I’m working on a Photon-based control for a meat smoker. I’m very much in the noob/hobbyist/tinkerer camp when it comes to electronics. I’m turning to y’all since you’re some of the most friendly and awesome people I know.
I’d love some feedback on the design and components as well as some generic, non-Particle-related questions. I’m trying to use parts I have on hand, if possible.
First the schematic:
Second the questions:
- How do I pick a good value for a flyback diode? Some advice I saw on the 'net suggested look at the motor’s voltage and current usage and multiply by 2. Is that enough? Seems like momentary spikes could be bigger.
- On the 12vdc -> 5vdc regulator, I got the cap values from the datasheet’s reference design. The problem is I don’t have those exact values on hand. I saw somewhere (electronics.stackexchange.com) that I could use 10uf and 1uf as substitutes for 0.33uf and 0.1uf respectively. Is this true, and what sort of issues would I run into potentially? Do I have to use electrolytic capacitors, or can I use ceramic caps?
- Do I need to do any type of filtering/protection between the motor and the LM7805 or the Photon? Is the power ‘clean’ (for lack of a better word)?
- Voltage regulator LM7805. (I know switching regulators are more efficient, but I have a handful of LM7805s, so I’d like to use them if I could. I do have a heatsink.)
- High-side power switch using:
- FQP27P06 P-channel MOSFET
- PN2222A or 2N3904 NPN transistor (I have both. I’m assuming either would work.)
- MAX6675 Breakout that talks to the Photon over SPI (Type K thermocouples to go with the boards).
- 12vdc / 360mA blower fan (specs on the mA, not tested yet) - unsure if it has a built-in flyback diode or not. If it does, I’ll take one off.
- 1n4148 diode for flyback, but I’m worried it’s too wimpy.
The breakout boards want 5v (they might be fine with 3.3v also, but I haven’t tried), so I wired them up to the 5vdc bus, but the pins in SPI mode aren’t 5v tolerant on the Photon according to the datasheet. So, I’ll have to address that somehow. If any of you have advice on this, I’d appreciate that too.
Thanks for any advice you can give!