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pulsar light sensor

For electronic related stuff like module repair, silver epoxy fixes etc.
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smokefrog

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pulsar light sensor

Post14 Nov 2010, 04:28

hello all

i got a question, i was looking at my pulsar p2 and the display looked dimmer then usual, after closer inspection the light sensor fell off, does this make a difference? what is the best way ti attach?

thanks in advance

gus
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bruce wegmann

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: pulsar light sensor

Post14 Nov 2010, 08:19

Thirty-five-plus years of aging and thermal cycling can take their toll... Believe it or not, these were originally attached to the circuit board with conductive epoxy, not soldered on. Presumably, this part has very limited tolerance for heat, so was put in place after all the other surface-mount components. You'll have to remove the module to retrieve it. Carefully remove the old epoxy (gentle scraping with a razor blade works well) until you get down to the contacts (they are metallic and have a bright sheen when clean). Then apply about a cubic millimeter of silver epoxy to each end, and drop it back in place. You can do this with tweezers without magnification. The part is not polarity-sensitive, so either orientation will work fine. Just be sure you don't short out adjoining traces, or you'll be trading one minor problem for a number of big ones. You can test the functioning of the sensor immediately by powering-up the module, but I would let the adhesive cure for a day before actually wearing the watch.
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coconutman351

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Re: pulsar light sensor

Post18 Mar 2021, 19:39

Will a missing light sensor cause the module to not display anything? I always thought the sensor would be that current limiter device that sources power to the LED module and if that path was broken it would not light up. I just acquired a P2 on eBay and the seller pointed out on the photo that something had fallen out and pictures showed the light sensor cleanly fell off the circuit board. Seller said he tried to add batteries but nothing lights up, I'm just hopeful that it may just be the sensor that's the problem.
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bruce wegmann

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Re: pulsar light sensor

Post19 Mar 2021, 01:43

Short answer to your question...no. It's just a variable (cadmium-sulphide) resistor that regulates the width of the pulses enabling the digits (part of the multiplexer circuit), in order to vary the display brightness in changing light conditions. If it's missing, the circuit reads that as low light level, and dims, but does NOT extinguish, the display. So, if your module is not showing any display, the primary problem lies elsewhere.
I guess, if you got the watch cheaply enough, you won't get hurt too badly on this, but, chances are you're going to need a new module. NEVER buy a non-working watch on the assumption that it will be a simple fix...in twenty years, I've managed to do that exactly five times (and, mostly in just the last few years, when I knew much more about what I was looking at). Sometimes, working condition is not a significant factor (like the 18K Tiffany Calculator that just sold for marginally over the "melt" value), but usually, unless it's obviously working, the value just isn't there. I would hate to miss a genuinely good deal because I had expended my resources on bargain-hunts.
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coconutman351

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Re: pulsar light sensor

Post19 Mar 2021, 02:28

Thanks Bruce for the explanation on how the sensor works that helps. In acquiring this P2 got it at a good price and the case appeared in good condition so I bought it with the expectations that I probably won't be able to get it to work and if I did it would be an unexpected bonus. It is easy to get caught up in bargain hunting for the cheap stuff only to end up with a pile of non working cases and bands so I pick my challenges carefully. Worst case I can send it to Strikes and Spares to get a SASM module and even factoring that cost it would still be a good value. I recently sent 4 of my pulsars to Strikes to get them in working order. Definitely better than having them in the parts box. One of these I managed to revive with an external oscillator circuit and that did work for a few years and then it finally croaked on me. I've come to expect this level of maintenance and operational expectations with these 40+ year old timepieces.

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