# Monostable 555

Discussion in 'The Projects Forum' started by ertiexd125, Mar 9, 2011.

1. ### ertiexd125 Thread Starter New Member

Jul 4, 2010
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We all know that a monostable 555 works as a one shot timing circuit. I have read an article that says when i use a low value capacitor on its RC connection on pin 6 and pin 7 the timing capability will be gone. Is this true? If yes, what is the electronic explanation of these?

help me pls...

2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
I think you've heard wrong. You don't want the resistor to be too low, since the current can blow the innards of the 555 if it is too high, but the concept of RC timing is pretty dependable, and extremely predictable.

555 Monostable

I've been writing a set of tutorials for the 555 over time, and am quite familiar with them.

Bill's Index

The 555 Projects

My Cookbook

Aug 12, 2009
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4. ### ertiexd125 Thread Starter New Member

Jul 4, 2010
16
1
yeah i'm aware of that but its not the resistor that will have a low value its the capacitor.

5. ### ertiexd125 Thread Starter New Member

Jul 4, 2010
16
1
i have attached a circuit diagram here can you explain how the timing capability of the monostable 555 is used.

The author of the circuit only says,

Here, Timing(T)=1.1*68K*0.01uF = 0.000748 seconds. So, as soon as you let light fall on LDR, the LED- D1 switches off.

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6. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Timing can be for extremely short durations, faster than the eye can see or the mind perceive. It still exists however. In the monostable link I provided the RC duration is set long enough that a human can see it.

This design has some flaws however. A 555 timer starts when pin 2 goes under 1/3 Vcc. If pin 2 stays there the timer will time out, and the 555 will stay forced on. At this point it is no longer timing, that aspect has finished. As soon as pin 2 goes above its setpoint of 1/3 Vcc the 555 will turn off, apparently without timing. The timing did occur, but it was invisible.

The theory of operation and instructions in the link I provided explain this. It is why this is generally considered an illegal condition, but like everything in electronics it can have its uses.

Speaking for myself, I prefer to calculate the values for a 555 with my calculator on my desk. I don't use or need tables.

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