How does putting an LED in parallel with an 8-bit computer load affect the voltage/current of a circuit?

Question

I was watching this video by Ben Eater on building an 8-bit computer: Youtube Video time: 1:51.

He has this circuit:

Right before the 74LS245 he has 8 LEDs in parallel with the 74lS245 IC. I am a little confused on what this does to the circuit exactly. How can he just put 8 LEDs in parallel without affecting the 5 V going into the 74LS245?

As far as I am aware, putting an LED in parallel with a load would lower the voltage of both the LED and load to be equivalent to Vf of the LED. How does this not affect the voltage of the 74LS245 IC?

Answer 1

You're right that the LEDs will limit the voltage to about 1.8 to 2.2V and the only current limit is that inherent in the outputs of the chips driving those pins.

But it's all TTL, so anything above 1.2V is considered "high" and TTL high outputs are kind of wimpy, so the LEDs don't reduce the voltage enough to cause a logic malfunction, and because they are TTL chips their outputs aren't strong enough to damage the LEDs, or to damage themselves.

Answer 2

I agree with everything Jasen said, but I would add (stress?) that this circuit demonstrates extremely bad practice. The danger is remembering this circuit technique without remembering its conditions and limitations.

Besides the stress on the output stages, this circuit will not work with many other logic families. For example, the nominal input transition level for standard CMOS is Vdd/2. In a 5 V circuit this is 2.5 V, too high for an input signal that is clamped by a standard LED.

Answer 3

You should maybe ask Ben Eater about his circuits. We have not checked if the circuit matches the schematic or if that is just a block diagram and not the actual schematics. We also don't know the LED color which plays an important part here.

So first thing is to be absolutely sure that there are no resistors in the video you watched. There are no bypass capacitors drawn either, which should generally be a part of any sane logic chip design. So if you are not sure, there might be resistors after all.

Having said that, there are LEDs that have a forward voltage higher that logic level input threshold for a valid logic high level.

And the output high current drive of LS logic is so weak that there is not much current through the LEDs anyway and the LEDs don't burn if there are no resistors.

So maybe a red LED might not work, but a yellow, green, white or blue could indeed work.

The diagram does convey they basics what the circuit is about with block level. It may even work enough with no resistors or capacitors for the purpose of making an educational Youtube video.

Answer 4

The highest voted comment, from Jason Smith, under the video points out this issue:

NOTE! Watch your forward voltage drop on your LEDS(they act like a voltage divider*). I needed to add resistors to keep the logic voltage high enough for the ls245 to recognize. A small bald spot ensued trying to debug :)

along with the follow up comment:

Your LED should drop approx 1.7 volts from your logic level, putting it below 2.0(logic high,ttl). Put a 1k resistor in series to limit current and keep your voltage up. ttl output - resistor - led. Test all 3 points with a voltmeter and you will see better.

Answer 5

Do not do that!

Always use a resistor in series with an LED to limit the current.

(Anything between 330 Ohm to 2.2 kOhm will work there.)

Once you do, your question becomes moot.

Answer 6

tl, dr: the LEDs, if a type with Vf around 2V (red, yellow), will drag the Voh below an acceptable level, causing the circuit to malfunction (or at least be out of LSTTL spec.) The LEDs should be buffered.

The more Ben Eater stuff I see, the less I like. This is no exception.

Let's dig in a bit. First, what does an LSTTL output look like? This:

From here: https://www.ti.com/lit/ds/symlink/sn54ls08.pdf

See those two transistors at the right? They are a totem pole NPN pair, with a 120 ohm resistor in series with Vcc.

So what will happen to that 'LS173 output type with Ben's LED glommed onto it, so rudely without even a courtesy of a current limiting resistor? When the output is high, the LED will clamp to its forward drop voltage (Vf), with the current limited by the 120 ohm series resistor.

This might work if Ben choose a green, blue or white LED with Vf > 3V. Red or yellow? Nope, those usually have Vf around 2V or less, so Voh won't be high enough (needs to be at least 2.4V to meet TTL Voh spec.)

Example LED Vf vs. current (from here):

With this LED, you can see that even at only 3mA, the output isn't making it to 2V, the minimum Vih spec, let alone the 2.4V Voh required for margin.

The fun part is, this V-I curve varies not only by LED color, but even within the same color from the same vendor, by the specific part. And, this can vary from LED to LED even within the same type.

It's literally luck o' the draw whether that red LED will let the '245 see an adequate Vih to work.

Flaky at best, non-functional at worst.