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Monochron Clock -- Extra Resistor
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Monochron Clock -- Extra Resistor

by adc1 on Wed Apr 09, 2014 6:18 pm

I seem to have an extra resistor in my Monochron clock kit. The kit arrived today. According to the parts list on the web site, there should be 9. I have 10.

I seem to have an extra 10K resistor.

The PCB shown on the Monochron website is slightly different. Mine has an R10 position just above the IC2. The one on the website does not.

Can anyone suggest how to handle these differences please?

Thank you.
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Re: Monochron Clock -- Extra Resistor

by jarchie on Fri Apr 11, 2014 4:59 pm

I've never used a Monochron, but since no one else has answered this post, I'll give it a go.

Like you, I saw no mention of R10 in the assembly instructions, but the design files on GitHub do include R10. The board design shows R10 above IC2, just as you wrote, and the schematic shows that R10 is a 10k pull-up resistor on the ATmega328p reset pin. The design files can be viewed with the freeware version of Eagle, if you'd like to examine them yourself.

The ATmega328p has a somewhat weak internal pull-up on the reset pin, and because the circuit would probably work without an external pull-up, R10 was probably omitted in earlier Monochron designs. But adding an external 10k pull-up resistor on the reset pin is good design practice with ATmegas and can help prevent unwanted spurious resets--especially in noisy electrical environments.

My guess is that R10 was probably added for robustness, but that Adafruit has not yet updated their assembly instructions. Although R10 might be optional, I suggest installing the 10k as R10. Doing so should help ensure reliable operation.

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Re: Monochron Clock -- Extra Resistor

by adc1 on Fri Apr 11, 2014 6:43 pm

Thank you jarchie for your reply and your helpful explanation. I sincerely appreciate you including the links. Thank you! :)
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Re: Monochron Clock -- Extra Resistor

by jarchie on Sat Apr 12, 2014 3:08 am

You're very welcome.

I'm a bit surprised that R10 was added two years ago (according to GitHub), yet the Adafruit docs are still out of date. Of course, I'm not familiar with the Monochron, so maybe I'm being unduly harsh. But if I'm not then please, Adafruit, fix the docs!!!

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Re: Monochron Clock -- Extra Resistor

by adc1 on Sat Apr 12, 2014 3:45 pm

Yes, I agree with jarchie. The documentation needs to be updated please: the parts list and the assembly instructions.

I also found that the recommended placement of the 3 mm red LED is incorrect. While it does need to be bent over so that it is pointing outward, the LED should still be over the PCB. Perhaps, at one time, the hole in the case allowed the LED to protrude through. The hole in the case is large enough so that the light can be seen, but not large enough to allow the LED itself to pass through.

I finished the project yesterday evening, 4/11/14. It took about 3 hours from soldering to putting it in its case. I am quite delighted with the result. I have it sitting on my entertainment center where I can admire it. I'm quite happy.

It's a product I would recommend to anyone, subject to my reservations about the instructions.
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Re: Monochron Clock -- Extra Resistor

by jarchie on Wed Apr 30, 2014 9:08 pm

...bump...

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Re: Monochron Clock -- Extra Resistor

by af3556 on Sat Jul 19, 2014 12:44 am

The assembly guide needs an update to include this resistor; my son had pretty much finished assembling the kit when he noticed the 'extra' part, and had to disassemble the case to fit it.

(a minor blemish on an otherwise excellent project - he's very happy with the result, and is not trying to figure out how he can change the code to actually play one of the pong players...)
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Please be positive and constructive with your questions and comments.