I am very disappointed at the lack of explanation regarding the role of capacitor C4 on the Make Electronics book exercise 18.
I have tried to find more detailed information about the role that capacitor C4 plays in the circuit but I have been completely unsuccessful. I have even asked about this in the electronic engineer stack exchange forum but no one seems to be able to figure out how all this work.
Is there any way for the author of the book could explain this topic in far more detail than a simple sentence in the book?
Or better yet, can the author of the book answer the question that was posted on the electronic engineer stack exchange forum (see link below)?
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If you are referring to capacitor C4 in Figure 4-41, it is a coupling capacitor. This was explained in the first printing of the book: "The capacitor communicates the sudden change from positive to negative, but the rest of the time it blocks the steady voltage from IC7 so it won't interfere with IC6." I just checked the current electronic edition of the book, and this sentence is missing. I'm sorry, but I don't know how or when the omission of this sentence occurred.
If the coupling capacitor wasn't there, the output from IC7 would constantly affect the reset pin of IC6, and consequently when IC7's output goes negative, IC6 would be put in reset mode indefinitely. The capacitor blocks that steady voltage and transmits only a brief pulse.
Hi Charles - (Thank you Chris)
I really appreciate you taking the time to answer my question, the information you provide here does help but I feel like more detail is needed to better explain what exactly is going on with this C4 capacitor.
If you don't mind, would it be ok for you to please take a look at the post I made on the stack exchange forum (link provided below) and checkout the 4 steps I listed there where I describe in more detail what I think is going on regarding the C4 capacitor? All I would like from you is to validate or invalidate what I say there (the 4 steps) so I know if I am going on the right direction or I am totally lost.
The link is this:
Ah, now, this is a different question! Initially you asked "how it supposed to work?" but now I think you are asking "why isn't my version of the circuit working?"
When the book was first published, I built this circuit, an intern at Make magazine built this circuit, and one other tester built this circuit, and we all had no problems. Years went by. Then suddenly I received a message complaining that it didn't work. After eliminating all my initial guesses, I was able to replicate the problem, which I believe is caused by manufacturing inconsistencies in 555 timers regarding pin 4, the reset pin. This pin must be pulled low for it to reset the timer. The pulse through the capacitor in my circuit does pull it low, but apparently not quite low enough for the version of the 555 that most people seem to be using right now (manufactured by Texas Instruments).
The fix that I have tentatively developed requires you to change one resistor and add another. In the schematic, R10, the pullup resistor for pin 4 of IC6, should be changed from 10K to 47K. Shift the 10K resistor so that you add it between pin 4 and negative ground.
The two resistors form a voltage divider. You can imagine conventional current flowing from positive, through the 47K resistor, and then through the 10K resistor, to ground. Pin 4 of the timer is connected to the point between the resistors. This point has a voltage slightly less than 2V assuming you are using a 9VDC power supply. So, instead of pulling up the pin to supply voltage, we are now applying a lower voltage to it--slightly less than 2V if you are using a 9V power supply.This is still sufficient to avoid resetting the timer, but when the low pulse comes through C4, now it pulls pin 4 low enough to do the reset.
C4 should still be 0.1uF, as shown in the original circuit.
The reason I didn't suggest that you make this modification is that, first, I didn't know you are one of the unfortunate people for whom the original circuit isn't working, and second, we haven't finished testing the modification yet. It works for me--but, the original version worked for me, too. It works for one other tester, but there is still a third tester who hasn't built it yet.
It's important to understand that the reset pin doesn't behave the same way as the trigger pin. The trigger pin requires the voltage to drop below 1/3 of supply (i.e. below 3V with a 9V supply). But the TI datasheet specifies minimum, typical, and maximum necessary voltages of 0.1, 0.7, and 1.0 for a reset. In other words, reset has to go much lower than the trigger pin, for it to work. The datasheet does not say whether these voltages depend on the power supply voltage. So, the manufacturer hasn't made this entirely clear (to me, anyway). Probably the manufacturer assumes that pin 4 will be activated by a direct connection through a pushbutton to ground, and didn't consider that we might try to activate it through a coupling capacitor to the output of another timer.
Please try this modification, and if you still have difficulties, you can try a lower resistance than 10K. But I think you will find that it does work.
When I am more confident of the modification, I'll publicize it more widely. I'm very sorry you had the frustrating experience of building it and finding that it wouldn't work for you.
Sorry for taking so long to reply, something came up and I totally dropped the ball on this one.
I tried the suggested change and things worked nicely. My only concern with the change is that we went from trying to trigger the reset pin using a value close to zero volts to using a value of around -2 volts (negative volts). I am not sure what this even means but it seems wrong to me, it feels like if the capacitor is momentarily being used as a battery connected the opposite way and the reset pin is sourcing current rather than sinking it (or the other way around).... I don’t like it but it is probably because I don’t really understand what is going on.
Also, you never responded to me if the 4 steps I listed on the stack exchange forum describing in more detail what I think is going on regarding the C4 capacitor where correct or not. I assume that the reason you did't responded is because they are completely wrong and trying to explain the concept is somewhat out of scope for beginners. No matter what, I would like to suggest to you to include a much more detailed explanation of what goes on with the C4 capacitor on your next edition / revision of your book. If you decided to do so, please make the information available to people that have already purchase an older version of the book.
Not sure how you measured -2 volts. Relative to ground? That would be interesting!
A capacitor can function in two ways. It can gradually accumulate a charge, while blocking DC current. Or, if there is a sudden fluctuation in voltage, it can pass this fluctuation straight through. This is why a capacitor is often said to block DC while passing AC, as AC is a series of fluctuations. A coupling capacitor (which is how C4 is used) passes the fluctuation from one timer to another.
If I place my multimeter between IC7 pin 3 and IC6 pin 4 (across the C4 capacitor):
Before the change, my readouts during the pin transition would be somewhere around 1.3 volts (which is way pin 4 on IC6 never reseted since 1.3 volts is to high). However, with the new change (multimeter placed in the exact same way) my readouts during the pin transition are somewhere around -2 volts (depending on the voltage divider resistor configuration).
Well, now you got me doubting myself so I will try it again later on to confirm but I am pretty sure that is what is happening.
While I have your attention, if you don't mind, could you please take a moment to answer my other question regarding your other book (link to question posted below):