| k0jdd | Posted: 18 Apr 2025, 01:45 PM |
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 Member Posts: 7 Joined: 6-April 25 |
My new Clock works fine. [ Assembled from locally sourced parts and ordered your board online ] Unfortunately the GPS fix LED is not working. Never lights at all. PPS LED on the GPS module does flash as do the colons. This is more an annoyance than anything, as I have certainly done something wrong in assembly. Note that this is the newer ATGM336H style GPS and not the NEO-6M. Replaced LED with no change in result. 20K resister tests OK. Double-checked LED polarity - twice. I suspect a soldering issue somewhere else but would like a little guidance on what to test/measure and where to look before starting to randomly tear the display digits off the front. Appreciate any assistance you can provide. (I have an o-scope available if needed to help diagnose) ------------- |
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| k0jdd | Posted: 18 Apr 2025, 07:00 PM |
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Member Posts: 7 Joined: 6-April 25 |
UPDATE - Resolution Spent some more time on this because I hate loose ends. I looked at the clock schematics for this LED. Very simple design with it being directly driven off the PPS pin on the GPS. Did some more reading here on the forum and uncovered a similar issue which seemed to be attributable to a bad GPS module. Their LEDS were working but oh so faint. Went to take some measurements, but first I triple-checked and indeed the LED was actually flashing, but so dim as to be almost unnoticeable unless you look directly into the led. So.... LED dim = low current. I then experimented with alternative resistor values by poking the ends of the long resistor leads into the board at base of the existing 20K resistor. Sure enough, by providing a path of lower resistance (in parallel) I was able to bring the LED brightness up to where I thought it was useful. 4.7K was the value I settled on. So now it is working and I am satisfied and relieved that it was not something I did incorrectly. Theory as to why this was change was necessary. 1. Differences in GPS module from the original design. Mit tested these alternate modules but I am not sure he ever paid mind to the LED. (He probably did...) Also, the GPS module I have could actually be a clone of the module he tested. 2. Differences in LEDs. I don't know the specs on the LEDs that shipped with my first clock. The LEDs I used for the new clock have a Vf of 2V and a test current of 20mA. They are also diffused which reduces the viewing angle considerably along with what I will call 'apparent brightness'. 3. Something else which is beyond my knowledge level. John ------------- |
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| mit | Posted: 19 Apr 2025, 11:17 AM |
 yeah whatever Admin Posts: 594 Joined: 4-May 16 |
Well done for figuring it out. There's a huge variation in brightnesses with LEDs, and the diffuse LEDs can be difficult to see when they're dim. It's absolutely fine to change that resistor to make it brighter. I deliberately erred on the side of being too dim, as it's not controlled by the brightness circuit and at night it might be too bright. It's easy to make it brighter by adding another LED in parallel, whereas making it dimmer is a pain, unless you resort to putting tape over it. To be honest that whole section is redundant since there's a PPS LED on the module now. ------------- |
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