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

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.

• 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. – PlasmaHH Nov 29 '16 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? – marcelm Nov 29 '16 at 12:49
• Hmmm. 1.32 is +20% , which is what I would expect from the tolerance of an electrolytic capacitor. – JIm Dearden Nov 29 '16 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 – razzak Nov 29 '16 at 13:07

## 1 Answer

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.

• Thanks for your answerer, I'm only a hobbiest and wasn't aware of the huge tolerance of the electrolytic capacitors. I'm trying to use a parallel cap with R2 as you suggested but just want to clarify, should it sit between Vcc and Pin2? – razzak Nov 29 '16 at 13:46
• Yes, parallel to R2 so that is just like R2: between Vcc and pin2. – Bimpelrekkie Nov 29 '16 at 14:12
• I lost two IC's so far trying to do that for some reason, I used 10nf ceramic capacitor! – razzak Nov 29 '16 at 14:21
• If you break the ICs then you're doing something wrong. Since you use a rechargeable battery, a large current can flow in a fault situation. I'd use a cheap non-rechargeable battery if I were you because their maximum current is much lower. Even better would be a lab supply with current limiting but those are not so cheap. If treated properly the 555 is very robust even against ESD etc. But for example exchanging + and - on the battery will destroy almost any IC. – Bimpelrekkie Nov 29 '16 at 14:40
• Turned out the charging resistor R1 had a loose connection on the breadboard and by moving it to another location the triggering problem was fixed. I took out the resistor to see if it cause the same exact problem and it does. Now I'm puzzled how does the timer reach threshold without the capacitor being charged? Shudnt the output stay always high when there's no R1? – razzak Dec 1 '16 at 0:21