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Waltec Elecronic Engineering

Electronics Blog

October 14 2009

I have been using a Fluke 123 for many years. Last week I stumbled on an IR Scopemeter interface from Jan Wagner (see here).

From one of our previous projects we had some IRED's (TSKS5400S)  and Photo detectors (TEKS6400) from Vishay and decided to try to build a Scopemeter interface.

(Some people talk about IR LEDS but we use the term IRED. Check the acronym!)

After some tinkering we got the receiver working. When printing we saw a serial data stream on the RS232 RX input. But there was no way we could send any data or commands to the Scopemeter. Lowering the current limiting resistor in the anode of the transmitting IRED did not help.

So we added a battery to boost the output of the IRED and the interface worked.

After some research we found that IRED's come in 2 different flavor wavelengths 875 and 950 nm.

So we ordered some of both types an checked them with a digital camera. (You now that digital camera's are also sensitive to the near IR spectrum!!)


You can see in the pictures the difference. After replacing the 950 transmitter with the 875 transmitter the interface worked beautifully.

My guess is that the Fluke boys (or rather the Philips boys who designed the first Scopemeters in the 90's) have used 2 different wavelengths for TX and RX to prevent crosstalk as much as possible.

So here is what we learned. For the Scopemeter interface Use 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 (bothe 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.





In this blog we describe some of our work. As a reference for ourselves and our customers, to let other Electronic Engineers learn from our mistakes' and to share the fun we have at working with electronics.

A 875 nm IRED

A 950 nm IRED

Pictures taken with a HP307 Photosmart camera