## Choosing pulldown resistor for 74LS86

 I am trying to build a half adder using a 74LS86 XOR gate. Initially I connected both input pins that I am using (9 & 10) on the 74LS86 to the ground rail using 10K ohm resistors. I then connected 5V to pin 9 and expected the LED fed from pin 8 to light. No luck. I then pulled pins 9 and 10 high by connecting the 10K ohm resistors to 5V. When I pulled pin 10 low by connecting it to ground, the LED lit. I substituted 330 ohm resistors for the 10K resistors and reconnected them to ground. Now the circuit works properly when I pull either pin 9 or 10 high. Is 330 ohms an appropriate choice? Are pins 9 and 10 sourcing current when they are not active (pulled high)? I read that low power schottky ICs can only source 8mA. I am calculating that with 5V and 330 ohms that the current draw could be 15mA. I am wondering where I should look on the datasheet to properly size pulldown resistor for TTL ICs. 74LS86 Datasheet:https://www.apogeeweb.net/circuitry/74ls86-xor-gate-digital-ic-2--simple-using-and-other-information.html by Deion_Adel September 23, 2021 You should not need a pullup or pulldown resistor with a logic gate, when the output goes from a logical 1 to a logical 0: the gate is able to sink the current. You must use a resistor to limit the current through a LED (or use a constant current sub-circuit). Some LED becomes visibly turn on with 20 mA, in sun light, others with just 5 mA, in a poorly lighted room. With a 10k resistor, assuming you are using 5V, that leaves a current way too small to turn the LED on (to the naked eye). Have you look at the many videos by Ben Eater on Youtube? Four bits adder, Ben Eater by vanderghast September 24, 2021

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