I am designing some digital circuits for a homeschool course in logic. One of these uses 2 74LS173 ICs to implement an 8-bit D flip flop register. The circuit is shown below:

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When I breadboarded the circuit, the second of the two 74LS173 chips was fried. Upon reflection, I hypothesize that hardwiring the two input enable pins (1 and 2) and the two output enable pins (9 and 10) on both chips to ground, thereby enabling both inputs and outputs on both chips and then connecting the two clock pins (7) together with the output of a momentary switch (and a 1K pull down resistor) is the problem.

If so, then one solution would be to use a DPDT momentary switch with separate connections to +5V and separate pull downs on each throw, connecting one throw to the upper chip and the other to the lower chip. The problem with that approach is it doesn't scale well. For example a 16-bit register would require a 4-pole/4-throw switch, something that isn't common. If I have analyzed the problem correctly, are there any other possible solutions that don't require the use of other ICs.

If I haven't analyzed the problem correctly, would someone suggest what the problem might be?

 

I hypothesize that hardwiring the two input enable pins (1 and 2) and the two output enable pins (9 and 10) on both chips to ground, thereby enabling both inputs and outputs on both chips and then connecting the two clock pins (7) together with the output of a momentary switch (and a 1K pull down resistor) is the problem.

Why do you think that's a problem?

 

This may not be the cause of your problem, but 74LSxx devices can only source minute amounts of current, but sink significantly higher.

If you are attempting to drive directly a LED array, it should be common anode and tied to 5v.

 

Why do you think that's a problem?

I am guessing. I don't know what the problem is. schmitt trigger suggests I am trying to source too much current from the 74LS173.

 

I make the following criticism because you intend to use this material for teaching. This is a poorly conceived and poorly drawn design. A more suitable design would use standard schematic symbols, have inputs on the left, outputs on the right and convey intuitively the intended function of the device. Next you have TTL inputs with pull down resistors. The is a major BOZO no-no in using TTL chips. TTL inputs SOURCE current, making pull down resistor inappropriate at all times. This is because the current sources will force a voltage on the resistors that might put the voltage level above the logic low threshold. Then you are trying to use the outputs to source current into the LED bar. In short, a TTL output simply cannot do this. An LS chip is capable of maybe 400 μA which is not enough to light an LED. Lastly you have an un-debounced switch with a pulldown resistor on the CLK line. This is problematical in all but the most trivial designs. It tells me that it reflects a naivete that will mislead the consumers of this material. If you want to make teaching materials for educational purposes, then you should seek the advice of someone who knows something about the material.

 

For LS TTL you should really be tying your clock inputs high, and pulling them low with the switch. If you look at the typical low level current for an LS input it's 0.4mA. With 2 tied together, that's 0.8mA flowing through the 1k resistor, which means the inputs are right on the limits of being a valid LOW, which is 0.8V max.
The switch also needs to be de-bounced, to avoid multiple trigger events. This won't matter much with this circuit connection, but will if you go on to connect one output to the following input stage!

 

This may not be the cause of your problem, but 74LSxx devices can only source minute amounts of current, but sink significantly higher.

If you are attempting to drive directly a LED array, it should be common anode and tied to 5v.

I can drive 4 LEDs in the array with one 74LS123. So, if drive current is the problem, why couldn't I drive 8 LEDs in the array with 2 74LS123s?

 

For LS TTL you should really be tying your clock inputs high, and pulling them low with the switch. If you look at the typical low level current for an LS input it's 0.4mA. With 2 tied together, that's 0.8mA flowing through the 1k resistor, which means the inputs are right on the limits of being a valid LOW, which is 0.8V max.
The switch also needs to be de-bounced, to avoid multiple trigger events. This won't matter much with this circuit connection, but will if you go on to connect one output to the following input stage!

According to the data sheet (https://www.ti.com/lit/ds/symlink/sn54ls173a.pdf?ts=1709352553263, page 2) to set each D Flip Flop in the register requires CLR (pin 15) to be low and then assert a rising edge on CLK.

 

This may not be the cause of your problem, but 74LSxx devices can only source minute amounts of current, but sink significantly higher.

If you are attempting to drive directly a LED array, it should be common anode and tied to 5v.

