How T=R*C tolerance of capacitor?

Thread Starter

mfr

Joined Apr 24, 2011
19
Hello,

I have a circut that I found on this forum, that I don't want to keep powered up, rather I'd like the power to be switched on via the same switch that triggers the timer delay. I thought that if I introduced a 50ms delay to powering up the 555-2 and that if the Frequency of the 555-1 clock is high enough, then the circuit would still generate a 'random' delay anyays due to the hopefully insconsistent (tolerance) behaviour of a capacitor in serises with a resistor used to delay the power of 555-2 by 50msec or so. I was thinking at 3.9K resistor and a 4.47uF capacitor to ground the LM555-2 for 50-60msec.

Is that a possible route?

The circuit:
https://cid-9c1723531baec47d.skydrive.live.com/redir.aspx?page=play&resid=9C1723531BAEC47D!204&authkey=R1KjyQY!CJs$

Basically I'd like the ON/Off switch to trigger the random delay without a momentary-on switch, hence only one shot at it until powered off manually.

Thanks,

Mihai
 

Thread Starter

mfr

Joined Apr 24, 2011
19
Well if the 4017 counter clock would be at 1MHz, then would you not expect some level of inconsistency on the part of the RC network bringing the second 555 online 50 ms later ? I'd expect the counter to hopefully not be on the same count at that moment...
 

Adjuster

Joined Dec 26, 2010
2,148
As I said, try it and see. Some inconsistency is probable. Whether it would be enough to get a really random result is hard to say. My gut reaction is probably not, but I do not have any maths to support that - do you? Unless you can model or otherwise calculate the statistics for the RC timer, I don't see much alternative to trying it.

That's not the usual sense of "tolerance" as applied to resistors and capacitors, by the way. Different examples of a 10% capacitor may vary by that amount, but an individual capacitor would not be expected to drift by that amount, at least not in the short term. If you want to achieve true randomness (which is actually very difficult) you might try looking at a noise generator.
 
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