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I want to adapt the output of a board to drive additional logic instead of an LED. The driver board has a current limiting resistor before the signal is routed to the connector. Is there any problem leaving the resistor in the circuit? I can modify the driver board, but would prefer not to.

The logic gates are TTL 74xx in the Low power Schottky family.

(A simplified schematic of what I have in mind).

schematic

simulate this circuit – Schematic created using CircuitLab

The full schematic is here

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Datasheet says: VIL max = 0.8V, IIL max = -0.4mA, VOL max = 0.4V
VOL + (-IIL) * 470ohm = 0.4V + 0.4mA + 470ohm = 0.588V < VIL (= 0.8V)
So, when the output is low, the voltage on the input side of 470ohm is 0.6V worst case, below 0.8V, thus it registers '0', good!

In the datasheet: VIH min = 2V, IIH max = 20uA, VOH min = 2.5V VOH - IIH * 470ohm = 2.5V - 20uA * 470ohm => 2.49V > VIH (= 2V)
When the output is high, the voltage on the input side appears as 2.49V (and higher) worst case, good!

You will be fine with 470ohm there, as long as it was only for "blink blink" or like (slow).

In case, if you want more operating margin, you may use "74HC" series.

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  • \$\begingroup\$ Thanks! I was hoping it was that simple. \$\endgroup\$
    – Lee Morgan
    Aug 18, 2021 at 1:53
  • \$\begingroup\$ @LeeMorgan Thanks for the double tick. 470 ohm is indeed large enough to worry, especially for the '0'. But, the datasheet (LS type) says we should be okay. It will have less "noise margin", not suitable for a noise environment. If noise becomes problem, use 74HC14 at the receiving end. \$\endgroup\$
    – jay
    Aug 18, 2021 at 1:59
  • \$\begingroup\$ That makes sense. I am worried about noise, I may be on the other end of a non-trivial length ribbon cable. The good news is I'm only using this to read and display status via an AVR; if it's flaky, it won't effect the actual operation of the display. I'll also use the HC family on the other end for added insurance. \$\endgroup\$
    – Lee Morgan
    Aug 18, 2021 at 2:06
  • \$\begingroup\$ @LeeMorgan That sounds like a right plan. For a severe noise case, since you already have 470ohm, you can add a small cap (470pF or 0.01uF) at receive side (between input & ground), along with 74HC14 (a schmitt trigger). That 470ohm and the cap become a noise filter, while the schmitt trigger re-shapes the signal edge back. \$\endgroup\$
    – jay
    Aug 18, 2021 at 2:12

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