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I need help or maybe an explanation on a particular issue I am experiencing when porting a small and relatively simple circuit from a breadboard to a PCB. The circuit is for a school project used to receive infrared pulses coming in at a frequency of 4 Hz. The circuit is composed of about 5 or 6 logic ICs, it works flawlessly on a breadboard. However, as soon as I transpose this circuit to a PCB a problem occurs. I have isolated my problem to a single IC which is a 74LS93 4-bit binary counter.

I decided to try and reproduce the problem with as little components as I could and it turned out to be fairly easy. I simply set up a 555 timer IC in an astable configuration to generate a square wave with a frequency of 4 Hz (to simulate the signal input) and fed the output directly into the counter's input:

schematic

simulate this circuit – Schematic created using CircuitLab

Again, this circuit behaves as it should on a breadboard; the counter counts on each falling edge of each pulse it receives from 0000 through 1111 and then resets back to 0000. Now, the problem when I transpose this tiny bit of circuitry to a PCB appears on QA, the least significant bit, or rather, the counter's first Flip-Flop. It seems as if the counter counts both rising and falling edges.

The good news is I can fix this problem by adding a 100nF cap between VCC of my counter and GND. I examined it with an oscilloscope on the picture below. To the left of the middle line, you can see the behavior of the counter without the cap and, to to the right, the 'fixed' counter with a cap between VCC and GND. (Yellow is the input signal, Blue is QA. Please don't mind the input fluctuations as I was powering using my Arduino, the same occurs when I use a steady voltage source.)

enter image description here

The bad news is, when I reassemble the circuit as a whole on a PCB, the problem reappears. I have tried putting decoupling caps on every IC, it seems to help but the output is consistently... well, unpredictable. Sometimes it counts as it should but most of the time it is random. I know breadboards might act like very small capacitors at some points, but I have a hard time understanding and figuring out what is the source of my problem here.

I know it may seem like basics, but please excuse me as I have looked around and am unsure as to what to search for, I am also a rookie about electronics as a whole. Anything would help!

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  • \$\begingroup\$ I don't think it's the decoupling. Can you verify that you have no shorts on your PCB? \$\endgroup\$ – Tom L. Nov 27 '13 at 8:30
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    \$\begingroup\$ Is that "4.12V" shown on the scope supposed to be a 5V logic signal? Are your supply rails sagging? Can you post a full schematic or at least a couple of clear photos of each side of the PCB? \$\endgroup\$ – pjc50 Nov 27 '13 at 9:55
  • \$\begingroup\$ So, what do you think is causing that little voltage step-down on QA at the clock rising edge? That doesn't look right. \$\endgroup\$ – user28910 Nov 27 '13 at 12:53
  • \$\begingroup\$ Showing us a scope photo and then saying that we should disregard the sagging power is confusing and not very helpful. Give us a decent scope photo so we can judge what can be disregarded and what can't. I have a feeling that your 555 is disturbing the power supply. \$\endgroup\$ – Joe Hass Nov 27 '13 at 13:17
  • \$\begingroup\$ @TomL. I will double check the soldering on the PCB and post a picture if possible. Since it's the only thing I could check for myself, I had done it thoroughly without finding flaws and still had the issue so I kind of discarded the possibility of being that. Sorry I didn't mention that. Thank you for your input. \$\endgroup\$ – user33355 Nov 27 '13 at 16:26
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At first glance, this appears to be a power problem. It appears that your "5 V" power supply droops to less than 4 V in operation.

Unfortunately, these 3 problems bite extremely often, and cause all kinds of strange and not-quite-working-right behaviors.

(How do we add "decoupling and bulk capacitance" to the Electrical Engineering site FAQ ?)

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I taught a mechatronics course for a few years and several students saw problems migrating from breadboard to PCB. Most problems were fixed by adding capacitance.

What is likely happening is that your breadboard has a fair bit of parasitic capacitance from how the traces in the board are constructed. As a result, the breadboard can be more forgiving for beginner circuits. The reverse is also true, breadboard will kill advanced high-speed circuits.

It is always good practice to place a 100 nF cap across VCC and GND as close as possible to the IC. You want as small a loop as possible since a larger loop creates inductance. Try to keep the cap on the same side as the IC, vias will introduce inductance. Finally, package size also counts, try to use a physically small cap. You notice some caps are labelled with low ESL and might have their traces on the long sides of the chip all in the name of providing a clean voltage source to your IC.

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