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Old 2nd May 2020, 7:12 pm   #1
Uncle Bulgaria
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Default Reverse engineering Lumenition optical switch

I have a broken optical switch for a Lumenition 'Performance' ignition which sits in the distributor with a chopper under the arm to break the IR beam and send a signal to the power module to fire the coil.

A new one costs £100, and as I'm not exactly driving anywhere at the moment I thought surely I could fix it for less than that!

When it's working, it should have:

about 7.5V on red. When the beam is not interrupted, black-blue should be about 7.5V, dropping to 1V when interrupted.

I drilled out the potting compound and managed to salvage the majority of the circuit board inside without breaking too much. However, I've reached my limit when it comes to working out the circuit. Can some kind soul see the wood for the trees here?

I attach my drawing of the board and the continuities I've metered, and my interpretations of the somewhat obscured SOT codes. I suppose it's some sort of amplifier, though currently the black wire (my assumed ground) is missing any connections.
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Old 3rd May 2020, 5:08 pm   #2
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Default Re: Reverse engineering Lumenition optical switch

The usual disclaimer here, please keep to the intended topic as posed by the thread starter.
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Old 3rd May 2020, 5:17 pm   #3
Electronpusher0
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Default Re: Reverse engineering Lumenition optical switch

The drawing seems to be missing.

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Old 3rd May 2020, 5:22 pm   #4
Uncle Bulgaria
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Default Re: Reverse engineering Lumenition optical switch

Here are the attachments. As AC/HL says, I've tried to keep it all electronic.
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Old 3rd May 2020, 5:30 pm   #5
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Default Re: Reverse engineering Lumenition optical switch

Hello Uncle, I’ve done the same thing and I have the circuit somewhere. If I can find it I’ll post it on here..
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Old 6th May 2020, 2:51 pm   #6
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Default Re: Reverse engineering Lumenition optical switch

Any luck, Richard? I've been trying to find datasheet circuits for possible IR receivers but haven't spotted the right one yet. As it only has two pins I presume it's a photodiode rather than a transistor. So the larger resistor should be a limiter for the IR LED, with the other part of that circuit being the diode. But then why is the diode attached to the +7.5V via the 250R resistor? I'm going to try to reconstruct the grounding if I can work out the amplifier output which should go to the blue terminal... It's moments like this when I realise how ropey my circuit theory logic is, and have to read up on Ohm and Kirchoff yet again!
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Old 6th May 2020, 3:41 pm   #7
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Default Re: Reverse engineering Lumenition optical switch

It looks like an infra red photo diode like this this and equivalent photo transistor.

No need for a base connection to the photo transistor. The rest of the circuit looks like it is just getting the logic the right way round so you get a 3V switched logic signal.

Eddie
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Old 6th May 2020, 4:28 pm   #8
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Default Re: Reverse engineering Lumenition optical switch

His!

From my industrial electronics experience, IR emitting LED devices almost always have a "water clear" package encapsulation, whereas IR photodiodes have a purplish (or dark blue/grey or even black) transparent package!

It's easy to test which is which if you have a current limited supply, or a 9V battery and a 1k resistor available – simply connect your water–clear device to a 9V battery via a 1k resistor in the forward direction and point your 'phone camera at it!

For circuit–drawing purposes, the purple–tinted device can simply be represented as a diode with two inward–arrows at 45į pointing towards it!

Photo–diodes are normally operated in the reverse–bias direction as a rule!

Chris Williams
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Old 6th May 2020, 6:17 pm   #9
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Default Re: Reverse engineering Lumenition optical switch

Something like this... https://uk.farnell.com/optek-technol...tical%20switch
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Old 6th May 2020, 6:57 pm   #10
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Default Re: Reverse engineering Lumenition optical switch

LEDs make whatever wavelength their diode material dictates. If the lens is transparent at all frequencies, it makes no difference. Photo transistors and photo diodes respond to a range of frequencies of light, but higher frequency photons are more energetic (E=hf) s to keep background illumination and noise out, a good bandpass filter is essential.

Sofr IR, as stated above, expect clear on LEDs, dark (to our eyes) on the detector.
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Old 7th May 2020, 8:16 pm   #11
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Default Re: Reverse engineering Lumenition optical switch

Quote:
Originally Posted by valvekits View Post
It looks like an infra red photo diode like this this and equivalent photo transistor.

No need for a base connection to the photo transistor. The rest of the circuit looks like it is just getting the logic the right way round so you get a 3V switched logic signal.

Eddie
Thanks, Eddie. How does it function if the base is unconnected? Doesn't the base supply the current?

It's good to have consensus on the clear being the emitter and the dark (in this case orange-ish) being the photodiode/transistor. That is the impression I had gleaned elsewhere so my guess on the circuit diagram looks to be right.

Presumably the circuit in the power module is switched by the blue wire going low, as there should be 1V on there when the beam is broken. Am I to assume from this that the receiver conducts when the light falls on it, and this current is amplified and converted to a voltage with one of the resistors, such that 7.5V is present on blue when the beam is unbroken?
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Old 7th May 2020, 8:21 pm   #12
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Default Re: Reverse engineering Lumenition optical switch

Phototransistors use the collector-base junction as the photodiode. Hit it with light and the transistor itself leaks current from its collector to its own base. This is the current that biases up the base and is amplified by the Hfe of the transistor.

