Let's start out by laying out all the materials needed
Photo-transistor
Visible LED
2.7kohm 0603 resistor -Optional-
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Start by removing the old infrared LED and infrared photo-transistor. Use your iron to heat both leads at the same time. A large iron tip helps here. You can also add a blob of solder so that it bridges between the two leads, then heat the blob. When the leads are free, the board will fall away. Do not pry the LED/Phototransitor off the board. The through-holes are very delicate and could get damaged if you use too much force or leave the heat on the pads for too long. A good rule of thumb is, if the heat is on the pad for more than 10 seconds straight, remove the iron, let it cool down, then try again.
Once you've removed the old phototransistor and LED, clean out the holes. The easiest way to clean solder out of a through hole is to heat the through hole, then smack the board on a table while the solder is still melted. After the holes are clean, check the entire board for solder splashes.
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Grab the phototransistor and look at the base of the plastic package. You'll notice the base is flat on one side. The lead on the same side as the flat spot is the collector. The other lead is the emitter (another way of telling is the emitter lead is longer). Bend the phototransistor leads as close as possible to the base. the closer to the base, the easier it will be to remove/replace the eye board into the breech. You will want the bend to be slightly more than 90*. Look at the next step showing which way to bend.
Here is the phototransistor's datasheet:
PT204-6C.pdf
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The image shows the correct direction to bend the phototransistors leads. The top lead is the emitter (long lead, round side). The bottom lead is the collector (short lead, flat side)
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Bend the LED the same way but in the opposite direction. The LED leads are identified in the same way but they have different names. The lead near the rounded base (long lead) is called the anode. The lead near the flat base (short lead) is called the cathode. In the image, the anode is closer to you. The phototransistor is on the left and the LED is on the right.
Nows a good time to practice some preventative maintenance. In developing this tutorial, I bought 6 non-working eye boards from various people on the forums so I could run the tests. I was surprised to find that 5 of the 6 had no problem other than exposed tracks and through holes on the printed circuit board. Insulating the exposed copper from the breech made the "dead" eye boards start working properly again. I just used a thin layer of clear nail polish. Do not get any in the socket or in the phototransistor/LED through holes.
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Inser the phototransistor and LED into the board. In the image, the phototransistor is on the right and the LED is on the left.
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Solder the leads. Only a small amount of solder is needed to make a good connection. Be sure to push the phototransistor and the LED to the outside edges of the board while you solder them in. This will increase the space between them and make installing/removing the eye board in the breech easier.
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Cut the leads as flush as possible to the board. Clean off ALL the flux. You can safely submerge the eye board in hot water and gently scrub with an old toothbrush. Allow to completely dry before applying power to it.
Check the eye's for alignment. Make sure they are pointing directly at each other when viewed from all axis.
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Slide the eye board back into the breech. You may have to spread the gap between the photodiode and LED to get it to fit. Do not force it too hard. Plug the eye board into the main board and power it up making sure the main board is "eyes on". The LED on the gauge side will light up and illuminate the photodiode on the opposite side.
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Scope data for the eye board with STOCK IR LED. The top ghost trace is the LED turning off once every millisecond. The bottom trace is the photodiode output that the microprocessor sees. You can see by the cursor's that the rise time (up to 100% for clarity) is 570uS.
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Scope data for the eye board with RED LED. The rise time (up to 100% for clarity) is 240uS.
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Now I figured I can get an even better rise time out of the new photodiode. The current limiting resistor inline with the phototransistor is 24Kohm. That gives a collector current of approximately 100uA. The datasheet tells me that the ideal collector current to balance the new photodiode is about 2mA. I changed the resistor labelled R1 to a 2.7kohm which gives me 1.8mA through the collector.
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Scope data for the eye board with RED LED and 2.7Kohm resistor. The rise time (up to 100% for clarity) is 80uS. Now that looks like a good sharp edge!
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UNDER CONSTRUCTION!!!
Next I tried the blue LED. I didn't anticipate any problems because of the super wide spectral sensitivity of the photodiode but apparently the manufacturer fudged their numbers a bit (not too uncommon in the industry).
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Scope data for the eye board with BLUE LED and
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TO BE CONTINUED
-blue short
-blue scope
-other colors
-final comments can't condone shorting current limiting r being a datasheet abiding ee
- Not sure yet is Smart Parts designed the board with such a sluggish response as a cheap implementation of eye delay.
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