I designed a simple 555 astable 50% duty cycle square wave generator, assembled in China on SMT PCB. It works well: Except when you touch it, or even put your fingers near it, the timing changes radically - it seems to approach a very high frequency.
I understand the human body has capacitance - is that the cause? Is there a way to limit this? It's an open circuit board, not in an enclosure. The board has a ground plane.
Related: On the scope, I see a nice square wave where expected. But the trigger wave doesn't look like the shape you see in the diagrams (exponential charging and uncharging), but small and noisy. Yet, the LED and square wave look proper. How do you explain that?
Here is the schematic:
And here is the PCB. There is also a ground pour (not shown) on the bottom:
Regarding the scope:
- With additional manual tuning, I've been able to get the square wave on the scope.
- I've also been able to get the trigger wave on the scope. It appears like a triangle wave, slightly bent - not the shape I expected.
- In both cases, DC coupling was needed. Under AC, results were very inconsistent. And AUTO didn't work - I had to manually tune and retune till I got it.
- Frequency seems to be between 1.3 Hz to 7.6 Hz, roughly double what I calculated (between 0.6 Hz to 3.6 Hz).
This is my first PCB, so I'm appreciative of any suggestions and criticism, in any capacity. I've never posted schematics before, so if there's more or better information required, just let me know.
Spehro Pefhany and Justme write to connect RESET and CV, and not leave them floating. I will do that. Does that require a whole new PCB? The parts are SMT and very small, it would be hard to work with manually. Is there away to experiment anyway?
For my knowledge: Why could leaving those pins floating cause the behavior I'm seeing? Particularly the change when I go near it. Would leaving RESET floating cause the IC to spontaneously RESET? And is that what I'm seeing? How does touching the board (or putting my hands near it) affect that?
Did I calculate the frequency properly? I get between 0.6Hz to 3.6Hz.
Justme asked how I'm probing. I've plugged Dupont wires into the Jack (GND, OUT, and TRIG, not Vcc) and a breadboard. The scope has two 10x probes. On each probe, I've attached one Dupont wire to the GND clip, and plugged it into the breadboard's GND row, and a Dupont wire to the probe's hook, and plugged it into the corresponding breadboard row. One probe is compensated well, the other is not (the trimmer screw is broken) - but switching the probes didn't seem to make a difference.