Decoupling Capacitors - when to use them?

[higginsdj]

So I was given this link to find out more about building and using transistor and diode logic circuits.  My question is regarding the use of de coupling capacitors.
https://www.nutsvolts.com/magazine/article/transistor_clock

A resistor and capacitor have been used between all the different logic units in this circuit but I don't know why.  In all my experimentation, doing this coverts the outgoing DC signal into an AC signal and so inserts an AC signal into the next logic unit.  Why not simply connect each logic unit together without the capacitor so you get DC signal in and DC signal out?  In my experiments the 2 diode AND gate with following resistor and capacitor turned a 0-5v Square wave output signal (I used a 5v VCC) into a +/-4.1v Cap charge, discharge signal.

Now I do understand why they are used in audio and amplifier circuits - just not in this case.

What am I missing?

[BillyO]

As it stands your question is rendered meaningless by fortune of the picture you post.

Can you supply a schematic, or better still, a DETAILED (as in in-focus and appropriately scaled) picture of your test circuit in which we can see every detail?

It really looks like you put the capacitor in series with the device rather than in parallel. 

[ataradov]

Those are not decoupling capacitors. They are there precisely to do what you see on the scope - convert square wave into short pulses. This circuit specifically toggles on the negative edge of the square wave.

Normally the toggle voltage is kept at 1/2 VCC by the resistive divider. When the negative edge happens, it shortly pulls the cathodes of the diodes low, those toggling the flip flop.

But yes, it is better to specify what capacitors you are talking about. Because the ones shown at that link are only necessary before the flip flops. Logic gates should have stable levels, which is shown on the FIGURE 6.

[higginsdj]

As it stands your question is rendered meaningless by fortune of the picture you post.

Can you supply a schematic, or better still, a DETAILED (as in in-focus and appropriately scaled) picture of your test circuit in which we can see every detail?

It really looks like you put the capacitor in series with the device rather than in parallel. 

The schematic is in the link.  I looked them up again in my textbook and have mistaken AC Coupling with Decoupling - so much to learn. 

Cheers

[higginsdj]

Those are not decoupling capacitors. They are thee precisely to do what you see on the scope - convert square wave into short pulses. This circuit specifically toggles on the negative edge of the square wave.

Normally the toggle voltage is kept at 1/2 VCC by the resistive divider. When the negative edge happens, it shortly pulls the cathodes of the diodes low, those toggling the flip flop.

But yes, it is better to specify what capacitors you are talking about. Because the ones shown at that link are only necessary before the flip flops. Logic gates should have stable levels, which is shown on the FIGURE 6.

OK, thanks.  More to learn and experiment with.