Nobody teaches this clearly enough early on. Decoupling caps (usually a 100nF ceramic close to each IC's power pin, sometimes with a larger 10uF in parallel) exist to absorb the tiny voltage fluctuations that happen when an IC switches.
Digital ICs switch fast and draw sudden current spikes. Without decoupling, those spikes travel through the power rail and can cause other parts of the circuit to misbehave like glitching, false triggering, noise in analog sections. It's one of those things that works on the bench and then gets mysterious when you put it in an enclosure or add more components. Add the 100nF near every IC power pin. It's cheap and it almost always helps.
Digital ICs switch fast and draw sudden current spikes. Without decoupling, those spikes travel through the power rail and can cause other parts of the circuit to misbehave like glitching, false triggering, noise in analog sections. It's one of those things that works on the bench and then gets mysterious when you put it in an enclosure or add more components. Add the 100nF near every IC power pin. It's cheap and it almost always helps.
