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What is the purpose of the capacitor , the resistor , and the variable resistor ?

Do they work like clock pulses for the 555 timer?

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10K+100K is the charge path ,and 100K is the discharge path. – sandun dhammika Apr 18 '12 at 7:47
What's the purpose of the red box around the schematic? – clabacchio♦ Apr 18 '12 at 8:04
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Oh, come on, clabacchio, you know as well as I do that it's to protect the schematic against cosmic radiation. (Remove it if you don't like it, it doesn't bother me at all) – stevenvh Apr 18 '12 at 8:07
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@sandun - what you say is correct, but it doesn't explain the oscillation. – stevenvh Apr 18 '12 at 8:22
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1 Answer

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They let the 555 generate the clock pulses for the Johnson counter (the 4017). It's a typical AMV (Astable MultiVibrator) setup. The capacitor gets charged through both resistors until 2/3 Vcc is reached (internal comparator level). This sets a flip-flop which activates the Discharge output (DIS). Through DIS and the potentiometer, the capacitor is discharged until the voltage on the Trigger input (TRI) is 1/3 Vcc, which resets the flip-flop and switches off DIS, so that a new charging cycle starts. So the capacitor voltage swings between 1/3 Vcc and 2/3 Vcc, and each time one of the limits is reached the output toggles.

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555 AMVs often have Trigger and Threshold connected. By using a potentiometer to set the Trigger level you can vary the frequency.

It's worth it to study the 555 block diagram and its typical applications, like this AMV or the MMV (Monostable MultiVibrator). The 555 is one of the most versatile ICs around.

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so many guys failed to explain this simple thing correctly at the interview. Basics++ – sandun dhammika Apr 18 '12 at 8:16
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I would fail that at the interview. It's possible I used one at school, but I can't remember, and that was many many years ago. – Rocketmagnet Apr 18 '12 at 9:14
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Never having used a 555 is a plus at interview. – Russell McMahon Apr 18 '12 at 14:19
Being able to draw the schematic (transistor level) is a plus at interview. – stevenvh Apr 18 '12 at 14:27
Understanding how to use a 555 from its datasheet despite not having used one since school is a plus at interview. – noah1989 Apr 23 '12 at 8:20

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