-switches 2601'

There is also a document number in the header: 232-2447.

I think for your application a slotted sensor like RS stock number #304-560 would be suitable. All you have to add is a current limiting resistor for the LED, and you even do not need a torch... You will get TTL-compatible signals if you use a supply voltage of 5VDC (regulated, please).

In case you cannot place the cutout or slot for the opto switch conveniently (the depth of the slots of the mentioned sensors is limited to a few mm) I would suggest using a reflective opto switch (some are mentioned in the document). You should blacken the rotating disk and glue a small piece of reflective material (aluminium wrapping foil or chrome adhesive tape will do) on the disk such that it will pass in front of the sensor surface.

Sorry but I am neither a consultant nor a free-lance engineer. In case you need more detailed support you should apply to a consultant having some experience in optoelectronics.