So copying a little bit of what this guy showed on youtube, I tried to get my HD74LS86 ic. Here's what I tried to do:

enter image description here

Edit: I forgot to draw a resistor between the led and ground.

I put switches in-line for the B3 and A3 ports. And I used an Arduino to power it with 5V.

I hardly know anything about integrated circuits (but I've read on transistor and diode logic gates), so I'm not surprised it didn't work.

Here's the datasheet if it helps

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    \$\begingroup\$ Other things you need to read up about: 1: Decoupling Capacitors, 2: Pull down resistors, 3: Current limiting resistors (for LED). \$\endgroup\$
    – Majenko
    Mar 22, 2015 at 16:54
  • \$\begingroup\$ Lack of pull down resistors are the main problem as @Majenko says. When the switch goes open circuit, the voltage previously connected to that input via the switch remains there as charge so, adding a 10kohm pull down to ground discharges that voltage and will make your circuit work (hopefully). \$\endgroup\$
    – Andy aka
    Mar 22, 2015 at 17:04
  • \$\begingroup\$ @Andyaka: he's using a 74LS86 (bipolar TTL) - he'd need a 500 ohm pull-down to produce a Low. \$\endgroup\$ Mar 22, 2015 at 17:11

1 Answer 1


One very important thing to know about bipolar TTL logic (74xx, 74LSxx) is that the inputs source current, and if left unconnected will normally appear as a logic High. For the 74LS family, you must draw about 0.8 mA (or more) from an input to make it appear as a logic Low.

Because of this, it is common practice to put switches between an input pin and ground to ensure that the input will be seen as Low. It is also good practice to put a 5K or so pull-up resistor between the input pin and Vcc to ensure that an open switch will be seen as a High.

Also, because the inputs source current, outputs are designed to be much better at sinking current (pulling low) that sourcing currrent. Therefore, it is best to put LEDs between an output pin and Vcc, rather than between an output and ground.

Bipolar TTL is designed to operate from 5 volts (recommended Vcc range is 4.75 to 5.25 volts). The Absolute Maximim Vcc is 7 volts, if I recall correctly. At some point below 4.75 volts, it will not operated correctly, but the parts will not be damaged.


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