Dick
The TS is your stated exception, a one off. Unless you are using military or medical grade components the spec sheet is to be taken with a grain of salt and does not guarantee 100% compliance (check the bottom of the sheet for how to order guaranteed compliance). If this "unnecessary component" fails, you are just back to what you are proposing and wondering what the hell went wrong "because every component is totally up to snuff". Like the name says I'm just a duffer but in this case I wasted a lot of time trying to make a similar setup work before someone suggested that pull down resistor and EUREKA it worked. Maybe it was a cheap Chinese part )

 

I'm still trying to figure out what is wrong with the circuit, but I have to agree that, at least in my case, in the end crutschow was right. I have been reading about decoupling capacitors and RC filters thanks to the advice I got here and my circuit isn't up to 24 / 4 seconds yet, but the time on/off is always the same, the led is no longer flickering and the pull down resistor doesn't help at all anymore. Everything is still on a breadboard. I put some 0.1 decoupling capacitors between ground and Vcc and the improvement is huge.

 

What is a TS anyway? My language is not English.

 

Hello,

TS stands for "Thread Starter" , The person who started the thread/topic.

Bertus

 

Thanks

 

Roxi
Glad you have got it working. If the 24 / 4 timing is critical, go through your stock of components and pick the R C parts that have a measured value closest to what you need and use them to get the timing you require. Push comes to shove, you can always use multiturn cermet pots. Don't be too surprised if the frequency changes when you move from breadboard to protoboard or pcb. Good luck and have fun!

 

Roxi
Glad you have got it working. If the 24 / 4 timing is critical, go through your stock of components and pick the R C parts that have a measured value closest to what you need and use them to get the timing you require. Push comes to shove, you can always use multiturn cermet pots. Don't be too surprised if the frequency changes when you move from breadboard to protoboard or pcb. Good luck and have fun!

And hence the importance of using accurate multi-turn pots. With them, you can compensate for small differences and don’t have to start over.

 

22.3, 22.8, 22.4 were three on times I got on an online stopwatch. 3.8 was the off time I got. The times vary a little bit because I am starting/stoppIng the watch manually. What's the advisable thing to do? Make the project work perfectly on the breadboard before transfering it onto the PCB (yes Duffer I know there are changes, I've done this before a few times) or transfer it now and fine tune it after it is done? Would a thorough engineer transfer it before it works perfectly? I'd like to hear what people here have to say. Thank you.

 

Hi,

LMC555 datasheet states 1% timing accuracy, SE/NE/LM/etc. are less accurate, check the datasheet of what you are using for the timing accuracy specs.

Hint: Use as small a capacitor value as you can as most of the timing error (ignoring "extreme" temperature fluctuations or supply voltage dips or ground level bouncing around and stuff) comes from the imprecise capacitor discharging and charging to 1/3 and 2/3rds Vcc never quite exactly the same each time. Tiny C, large R.

I've done that stopwatch + LED thing before but it's a stupid method - by the time your eye responds to e.g. the LED turning on/off (and let's acknowledge that the LED turned on before your eye even registered the event) and your finger/thumb responds to your brain's signal to press the stopwatch button, (hundreds of) milliseconds have passed, and you and I are about as repeatably accurate as capacitors, I'm afraid .

Another way is to set up a homebrew frequency counter/pulse counter with a precision clock that counts e.g. milliseconds and is triggered at on time to start counting pulses and at off time to latch the total counted - sensible people do this with an MCU. What I described is a large circuit that needs to be made with care and takes longer than the actual circuit being measured!

Trust me, I've been down this road many times and it just leads to frustration and well-deserved doubts as an imprecise measurement tool will always give imprecise results, unless loose timing is acceptable.

Probably not a workable idea, but still: Got a function generator and an oscilloscope that are reasonably "good"? Set the function generator to the same timing of your circuit and put the probe on the timer output and compare the data obtained. Could work with a little patience, and a calculator if need be.