555 timer

When you start dealing with time-frames like 20 seconds, you really ought to consider a micro. Something like a PIC10 or PIC12 series is available in the same package as a 555 and can do all sorts of fancy things that a 555 can never do. And the best part, no external components other than a bypass cap!

---55p

 

R2 comes out to be 290 Meg. How exactly do you get that out of a 1Meg pot in series with a 100K pot?

Hint: You can not do 20 seconds of anything from a 555 timer.

At these durations you need to count down from a faster clock or use a micro.

---55p

 

I'm really sorry i took it for an astable one. i havent tried monostable yet. but from what i calculated from 555timer pro it gave me the following
R=2M
C=10uf
pulse time is 21.97sec

http://i820.photobucket.com/albums/zz124/rohitdhamal/monostable.png


so the best option remains is use a 1.5Mresistor with a 500k pot. you will get it. i think i'm not wrong this time.
really sorry for the misguidance in the previous post.

 

As 55p has pointed out, the 20 seconds is just too long for a 555... 2 megs of resistance, charging a 10uF cap? That wouldn't be close to stable (even possible?) unless you got a cap with nigh ZERO leakage. You're talking 6uA of current charging your (laaaarge) cap.
That's not practical at all.

Note the little warning in the lower right part of that wizard about pin 6? Heed it!

 

Actually, it would be under 1 micro-amp, not 6. What you have to worry about is just before the cap reaches the high side comparator threshold of 2/3 of the input voltage. Assuming you are running at 5V, the threshold is at 3.33V, the resistor has about 1.6V across it and is delivering under 1 micro-amp. Not possible with any technology that exists today.

---55p

 

Oh yes you are! At least you are consistent. You are consistently wrong!

---55p

 

there was a warning abt current not met by pin. there r tolerances which account for changed timings. but still it should work like it. and if it is in the off position then reverting the polarity of connection ie positive instead of ground. i just made one timer of 4min 45 sec off and 2 min 10 sec on astable from the 555 timer pro. the 555 timer pro should work fine this time also. well if its simulation data that proves wrong then go for actual practical. i was nt able to do it because i'm going for my PhD.entrance this sunday. so i dont have much time from studies. but i'l definitely look over this. this time i made the timing calculation with 47uf capacitor and 365K resistor. now there is no warning also. please see if it works...

 

The little screen cap he posted said 12V. Either way, we're in agreement... no way will it work.

 

You are right. I missed the 12V part.

But, the point I wanted to make got lost in other things. Your analysis of 6uA is valid when the cap is discharged. But that is not the state we need to worry about. The worst case happens when the cap is charged and we are just below the upper trigger threshold. At that point, the voltage across the resistor is 1/3 of the supply voltage and the voltage across the cap is 2/3 of the supply voltage. It is at that stage that the cap has the highest leakage current and the resistor is providing the least current, about 2uA in this case with 12V supply.

Chances are that the cap will never get this charged. The resistor and the leakage currents will reach equilibrium at some voltage less than the 2/3 point and the system will just sit there forever with the cap charged part way and nothing happening.

---55p

 

personally, I would use a PICAXE -08M microcontroller, switching a relay on and off.