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I'm trying to get this circuit using a SN74HC191 chip to work but it's producing no output to the LEDs.

My current setup so far is:

enter image description here

What makes this more difficult is that the datasheet for the SN74HC191 has no truth/function table so I'm not sure which switches needs to be high/low. Please could anyone provide some help?

Thank you!

Edit: This is the datasheet.

I have now actually found another datasheet with a truth table. However, even with the same H/L settings for "Count up", it still does not produce any LED output.

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    \$\begingroup\$ There are not pull-down resistors at the switches and no series resistors at the LEDs. Can the IC supply enough current for the LEDs? \$\endgroup\$
    – Curd
    Commented Jan 14, 2017 at 15:39
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    \$\begingroup\$ Have you applied +5v to Vcc pin? And connected GND pin to the supply as well? \$\endgroup\$
    – glen_geek
    Commented Jan 14, 2017 at 15:45

2 Answers 2

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Your LEDs are backwards, and require ~330Ohm resistors for protection.

Also, your enable signal (~CTEN) is floating. Tie this to ground to enable the outputs. Tie all other logic inputs (D0-D3, D/~U, ~PL) that you're controlling with the DIP switch to ground with ~4.7k resistors so that they aren't floating, too.

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enter image description here

This shows all the info you need for a truth table.

74HC' Ron driver resistance is approx 50 ohms at 5V to estimate your LED current, add suitable R for current limit. eg if Blue = 3V , then 2Vdrop/10mA=200 Ohm, so use 150 ohms. then verify voltage drop on R=150 = 1.5V. thus Voh =4.5V. (for example).

Include cap across Vss to Vdd for 5V and twisted pair for Clk //Gnd if using long wires.

  • Observe LED cathode towards gnd. (-ve is always the reflector cup side and may have marking (flat edge on body)

enter image description here

  • Never leave CMOS input floating. Pullup 1M to 10k.
  • Beware of ESD damage.
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