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#10 Design an LED Chaser [Post Homework Here]

Posted: Thu May 23, 2013 7:16 pm
by ThePyroElectro
A new lesson was posted today:
http://www.pyroelectro.com/edu/digital/binary_counter/

Post your homework answers here to compare with everyone else!

Re: #10 Design an LED Chaser [Post Homework Here]

Posted: Thu Dec 11, 2014 6:16 pm
by Raffles
*Not homework, but a question on the lesson. Sorry if it's not the right place!*

Hi there,

I'm having problems with the LED chaser circuit... any thoughts/ideas gratefully received!

In the circuit I put together, all 8 outputs of the 3-8 line decoder are registering a continuous positive output (all LEDs on). I've checked the outputs of the binary counter with LEDs, and they are incrementing as intended, so I assume those inputs to the 3-8 line decoder should be OK... I'm using a 74HC138N, and I see that the one in the schematic is a 74LS138, but the pinouts in the data sheets look almost the same to me, so unless there's something more nuanced that I'm missing here... (different use of enable pins perhaps?)?

Also, I note that the pulldown resisters for the binary counter and 3-8 line decoder in the schematic are different values. Could someone explain why I might use one resistance value over another? Also, should I be closing off any other unused pins on these ICs by connecting to ground? So many questions!! :)

Great course. I'm really enjoying it.

Ralph

Re: #10 Design an LED Chaser [Post Homework Here]

Posted: Sun Dec 14, 2014 8:57 pm
by ThePyroElectro
Raffles wrote:*Not homework, but a question on the lesson. Sorry if it's not the right place!*

Hi there,

I'm having problems with the LED chaser circuit... any thoughts/ideas gratefully received!

In the circuit I put together, all 8 outputs of the 3-8 line decoder are registering a continuous positive output (all LEDs on). I've checked the outputs of the binary counter with LEDs, and they are incrementing as intended, so I assume those inputs to the 3-8 line decoder should be OK... I'm using a 74HC138N, and I see that the one in the schematic is a 74LS138, but the pinouts in the data sheets look almost the same to me, so unless there's something more nuanced that I'm missing here... (different use of enable pins perhaps?)?

Also, I note that the pulldown resisters for the binary counter and 3-8 line decoder in the schematic are different values. Could someone explain why I might use one resistance value over another? Also, should I be closing off any other unused pins on these ICs by connecting to ground? So many questions!! :)

Great course. I'm really enjoying it.

Ralph


It sounds like the 3-8 decoder is never enabled. You might have an issue with the pull-down/pull-up resistors on the 3 enable lines feeding to the 74_138.

The pulldown resistors being different values is probably a type-o. A lower resistance pull down is stronger since more current can flow, 10kΩ > 4.7kΩ.

If you could take a photo of your setup, or give further details I might be able to help you more, debugging is really the tough part.

Re: #10 Design an LED Chaser [Post Homework Here]

Posted: Sun Jan 25, 2015 4:21 pm
by Raffles
Hi there... yep, debugging is apparently quite a challenge! Spent a fair amount of time on this circuit now... still no luck :) but I think have isolated the issue (s).
3-8 decoder works OK. Also, I that there is another typo on the schamatic as the 3 enabling pins should be held high/low in the opposite sense (i.e. high should be low etc.). But that's not the problem:
Bit counter... by the way, in the schematic i think it is a decimal counter, I assume no issue, but I'm using a binary counter. The issue I have is that my IC requires that the counter that is not being used (up/down), should be held high, while the other is counting. When I try this, it works. In the video, apparently the IC being used does not require this. The error I get is that it counts in twos, which is consistent with the information in the data sheet. I tried using LS ICs as well, but it didn't work at all.
The second part of the problem is that the NOR gate latch does not seem to drive enough power, it switches, but the current coming out of the latch gets progressively weaker and fades to low. I see in the video there are some resistors going into the NAND gate, (pull/up down?).

Anyway, any thoughts on the above would be gratefully received. I have to say that through the process of tyring to solve this issue, I've learned a great deal. Also, I think that the circuit *might* be simpliefied :) ... perhaps the use of the NOR gates as inverting input into the latch could be removed, if the inputs from the 3-8 decoder (Y0 and Y7) are simply swapped around...

if you do have any advice on the counter, I will have another bash. Otherwise, I'll give it a week or two and keep going with the other lessons.

All the best!

Ralph

PS: this query is for the Lesson 11, not the LED chaser. Sorry, you might want to move it.

Re: #10 Design an LED Chaser [Post Homework Here]

Posted: Fri Feb 20, 2015 11:44 pm
by ThePyroElectro
Raffles wrote:Hi there... yep, debugging is apparently quite a challenge! Spent a fair amount of time on this circuit now... still no luck :) but I think have isolated the issue (s).
3-8 decoder works OK. Also, I that there is another typo on the schamatic as the 3 enabling pins should be held high/low in the opposite sense (i.e. high should be low etc.). But that's not the problem:
Bit counter... by the way, in the schematic i think it is a decimal counter, I assume no issue, but I'm using a binary counter. The issue I have is that my IC requires that the counter that is not being used (up/down), should be held high, while the other is counting. When I try this, it works. In the video, apparently the IC being used does not require this. The error I get is that it counts in twos, which is consistent with the information in the data sheet. I tried using LS ICs as well, but it didn't work at all.
The second part of the problem is that the NOR gate latch does not seem to drive enough power, it switches, but the current coming out of the latch gets progressively weaker and fades to low. I see in the video there are some resistors going into the NAND gate, (pull/up down?).

Anyway, any thoughts on the above would be gratefully received. I have to say that through the process of tyring to solve this issue, I've learned a great deal. Also, I think that the circuit *might* be simpliefied :) ... perhaps the use of the NOR gates as inverting input into the latch could be removed, if the inputs from the 3-8 decoder (Y0 and Y7) are simply swapped around...

if you do have any advice on the counter, I will have another bash. Otherwise, I'll give it a week or two and keep going with the other lessons.

All the best!

Ralph

PS: this query is for the Lesson 11, not the LED chaser. Sorry, you might want to move it.


Looking at the schematic. I believe I made an error and the two AND gates (IC5A, IC5B) should in fact be NAND gates.