Flight Computer Schematic Review Request

[ThomasRocketMan]

Hey !

I am trying to design a flight computer for a model rocket using the RP2040 MCU. I have gotten some feedback and adjusted things so far and I believe this is my final design. The computers main goal is to gather data from the onboard sensors and actuate two 9g servos for thrust vector control, as well as having two spare servo ports for any projects down the road. I am fairly certain I have the wiring of my sensors down, however would appreciate a brief glance over them. My main areas of review include my pyro channels and my power delivery. I am going to be using two 2s LiPos, one to power the board at 3.3v, and one to provide logic to my servos at 5V.


I have attached photos of my complete schematic. Please let me know if you see any glaring errors, have advice on where to research more information, or just have advice on what to change/adjust.
Thank you all!

*Note, I just noticed my barometer is incorrectly labeled in my schematic, it is a BMP390 NOT a BMP180.

 

[Voyager1]

Not sure why you’d want a comparator on your pyro channels, rather than just a continuity feedback circuit to your processor.

I would be using the full battery voltage on the pyro outputs, not +3V3.

Check the placement of C49 and C51 in the pyro channels.

 

[RocketDestroyer]

You have grounded the Vcc pins on your TLV3691 comparator. It won't operate with no Vcc supply.

 

[ThomasRocketMan]

Not sure why you’d want a comparator on your pyro channels, rather than just a continuity feedback circuit to your processor.

I would be using the full battery voltage on the pyro outputs, not +3V3.

Check the placement of C49 and C51 in the pyro channels.

Thank you for the reply! I think im going to ditch the pyro channels for this computer, this is the first computer Ive designed and the pyros are the ones Im struggling with the most to understand . This computer is going in a small diameter TVC experimental rocket and Im now planning on doing a parachute release with a one of my spare servo channels and a spring release mechanism.

 

[Voyager1]

You have grounded the Vcc pins on your TLV3691 comparator. It won't operate with no Vcc supply.

Exactly! I think the OP's intention was to have the caps to GND and the comparator supply to +3V3. Even experienced eyes miss stuff occasionally.

 

[ThomasRocketMan]

Exactly! I think the OP's intention was to have the caps to GND and the comparator supply to +3V3. Even experienced eyes miss stuff occasionally.

Yeah that was my intention, I wouldn't have caught this! Thank you both.

 

[RocketDestroyer]

Exactly! I think the OP's intention was to have the caps to GND and the comparator supply to +3V3. Even experienced eyes miss stuff occasionally.

That was my guess too. It's easy to do and easy to miss. We all make silly errors at times.

 

[cerving]

Thank you for the reply! I think im going to ditch the pyro channels for this computer, this is the first computer Ive designed and the pyros are the ones Im struggling with the most to understand . This computer is going in a small diameter TVC experimental rocket and Im now planning on doing a parachute release with a one of my spare servo channels and a spring release mechanism.

That's a highly advisable path. Pyro is harder than it looks to do safely. Once you get the recording and filtering part down then you can work on the pyro. It also wouldn't hurt to do a ride-along with a commercial altimeter to compare data.

 

[Leo Deelman]

Here’s a simple schematic to activate e-matches using a dual channel P-channel mosfet using a 3.3v MCU. Please note that the pyro charges get the full battery voltage and not the 3.3v.

P-channels are more suitable for e-matches as most N-channels are not fully opened by 3.3v logic signals.

Digital outputs D10 and D12 control the mosfet through a simple transistor. Continuity of the charges is checked by injecting a small current through the Resistors R9 and R12. E-matches are connected between OUT and GND . If the E-matches has continuity then A1 or A2 are low (close to 0V). If there is no continuity, A1 and A2 will see 3.3V. Connect A1 and A2 to an analog input on the MCU

The extra voltage dividers using the 4k7 resistors protect A1 and A2 from higher battery voltages (for example a 2S lipo). Hope this helps you in improving your own design.

 

[cerving]

We use ST Micro VN5E160S automotive drivers in our Eggtimer altimeters, in one SOIC-8 package you get 10A current limiting, continuity checking, brownout protection (it shuts off the load if the input voltage drops below 4.5V), overtemperature protection, and they will work with up to 40V input. You only need one 100K resistor for support if you use the internal pullups in your processor (the status output is open-collector active-LOW so you need to pull it up). Very simple to use... and under $2 in single quantities. (We go through a lot of them... we buy them by the reel).

 

[Leo Deelman]

We use ST Micro VN5E160S automotive drivers in our Eggtimer altimeters, in one SOIC-8 package you get 10A current limiting, continuity checking, brownout protection (it shuts off the load if the input voltage drops below 4.5V), overtemperature protection, and they will work with up to 40V input. You only need one 100K resistor for support if you use the internal pullups in your processor (the status output is open-collector active-LOW so you need to pull it up). Very simple to use... and under $2 in single quantities. (We go through a lot of them... we buy them by the reel).

That is indeed an interesting simple solution, having all those features in a single SOIC-8 package.
Unfortunately not available for shipment to Europe, I see on the Mouser website. Would be great if there was a version with a lower brown-out voltage to allow for a 1S lipo battery.

 

[Spacedog49Krell]

The VN5E160S is an obsolete part, replaced by the VN7140AJTR.

 

[cerving]

That's a completely different part, it's a 4-output driver with a common current sense status output vs. a single driver with an open-collector logic status output. According to Mouser, DigiKey, and ST Micro, the VN5E160STR-E is a current part. There was an older version of this chip that WAS replaced... that may be what you're thinking of. I bought a reel of them a month ago, and DigiKey has about 30,000 of them in stock.

 

[Spacedog49Krell]

The VN5E160S is obsolete. The VN5E160STR-E is current production. I'll order a few for my flight computer design. I need to work on DD.