Can someone tell me what the point of C1 is in this circuit....

mik3

Joined Feb 4, 2008
4,843
Its 10nF not 100nF. C1 does not get involved in the determination of the output frequency value, thus why if you remove it you see no change in the operation of the circuit. Its purpose is to bypass the 2/3 of Vcc point (internal) to ground as to minimize noise variations on it and keep the frequency more stable.
 

Wendy

Joined Mar 24, 2008
23,408
Check this project out, it has the same capacitor.

http://forum.allaboutcircuits.com/showthread.php?t=14018

Basically it is something to help with noise, especially on the power supply. You're right, you probably don't need it unless you are using the battery for other devices. Techs and engineer throw it in almost as a habit. Occasionally it actually does something, but it is better not to need it and have it than visa versa.
 

KL7AJ

Joined Nov 4, 2008
2,229
Check this project out, it has the same capacitor.

http://forum.allaboutcircuits.com/showthread.php?t=14018

Basically it is something to help with noise, especially on the power supply. You're right, you probably don't need it unless you are using the battery for other devices. Techs and engineer throw it in almost as a habit. Occasionally it actually does something, but it is better not to need it and have it than visa versa.

They are commonly called decoupling capacitors. They are extremely crucial in R.F. circuitry, but might not be notices in most digital circuits.
 

davebee

Joined Oct 22, 2008
540
A National Semiconductor LM555 datasheet says that pin 5 can be used to modulate the position of the output pulse.

So visual displays will probably look and work fine without the cap, but if you needed precision pulse generation for servo control or something you'd probably want it.
 

scubasteve_911

Joined Dec 27, 2007
1,203
They are commonly called decoupling capacitors. They are extremely crucial in R.F. circuitry, but might not be notices in most digital circuits.
Just because logic is usually in the sub-gigahertz frequency range, doesn't mean that decoupling isn't crucial. It is the edge-rate of the signal that matters. In the past, I had some digital circuitry that ran at 48MHz and decoupling capacitors were crucial because I was using a fast-edge rate FPGA.

Steve
 

KL7AJ

Joined Nov 4, 2008
2,229
Just because logic is usually in the sub-gigahertz frequency range, doesn't mean that decoupling isn't crucial. It is the edge-rate of the signal that matters. In the past, I had some digital circuitry that ran at 48MHz and decoupling capacitors were crucial because I was using a fast-edge rate FPGA.

Steve
With such sharp edges, you ARE working with R.F. circuitry! (Fourier is your friend!)

eric
 

hgmjr

Joined Jan 28, 2005
9,027
To add another voice to previous replies and reiterate many of the comments already provided, C1 is often recommended for use in filtering the internal DC voltage reference to which the pin is connected. The purpose of the pin is to permit a degree of control over the frequency by the application of a voltage.

hgmjr
 

Bernard

Joined Aug 7, 2008
5,784
Looking at pin 5 with a scope was verry revealing; showed a strong damped oscillation on ever output transition. The cap went back in.
 

Audioguru

Joined Dec 20, 2007
11,248
The circuit is missing an important supply bypass capacitor (try 0.1uF in parallel with 100uF). The 555 draws up to 400mA on each edge of its output waveform that a supply bypass capacitor will supply if the power supply or battery cannot supply. Maybe that is why pin 5 is bouncing up and down.
 

Thread Starter

critiera119

Joined Nov 21, 2008
66
Thank you all for commenting. Very interesting responses. As you can see, the circuit works beautifully without the aforementioned C1. Voltage to the ouputs for the flashing showed no change with or without it. I would agree with the response that it was probably included in the pdf for this specific circuit by way of habit.

http://www.youtube.com/watch?v=xTtj1xR80Zk My vid.
 
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