Opto interupters - Page 1
Forum section: electronics / beginners

author: Raff
author_rank: Regular Contributor
timestamp: September 09, 2012, 05:17:27 am
content: Hi all,I hope the day sees you well.My wife has een given a neat toy, its an older pfaff Embroidery machine a "Creative Designer 7570" with faults.Its a pretty flash approx 15 yer old electronic sewing machine doodad made in Germany. It came with some manuals and lots of PCMCIA cards with designs on them.I have found the self test menu and it conks out at the "button hole detector" and "syncroniser". I bought and downloaded a service manual and pulled it aprt to find that the btton hole sensor is a tiny phtot interuptor, and the syncroniser is a tiny assembly which has inside a marked transparent wheel and 3 sets of IR diodes (tiny) and 3 sets of I assume phtot transistors. They have 2 legs each and are mounted in a tiny assembly with 6 legs total.This bit reminds me of the innards of the VL Commodore Nissan engine distributer, only way smaller.Sorry for the ramble but on to my question.I want to test these things so I have set up my ancient Heathkit test bed with the following:The button hole bit first. I have 5 volts power applied via 470 ohms to the IR LED, it draws aboyt 8mA. Then as the circuit board has the postive leg of the IR diode and the phototransistor collector commoned, I have 4K7 from ground to the transistor emitter.With this setup, I get 0.9V at the junction of the 4K7 when the transistor is illuminated, and around 170mV when the IR path is blocked.That doesnt look to good to me, I would have assumed a more "TTL" level here like closer to 5V when illuminated.Any suggestions greatly appreciated.I am a retired communications tech and opto stuff is new to me, well I USED to repair Telex machines WAY back the 80's and they had optical sensors everywhere in them, but I'm getting funny in the brain box and my memory is rubbishI am going to test the "syncroniser" bit next. I'll report how that goesOh and one last thing, I have searched in vain for replacement parts, and cant even find surface mount parts that LOOK like the bits inside these things.I'll try and take some photos if anyone is interested.Thanks,Raff.

author: SeanB
author_rank: Super Contributor
timestamp: September 09, 2012, 06:00:16 am
content: Normally those have the common connection between the phototransistor emitter and the diode cathode, this being connected to 0V. The LED is normally driven from a 5V rail by a resistor of between 1k and 470R, really depending on the desired current. The transistor has a 4k7 to 5V from the collector. Often the path is obstructed by dust ( especially in an embroidery machine that makes it) so blow out both sides of the slit and test again. The other failure mode is the emitter diode going low emission, and a common quick fix is to increase the current through it a little, by lowering the value of the series resistor to around 330R. Commonly done to HP lasers as the diodes there are run at under 1mA and age does make them weak.

author: David_AVD
author_rank: Super Contributor
timestamp: September 09, 2012, 06:33:06 am
content: The IR diode is most likely the culprit. The same thing (IR LED low emission) happens in some (now 20+ year old) pinball machines.Try the same test but with a "stand in" IR LED.

author: Raff
author_rank: Regular Contributor
timestamp: September 09, 2012, 10:49:35 am
content: Many thanks for the replies.If the IR LED goes low emission, is it correct to assume that it wont be able to turn the IR transistor on fully?I have wired up the syncroniser innards to the test bed, and the IR diodes are wired in series, I have hooked up a pot to allow 10Ma of current through them.I hooked up the other half of the sycroniser assembly, the photo transistors via a 100k pot to 5v and no matter what they dont seem to conduct.Tommorrow when I can actually see in my downstairs dungeon workshop, affectionately know as the skunkworks, I'll search out a replacement IR diode and try that.Hopefully the IR diodes have gone low emission and I can rig up some suitable SMD ones in place.Ahhh pinball machines.. I have 6 of them downstairs I collected and have repaired over the years. I had 17 of them at one stage, but sadly had to sell the lot for a pittance in the 90's. None of mine are modern enough to have such hi tech features as opto sensorsnewest is a 1980 Hankin Howzat!Thanks again guys.Raff

author: David_AVD
author_rank: Super Contributor
timestamp: September 09, 2012, 11:39:14 am
content: Yeah, if the opto goes low emission, it won't fully saturate the transistor.No many Howzat machines around is there? I only have 3 pinnies.

author: Raff
author_rank: Regular Contributor
timestamp: September 13, 2012, 03:40:35 am
content: Just an update guys.Seems like I have 2 problems.The IR leds are definitely low emission, but it seems that the phototransistors have gone "deaf" as well.I have managed to find some parts that look like they will fit inside the syncroniser assembly, and I may be able to graft a slightly different assembly into the button hole sensor. Got the bits coming from RS theyll be a little while coming from the UK.I tried a test setup with the assembly I am going to graft into the button hole sensor and it works a treat, IR on I get 5 volts, IR blocked 70mv, much better.Now I just have to work out how to graft it in, its like microsurgery!And yes Howzat is a fairly rare pinnie, only 350 were made. In the world of pinball, a very low number. My others are regular system 1 Gotliebs and 1 electromechanical Gotlieb from 1976 along with 1 electromechanical Bally bingo machine.

author: David_AVD
author_rank: Super Contributor
timestamp: September 13, 2012, 09:30:13 am
content: The IR receivers can also go blind. Often this is due to excessive heat. I've seen this first hand in some stage lighting.