Sometimes the circuit works all the time without a bypass capacitor. Sometimes it works most of the time without a bypass capacitor. Sometimes it doesn’t work at all without a bypass capacitor.
As you can see, this chip didn’t have a bypass capacitor when the PC board was originally designed. Apparently the chip worked but had some problem which was cured by putting a bypass capacitor across the power leads later when the board was reworked. Bypass capacitors can make all the difference between a circuit that works and one that has a mysterious and elusive problem every so often.
I bought a kit of a “Dice Roll” circuit. It used a 555 timer chip and ran off a 9V battery. I noticed when I assembled it that there was no bypass capacitor across the power leads. I decided to try rolling the dice to see if the circuit was giving the same odds that a normal single die would give. It was off considerably.
I put a bypass capacitor across the power lines and tried it again, and the problem went away, the circuit behaved as it should have. Lesson learned.
Conclusion – The hardly ever noticed bypass capacitor is a very important part of the circuit; without it, the circuit may not function or function erratically.
