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I am building a "nocturnal solar engine" with two LEDs flashing independently, following the description at the old Solarbotics website. I am essentially rebuilding the same circuit. The schematic and a photo of my breadboard is below.

I am using two different resistors for R3 and R5 to get a nicely asynchronous blink from the two LEDs. What I am experiencing, however, is that when the two flashes get close to each other (around 0.1 sec), they actually happen in sync, and the two oscillators' phases keep restarting from this shared moment. After a while, the independent flashing even stops, and I am left with two LEDs flashing completely in sync.

Diagnostics so far: When the solar panel is off and the circuit on the right is working, I am measuring 1.99V in the IC's circuit (the bottom power line on my breadboard). I have tried various combinations of R3/R5 resistors, different types of LEDs, and 10uF instead of 22uF for C4/C6, but the two oscillators always end up in sync.

You can watch a video of the LEDs syncing up via this Dropbox shared link. To clarify, here's a table with each flash's timestamps (jotted down while watching the video frame by frame). Yellow's interval fluctuates but seems to be around 1.90 sec. However, when the "synchronized" flash occurs (the two highlights), it flashes over 100msec too early. It should flash later, not in sync with Red.

LED flash timestamps

A similar problem was reported in Q406793, but that question has not been resolved.

74HC14 pummer schematic 74HC14 pummer breadboard

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    \$\begingroup\$ Probably a combination of no current limiting on your LEDs, a supply voltage that is (just) below the bottom operational limit for the 74HC14 and a lack of decoupling. Try the right hand side with a proper bench supply and see if it works any better. \$\endgroup\$
    – Finbarr
    Commented Nov 2, 2020 at 12:14
  • \$\begingroup\$ Will do and report back! I did try current limiting on the LEDs with a 1k resistor, but it didn't make a difference. What would decoupling mean in this circuit? \$\endgroup\$
    – g.u.
    Commented Nov 2, 2020 at 14:24
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    \$\begingroup\$ Same as it did in the other answer you linked to, a 0.1uF capacitor across the supply pins as close as possible to the HC14. \$\endgroup\$
    – Finbarr
    Commented Nov 2, 2020 at 17:12
  • \$\begingroup\$ I watched the video - I don't see the LEDs "syncing up". The yellow LED is consistently blinking faster than the red. The fact that they occasionally overlap and blink at the same time is expected. What exactly is the problem? \$\endgroup\$
    – td127
    Commented Nov 2, 2020 at 17:53
  • \$\begingroup\$ The Rs are not exact multiples of each other. I don't think the two LEDs should regularly blink at exactly the same time. Also, after a while, the separate frequencies simply disappear and the two LEDs blink completely in tandem. \$\endgroup\$
    – g.u.
    Commented Nov 2, 2020 at 18:51

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