Need help with making a Fluke optical usb link. - Page 1
Forum section: electronics / beginners

author: lavo-1
author_rank: Regular Contributor
timestamp: October 21, 2011, 09:44:21 pm
content: This is my first post on here and apologies up front as this is a "where do I go next" plea.My background is more HT electrics than electronics and although I learnt about electronics at uni, it is far too long ago than I care to remember, but I still recall much of the basics.I am trying to make an optical to USB lead for my FLUKE 123 and have found a board that does most of the hard work for me in taking the TX and RX optical pulses and converts them to USB DATA so FlukeVeiw software can read it.http://www.sparkfun.com/products/198I have purchased the following Infra-red LED and Photo transistor to couple to the scope.http://www.maplin.co.uk/infrared-phototransistor-2252http://www.maplin.co.uk/miniature-infrared-source-2251The schematic looks like this( link is eagle format only)http://omapalvelin.homedns.org/fluke/usb/I have substituted the LED load resistor to a 47 ohm to suit the 3.3Vdd, 1.6Vd, 40mA. requirements of the LED from my local supplier.Unfortunately it dont work!After a few trials and tribulations I have managed to get the photo-transistor to conduct only when there is a constant light source. The intermittent data pulses from Flukeview (verified by substituting a Green LED for the IR) don't seem to switch the Photo-transistor fully!At this point I'm stumped.Is the Photo-transistor to slow to acknowledge the pulses from the IR diode or are the pulses not intense enough?What else should I try (Buy a Fluke lead at £100 and be done with it I hear you cry)

author: amspire
author_rank: Super Contributor
timestamp: October 21, 2011, 11:34:24 pm
content: It is a good idea to post schematics as a JPG rather then schematic files.

author: lavo-1
author_rank: Regular Contributor
timestamp: October 22, 2011, 07:46:30 am
content: Uploaded withImageShack.usHope the link worked

author: amspire
author_rank: Super Contributor
timestamp: October 22, 2011, 12:14:01 pm
content: It is likely the transistor does not have enough gain to switch without amplification.Here is what you do.Set up an IF LED with a resistor of about 3k3 and a switch to a 5-6 volts supply so you can turn it on and off. Put the transistor in about the same orientation as it is to the fluke diode.You will probably have to keep the light away from the LED/Transistor pair. The transistor probably responds to visible light and that will swamp out the effects of the IR light.Connect the transistor up to 5V but add your multimeter on current range in series with the 1K resistor. So the multimeter is reading the collector current.See what happens with the collector current as you turn the LED on and off. Once you have these figures, you can probably work out what you need to do to make it work.Richard

author: qno
author_rank: Frequent Contributor
timestamp: October 23, 2011, 10:21:33 am
content: The Scopemeter interface uses a 875 nm IRED as a transmitter. As a receiver the 950nm device works OK. If you do not know your LED and it looks broken (both wavelengths cannot be seen by the human eye) check it with a digital camera or the camera in your phone. A bright white color is a 875nm. A purple haze is a 950nm.Here ar some pictures taken with a HP307 Photosmart cameraThis is the 875 nmhttp://members.chello.nl/c.walters1/index_image3171.jpgthis is the 950http://members.chello.nl/c.walters1/index_image3481.jpg

author: lavo-1
author_rank: Regular Contributor
timestamp: October 23, 2011, 05:52:36 pm
content: thank you for the tips chaps. Appreciated.Had a play with it today on a bread board and put the LED and the PT in alignment with each other and also connected my meter in series with the 1k resistor and started Fluke software.I watched the DCmA fluctuating rapidly on /off average at 1mA. I was expecting a maybe 10-30mA to be honest, so not sure that the output from the IR LED is enough to hand shake with the scope-meter.Checked the LED as per last post and it seems it is of the 950nm type (purple light) so could this be my problem?

author: amspire
author_rank: Super Contributor
timestamp: October 24, 2011, 11:57:15 pm
content: 1mA is the sort of number I was expecting.Phototransistors designed for optical coupling are a different design - often with a flat lens on the front perhaps connecting with a light pipe to capture the light efficiently. But the one you have should be able to do the job.1ma may be enough - try a 22K series resistor instead of the 1K. See if you can get enough voltage swing.The other question is will it be fast enough? I have no idea how fast Fluke optical port runs at. If you have an oscilloscope, try and capture transitions on the phototransistor output and see if the edges of the transitions are sharp, or very rounded.If the voltage won't go high with no IR signal, then there is too much ambient light getting in.Richard

author: lavo-1
author_rank: Regular Contributor
timestamp: October 25, 2011, 07:40:27 pm
content: :)Cheers for all the help Richard. I have now got it up and running. Turns out that the board/hardware was fine just a software problem (I always seem to fall foul of data/software troubles).The version of Flukeview I had was for their meter and top end scopes, not my lowly Scopemeter 123, so poor research on my behalf.Managed to get the correct program from Fluke and bar playing with a few baud speeds, its all now working.Thanks again to all that helped.