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So today I assembled a little circuit to test my new shift registers (74HC595).

I took three breadboards and connected them as shown on the drawing here.

enter image description here

It is supposed to work as a user controlled shift register with buttons for CLK, DS and LATCH. I was too lazy to write a program for it to work so decided to make a hybrid one.

After going through a TEST #1, I encountered a problem. When I hit the button on clock, it can glitch and actually give me 2-3 clock pulses and read DS pin.

It was the first positive outcome in my experiment. I didn't even consider it in my primitive design.

So I reconsidered the design and thought that this could also happen due to the to many wires and loose connections.

So I rewired the project with less wires. Exactly as it is shown on diagram but with less jumpers.

At TEST #2, I realized that I have super powers. I have no idea what happened, but I didn't even need to press buttons anymore. All I had to do is put my hand above the circuit and it would live its own life. Th shift register would blink like crazy.

So after all my questions is:

Why did it happen ?

Wire electromagnetism caused an induction in a circuit that cause current the bad manufactured buttons ?

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2 Answers 2

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Three things. First, switch bounce, explained here, is a thing. It's a two part article -- read both parts. I wish I'd written it. Second, you appear to be using LEDs as pull-ups, with no current limiting on them when you push a switch -- I'm surprised you're not burning up LEDs and doing bad things to your power supply at the same time. Third, I don't see a ground connection on your chip.

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  • \$\begingroup\$ there is a 470ohm resistor which limits the current on LED which is connected after the led and connects 2 parts of the GND row, and ground is connected just forgot to add this wire on a diagram (blue wire next to GND pin) \$\endgroup\$ Nov 10, 2018 at 5:21
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enter image description here

Figure 1. Problems.

  1. Negative link going nowhere.
  2. Chip GND missing.
  3. The blue line on the breadboard indicates that a continuous connection through that row.
  4. A redundant resistor. Both ends of the resistor are connected to the same track. It is doing nothing.

The random switching is most likely due to capacitive coupling of mains hum through your body.

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  • \$\begingroup\$ oh again you. There is different types of breadboards, and the one i physically have is split in to 2. so resistor is connecting all 4 leds to ground. 1) as pointed in comment above it is CHIP GND which u cxan assume as 2 is connected to 1. but thanks for the answer. \$\endgroup\$ Nov 10, 2018 at 10:29
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    \$\begingroup\$ If you give misleading information then you waste everyone's time. \$\endgroup\$
    – Transistor
    Nov 10, 2018 at 10:45

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