# 555 timer monostable operation not so stable

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 $1.1R_{1}C_{1}$?

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.

• Now thats a tidy breadboard. You might be interested in simulating that circuit with e.g. ltspice to observe more details and get familiar with it. Commented Nov 29, 2016 at 12:39
• "... is larger than 1.1RC, I tried several different resistance values for R and found out the duration to be about 1.32RC instead ..." - What happens to the values if you look up the tolerances of the involved parts, and include those tolerances in your calculation? Commented Nov 29, 2016 at 12:49
• Hmmm. 1.32 is +20% , which is what I would expect from the tolerance of an electrolytic capacitor. Commented Nov 29, 2016 at 13:06
• @marcelm I'm using metalic resistors with 1% tolerance for R1 so that shouldn't affect anything, but not sure about he electrolytic capacitors but it says that usually the tolerance is about -20%-+20%, the calculations become correct if i add 20% to C1 value Commented Nov 29, 2016 at 13:07