I just reviewed the data sheet (https://www.ti.com/lit/ds/symlink/sn54ls173a.pdf?ts=1709352553263) and this may be the problem. I misread it the first time, noting that low level output current can source 24 mA. However, I missed the fact that high level output current can only sink -2.4 mA, which would require a common anode LED and a much higher value on the current limiting resistor (i.e, 2.2K). Whether there are common anode LED arrays that will light with such a small amount of current is something I will have to investigate.

 

I can drive 4 LEDs in the array with one 74LS123. So, if drive current is the problem, why couldn't I drive 8 LEDs in the array with 2 74LS123s?

Members of a logic family will often have different drive capabilities. It is not the case that one size fits all members. This is why they make a separate datasheet for each part. It is well known that TTL and LSTTL outputs are very good at pulling low, and very poor at driving high. Among the more insidious effects is the output voltage will drop under heavy loads and depending on the forward voltage of the LED may not illuminate it at all.

 

According to the data sheet (https://www.ti.com/lit/ds/symlink/sn54ls173a.pdf?ts=1709352553263, page 2) to set each D Flip Flop in the register requires CLR (pin 15) to be low and then assert a rising edge on CLK.

The clock input is edge triggered, so only responds when pulsed from low to high. The fact that it starts out being pulled high just means that when you push the switch, it first goes low, then responds when you release it, on the pull-up stroke! That will seem the same to you.

 

I just reviewed the data sheet (https://www.ti.com/lit/ds/symlink/sn54ls173a.pdf?ts=1709352553263) and this may be the problem. I misread it the first time, noting that low level output current can source 24 mA. However, I missed the fact that high level output current can only sink -2.4 mA, which would require a common anode LED and a much higher value on the current limiting resistor (i.e, 2.2K). Whether there are common anode LED arrays that will light with such a small amount of current is something I will have to investigate.

Actually, the LED arrays I am using don't have a common anode or cathode. Each LED has an individual anode and cathode. Nevertheless, it looks like I am trying to drive 22 mA on each output line and this seems to be way too much output current for the 74LS173, as many have pointed out.

 

I can drive 4 LEDs in the array with one 74LS123. So, if drive current is the problem, why couldn't I drive 8 LEDs in the array with 2 74LS123s?

What current do you think you're driving them at? Are the outputs being used for anything that requires proper logic levels? Schematic??

 

I am designing some digital circuits for a homeschool course in logic.

Teaching a child to draw a schematic like that is an extreme disservice. If you don't know the subject material, you should let someone who does do the teaching.

 

Teaching a child to draw a schematic like that is an extreme disservice. If you don't know the subject material, you should let someone who does do the teaching.

I have ignored these insults since I am trying to solve my problem, not yours. The students are not learning how to design logic circuits. They are learning how to apply propositional logic to a real world problem. They will only build these circuits, not design others. The schematic may not satisfy your aesthetic sensibilities, but they serve the purposes of the course.

 

I have ignored these insults since I am trying to solve my problem, not yours. The students are not learning how to design logic circuits. They are learning how to apply propositional logic to a real world problem. They will only build these circuits, not design others. The schematic may not satisfy your aesthetic sensibilities, but they serve the purposes of the course.

Taking offence here is childish. Accept your limitations first of all and, eventually, try to improve your knowledge.

 

They are learning how to apply propositional logic to a real world problem. They will only build these circuits, not design others.

You should, at the very minimum, try draw the circuit so they can see the logic:

 

Sorry, you are receiving expert advice from educators in the field.

I have taught electronics for over 50 years. We don't draw circuit diagrams as what is shown in post #1.
We draw circuits that reveal function, not wiring connections.

Here is a random example of what I mean by function.

 

Thanks to everyone who helped me find the problem.

 

I have ignored these insults since I am trying to solve my problem, not yours. The students are not learning how to design logic circuits. They are learning how to apply propositional logic to a real world problem. They will only build these circuits, not design others. The schematic may not satisfy your aesthetic sensibilities, but they serve the purposes of the course.

That is total BS. There is no propositional logic in your original post. Your own hubris is the enemy of what you are trying to accomplish. Why are you making this all about you? This may be the quintessential argument for why homeschooling is an inferior product that does a huge disservice to the victims.