555 timer monostable operation not so stable

Question

I'm experimenting with a bunch of Texas Instruments NE555P IC's that I bought recently from Amazon, I made a basic monostable operation circuit on a breadboard with the following schematics

^{simulate this circuit – Schematic created using CircuitLab}

When I push the button the 555 gets triggered and the LED lights up for few seconds before going off, pushing it again would repeat the process. This is the expected behaviour indeed, however there are two issues I'm experiencing with this circuit.

The first issue is that closing the switch doesnt always trigger the timer, what happens instead is that the LED goes on for only a fracture of a second, I sometimes can overcome this by holding the button down for a second or so and it will then trigger normally after releasing the button.

The second issue is that the trigger duration is larger than \$ 1.1R_{1}C_{1} \$, I tried several different resistance values for \$R_{1}\$ and found out the duration to be about \$1.32R_{1}C_{1}\$ instead (in the circuit above the trigger lasted about 7.40 seconds where it supposted to be 6.16 seconds).

What's causing this erratic behaviour? and why the trigger duration is not consistent with \$1.1R_{1}C_{1}\$?

Heres the breadbord layout:

Last thing I would like to mention is that while testing a 555 IC got zapped (it stopped triggering all together and started heating up), what could have caused that? I used insulated tweezers to avoid ESD damage.

Answer 1

Among experienced EEs it is a well known fact that electrolytic capacitors (like that 100 uF one) can have huge tolerances. These capacitors often have a 20 to 30 % higher value than their nominal value. As there caps are mostly used for supply decoupling that is usually irrelevant.

You are trying to make a (somewhat) precise timer with a timing of a few seconds. You cannot expect much precision from this circuit, the NE555 is not very well suited for longer timings. Most EEs would use a faster running clock and a counter, the CD4060 (14-stage ripple carry binary counter) is a candidate for that. You can make it monostable if you play with the reset.

To solve both circuit problems I would add a small (10 nF) capacitor in parallel with R2, this will force the Trigger to be slightly longer when the button is pushed. You could try a different combination of R1, C1 like 1 Mohm and 5.6 uF. That way the capacitor is smaller making it easier for the discharge transistor in the NE555 to discharge it.