The base connection is normally left floating. It's brought out so a filter capacitor could be added if needed, and so that the photo diode and transistor characteristics can be measured.

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Old 8th May 2020, 9:57 am   #13
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Default Re: Reverse engineering Lumenition optical switch

Quote:
Originally Posted by Uncle Bulgaria View Post
Am I to assume from this that the receiver conducts when the light falls on it, and this current is amplified and converted to a voltage with one of the resistors, such that 7.5V is present on blue when the beam is unbroken?
You can see from post 9 that everything can be contained in a small package but there is a limited space for an opaque flag to move in and out in that arrangement. Mindful of post 2, in your application there needs to be a larger working distance therefore an IR LED and an IR receiver (photo transistor or photo diode) are used.

There is a hole in your damaged pcb but assuming the switching (blue) wire is connected to the device above the hole, that would suggest an open collector switching arrangement that is provided with a voltage within the power module, so you aren't going to measure a switching voltage until that is connected. Is the pcb double sided by any chance?

Eddie
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Old 8th May 2020, 3:27 pm   #14
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Default Re: Reverse engineering Lumenition optical switch

Iím keen to contribute something to this group especially in view of the electronics help I received with my similar problem however as I demonstrated my electronics knowledge is rudimentary. I looked into my assorted other bits box and found a complete working example of this device together with an older version that I had picked apart 10 years ago in an attempt at repair, unsuccessful.

The working one demonstrates similar to your observations, the power to the optical detector is about 7.5v, the signal wire is at about 2.5v when unobstructed and 0.8v when obstructed. The trigger occurs when the beam is first obstructed. The photos show the detector powered up, my digital camera doesnít seem to be seeing any infrared, both lenses are clear.

The photos of the older dissected unit may give some clues if the manufacturer stuck to their design and just used smaller components. Iím afraid I cannot read any of the markings on the tiny 3 legged parts. On the older detector with the eye of faith you could say one half has an orange tinge.

If I can take any measurements or do any experiments I would be pleased to help.

Chris
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Old 8th May 2020, 6:49 pm   #15
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Default Re: Reverse engineering Lumenition optical switch

I've just found this in the instructions, how it works...

1. First is the optical switch, this contains a light emitting diode (LED) which sits in the switch bracket opposite a matching silicon phototransistor. When the ignition is switched on, the LED emits an invisible infra red beam towards the
silicon phototransistor which receives or 'sees' the beam.

Chris
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Old 8th May 2020, 8:03 pm   #16
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Default Re: Reverse engineering Lumenition optical switch

Thanks, Chris! That's very interesting to see the power module in bits, too.

As you can see in my initial post, my two optical bits are obviously different colours. However, either I recorded them entirely wrongly or in this case the LED and the detector are reversed (detector clear, LED orange) as there will be no emission if 7.5V is across the LED and the 5k resistor, as my circuit surmise suggested. I've had some work to do today, but have been attempting to draw out the circuit while looking at open collectors (thanks, Eddie and RadioWrangler!) and opto-couplers for ideas as to what the SMT components might be. The values of 250R and 5K appear in line with optical coupling circuits I've looked at, so I'm feeling confident I can work it out with a bit of SPICE modelling and some veroboard. I'm going to use other Chris's suggestion of battery and resistor to test the LED as well. My diode test is inconclusive, so I'll check the camera option.

Your dissection of the power module is helpful, as it suggests that I could rebuild this with through-hole components (I have no stocks or experience of SMD) before re-potting.
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Old 8th May 2020, 8:05 pm   #17
chris.oates
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Default Re: Reverse engineering Lumenition optical switch

...and here is the diagram from their 1979 us patent application, it may not be what was put into production but may show their thinking,
http://www.freepatentsonline.com/RE30418.pdf
is a link for the description of the claim.

Chris
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Old 8th May 2020, 11:24 pm   #18
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Default Re: Reverse engineering Lumenition optical switch

Chris, that's amazing! Well done. That looks just like the circuits I've been investigating, but with the diodes across the phototransistor and LED it's almost certain that one of the SMD components I have been wondering about is a BAW56 dual diode rather than a P-channel MOSFET. Time to test the new circuit...
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Old 9th May 2020, 12:43 pm   #19
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Default Re: Reverse engineering Lumenition optical switch

Two off topic posts deleted.
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Old 10th May 2020, 2:19 pm   #20
Uncle Bulgaria
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Default Re: Reverse engineering Lumenition optical switch

I was considering how to dig out the LED/receiver from their moulded inserts in case they are duff, and have a very clear recollection of looking at a parts sheet with spares numbers for the holders. Now as I can't find any reference to it I'm convinced I dreamed it last night. Bizarre! Memo to self: stop soldering late at night.

I'm making the circuit on veroboard to test. The LED shows up on my camera, so I'm assuming the IR transmission side is working.
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