Oscilloscope Effect on Timer 556 performance

[ElectronicsRookie]

Greetings Everyone,
I build a mono stable pulse generator in which I can adjust output pulse width and delay. It works in 100 to 300 ms range. I am using just one timer on 556 chip and output activate optoisolator and LED. I have a puzzling problem with it. The circuit alone does not work to my specs however when I connect two probes of oscilloscope to timer, one at input side of timer the other at output side of timer, the circuit works to my specs perfectly. My reaction was that probe loading somehow affected the set up of the circuit. I reasoned that if I hook up circuit that are equivalent of the oscilloscope probes then that will solve my problem ( x1 - 125pF and 1Mohm, x10 - 12pF and 10Mohm both in parallel). Unfortunately, it did not solved my problem. Did I miss anything? Insights appreciated.

 

[betwixt]

The loading effect of the scope probes and inputs shouldn't have much effect, even at x1 when the frequency is a low as that, assuming you have suitably proportioned timing components. It's far more likely that connecting the ground of the scope probes is completing a path, particularly as you are connecting to both sides of the optoisolator. Can you post a schematic and also confirm you have suitable decoupling capacitors across the supply line of the 556.

Brian.

 

[ElectronicsRookie]

Thanks for reply,
Yes, that is correct. I noticed that when I disconnect the probes but leave ground clips connected, the circuit is still working. Furthermore, I do not have coupling capacitors in place. As soon as I get software tool and drawing, I will post the circuit schematic to look at. Thank you for your time again.

 

[ElectronicsRookie]

Hi,
Here is the NE556 circuit.


---------- Post added at 15:32 ---------- Previous post was at 15:30 ----------

Yes, I posted the circuit schematic

 

[betwixt]

My first guess is the decoupling capacitors, you really should add one of at least 100nF directly across VCC and ground on the 556 (555 in the schematic).

The circuitry is a little unconventional but not incorrect, be careful how you drive the opto LED as the voltage across it will reach around 15V as C1 charges and discharges.

Brian.

 

[ElectronicsRookie]

My first guess is the decoupling capacitors, you really should add one of at least 100nF directly across VCC and ground on the 556 (555 in the schematic).

The circuitry is a little unconventional but not incorrect, be careful how you drive the opto LED as the voltage across it will reach around 15V as C1 charges and discharges.

Brian.

Hello Brian,
I tried the capacitors and it did not work however I read your preceding messages and I followed your hunch that the probes completed continuity of the path. Therefore I did something that did not follow my logic. The input circuit (see drawing before) has its own power source and the timer has its own. Hence, I connected two different grounds together and turned on the radio for soft rock music. It worked. One wire solved my circuit problem, and nice music relaxed my brain and brought me some optimism. The combination of two gave me a nice hurrah. Lesson I learned from it is that a little music is a good food for solving some problems.
Thanks Brian.
Adam