USB Triggered Power Latch

I am making a digital clock. You can see the progress on youtube:
DIY Clock powered by Arduino - YouTube

But I want it to only be on when the TV is on. The TV has USB ports for diagnostics and F/W upgrades.
These USB ports only output power when the TV is on. So, I'd like to use a few mA to trigger the clock's power latch.

This is what I've got so far. It uses just about the same latch as my power supply does but I moved some resistors to eliminate the voltage divider and still maintain t=0 short protection.

Please let me know what you think.
*** Edit: Updated schematic *** in comment 4

I haven't specified Q1 yet because I'm still shopping. It will be capable of passing at least 8A (exceeds clock's max draw) with a -4.5V trigger.

I also have a few questions:

  1. What should I do with pin 6 on U1?

  2. What should I do with the shield and data pins on J1?

  3. I didn't know what symbol to use for the 5VDC power supply input. Is that symbol clear or should I switch it to a 5V arrow like the other side and add an annotation?

I am not into videos, however maybe I am missing something, why not just power it from the USB. Telling what you are powering might help, from what I could see you are not using a lot of current. From your schematic do not be surprised if you have problems with Q1 and reset problems with the arduino. Your output circuit will turn off very slowly, I would suggest R5 be in the 4K range at most. R3 serves no purpose other than to slow down Q1. you state "I moved some resistors to eliminate the voltage divider and still maintain t=0 short protection." What is t=o short protection?

It has 128 WS2812B LEDs and I want to have the option of using all of them. It's way too hot for the TV's USB port.

I know you're not into videos but you can see the LED matrix in them.

Good note, that allows it to pull the gate up faster?

When this type of latch is first connected and the FETs have not been connected in a while there is a momentary spike at the output, at time = 0, and before C1 fills there is a short circuit to ground. C1 is necessary though as it narrows the spike to darn near nothing.

I could change R5 to something in the 4K range and R3 to 1K. That will keep the momentary short to 5mA.

I saw a few LEDs, nothing telling me what they were nor how much power they needed, now I know. What type of latch are we talking about. The Cap would eliminate your voltage spike and dissipate the heat in the MOSFET keeping it in the linear range, OK within reason, depending on the MOSFET. What is the value of the current spike the MOSFET sees, I would be more concerned about that. Checking the data sheet for the MOSFET you can determine what type of spike the MOSFET can safely support. From what you are describing you have a highly capacitive load, not a short circuit.

I went back and forth with several very helpful users in this exceptionally protracted and yet unfinished thread:
Arduino Controlled 7.4V Li-Po Power Supply Design - Using Arduino / General Electronics - Arduino Forum

I will dig through it for the important parts to answer your questions, they are close to the end, but you are welcome to look as well if you like. There are almost no videos.

The spike is from 0V to the input voltage, I'm pretty sure.
There are simulations of it that I will find for you.

I would much prefer to see a description of the project here with circuit, code and photos.

Is it already on the forum here, if so why this cross posting?

I am sure a clock needs to be powered all the time in order to keep time. Or perhaps you are using an RTC module.

In any case, the display will be controlled by your Arduino logic, so to turn it off, you simply use the display logic to tell the display to shut off. No need to add extra switching. What's the problem?

It is not on the forum yet. I was waiting until the project was finished and I could share the entire build and code for everyone.
I do my best to separate issues without cross-posting.
This latch has varied applications and is not exclusively tied to the clock. I'm just using the clock as it's the first high-current project needing an external power trigger.

Indeed, a DS3231

The tv it's connected to has a single, very simple remote and I am not trying to add any buttons or complexity.


However, as I type this I think I'm better getting what you're saying.
Instead of all this I could just run the 4n27 to hook the 5V back into a digital pin and just turn off the display (and possibly go into a low power mode) when the pin is low.
Still uses the USB but eliminates all the power latch stuff.
Sounds like a great idea.

In addition to that I'd still like to finish this circuit as it may have uses elsewhere too.

When the pin is HIGH actually.

The obsolete 4N27 (since you specify that) always goes from input pin to ground, using INPUT_PULLUP. You only need to give it a milliamp or so in this configuration.

Of course, you could alternately power the clock from the USB if it does not use too much current for your secret display. :grin:

OK, I just risked a peek at YouTube without actually playing it - which crashes this computer. I gather you are using a WS2812 matrix. You could quite likely run your "clock" logic from the TV and the display powered separately, but I suppose you would have to figure out some means of telling the display to blank as the logic lost power. Oh well. :worried:

Cool, thanks. Use it like a button. I knew it couldn't handle much which is why I added the FETs, but this makes sense.

Tell that to the huge pile of them in my inventory :laughing:
In all seriousness, they still work and this isn't a critical application so they will do.

You should fix that. No reason a modern healthy computer can't handle a youtube video.

No secret. It's in the videos and in comment 3

I think mingling my 8A power supply with the TV's USB without any form of isolation is foolish at best. The 4N27 might be old, and it might have been marked as obsolete, but unless there is a major design flaw it will do just fine to isolate the two devices while letting them interact.

No, your first advice about using it as a direct signal is the best, along with your follow-up about using it like a button. Thank you.

True, but some things are just not readily fixable. Both OS and Firefox are regularly updated, Firefox still causes memory consumption crashes, as does the Arduino IDE using Java runtime from time to time.

I bought a newer PC to replace it but its UEFI boot loader (Lenovo) for some peculiar reason fails to recognise the Linux partition and I have not been able to make sense of this.

Have you ever used Clover bootloader?
I use it to put White Box Linux as MBR boot along with Windows 10.
Works great.

Use Brave for an internet browser, or the Chrome based MS Edge which is surprisingly robust and secure. Firefox is a fad much like its progenitor Netscape Navigator. It constantly needs fixes because the developers want to push new standards rather than following the established ones.

In Windows set your connection to metered and stop updates until you want them. Then, only update when you can afford a day or two of downtime to test and fix anything that might need fixing.

I wouldn't let you on my WiFi with a Lenovo computer, nor would I do anything at all with it that I didn't want to put in an email and send straight to their headquarters.

Yes, I was just researching and note that Lenovo is named as a problem with bootloading. :roll_eyes:

I would try using the TV USB voltage simply as a signal via an optoisolator (the kind on relay boards) and power the clock separately.

That's because they don't use a standard EFI and theirs is full of crapware and spyware.
I'm sorry you're stuck with that one :frowning:

Maybe sell it on feebay?

I think that's pretty much what I've worked out based on advise from @Paul_B
but it's nice to get a reinforcing voice too, thank you.

I haven't marked it as the answer because while it's a solution to one problem it is not the solution to the circuit design, which I'd still like to figure out regardless of using it for the clock.

Where do you suppose I got it? :rofl:

Ashes to ashes
Ebay to ebay

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