It's been forever and a day since I've been here, but I'm dipping my toes back into electronics design
Anyways, I have a "dumb" device that's connected to a hat on my Raspberry Pi 4. Unfortunately, due to some signaling issues with the dumb device, if it's on when the Pi powers on, the Pi gets severely wedged (doesn't even get to bootloader). Because this setup is remote, I need it to be power-loss-tolerant, so my "ingenious" idea is to make a simple board that delays power on of the dumb device by about 5 seconds (that's enough time for the bootloader to get started, and from there it doesn't care about the signaling issues from said dumb device).
Here are my requirements:
When I think timed stuff, the first thing I think is 555. I know it can do a monostable pulse... so now I need to figure out how to switch the power on when the pulse ends... but I don't want the power to come on before the 555 starts its pulse. Since I need a way to switch the 20-odd watts the dumb device draws anyways, my mind went to a MOSFET. As a result, I'm thinking perhaps I can tie the output of the 555 to a P-channel MOSFET with its gate pulled high.
Any refinements to my idea (or alternatives if mine's completely off kilter) are welcome!
Looking at it now I'm already realizing that the 555 is perfectly content to run on 12V (datasheet says VCC≤18V) so I can eliminate the LDO and its freakishly large caps (I'm trying to stick with parts JLCPCB/LCSC has so I can have them do the whole shebang for me, and the LDO datasheet is entirely in Chinese but the application circuits all show 10uF caps on Vin/Vout )... that should also ensure that the "high" output on pin 3 is equivalent to the drain voltage on the MOSFET to keep it off.
Oh, and I'm not sure it's entirely necessary (or even correct), but I wanted to make sure the 555 triggers at power on, so I put the 20K/10uF RC on the trigger which I figure should be enough of a pulse to poke it.
As it stands now, I managed to make a rough-out of the PCB for this and it's like 43mm (1.7") by 20mm (0.8") so I'd be quite happy if I can keep it this small (or smaller!).
Anyways, I have a "dumb" device that's connected to a hat on my Raspberry Pi 4. Unfortunately, due to some signaling issues with the dumb device, if it's on when the Pi powers on, the Pi gets severely wedged (doesn't even get to bootloader). Because this setup is remote, I need it to be power-loss-tolerant, so my "ingenious" idea is to make a simple board that delays power on of the dumb device by about 5 seconds (that's enough time for the bootloader to get started, and from there it doesn't care about the signaling issues from said dumb device).
Here are my requirements:
- Needs to be plug and play (no microcontrollers or other stuff that needs to be programmed or might otherwise get fidgety)
- Something I can send off to a fab and get a board made (and possibly assembled if I use all SMD)
- Plugs in in-line to the dumb device (12V 2A power supply with bog standard 5.5mm/2.5mm barrel connector)
When I think timed stuff, the first thing I think is 555. I know it can do a monostable pulse... so now I need to figure out how to switch the power on when the pulse ends... but I don't want the power to come on before the 555 starts its pulse. Since I need a way to switch the 20-odd watts the dumb device draws anyways, my mind went to a MOSFET. As a result, I'm thinking perhaps I can tie the output of the 555 to a P-channel MOSFET with its gate pulled high.
Any refinements to my idea (or alternatives if mine's completely off kilter) are welcome!
Looking at it now I'm already realizing that the 555 is perfectly content to run on 12V (datasheet says VCC≤18V) so I can eliminate the LDO and its freakishly large caps (I'm trying to stick with parts JLCPCB/LCSC has so I can have them do the whole shebang for me, and the LDO datasheet is entirely in Chinese but the application circuits all show 10uF caps on Vin/Vout )... that should also ensure that the "high" output on pin 3 is equivalent to the drain voltage on the MOSFET to keep it off.
Oh, and I'm not sure it's entirely necessary (or even correct), but I wanted to make sure the 555 triggers at power on, so I put the 20K/10uF RC on the trigger which I figure should be enough of a pulse to poke it.
As it stands now, I managed to make a rough-out of the PCB for this and it's like 43mm (1.7") by 20mm (0.8") so I'd be quite happy if I can keep it this small (or smaller!).