I’ve been working on an arduino compatible macro focus stacking controller, dubbed Stackduino, for a little while now. The last revision worked fine, and I’ve since added a few new features to the latest board which I was hoping to double check the logic of before I get it made. Mostly I’m hoping to confirm my choice of logic level MOSFET is appopriate, and that the resistor network for each looks correct.
Here’s the schematic:
The schematic is attached.
And an OSH Park render of the board layout:
The MOSFETS I’m interested in are MP1, MP2 and MP3. I’m looking at using a NTR4501N (http://www.onsemi.com/pub_link/Collateral/NTR4501N-D.PDF) for all of these. All will be performing a basic occasional switching function, there are no high frequency requirements.
1/ MP1 switches a bluetooth add-on board on and off as required by toggling 5v VCC supply.
It is controlled by a pin on the MCP23017, through a 150 Ohm current limiting resistor (R4). A 10K pull-down resistor(R3) makes sure the switch stays off when it should be.
2/ MP2 intermittently connects the battery to an analogue pin on the ATMega via a voltage divider to read the current battery voltage. It is controlled by a pin on the ATMega, through a current limiting resistor (R10). A 10K pull-down resistor(R7) makes sure the switch stays off when it should be.
3/ MP3 is switched on by VCC. It allows unregulated input voltage (9-12v) to flow to the VMOT pin of the A4988 stepper driver board - essentially switching the a4988 on and off with the main board. There’s a current limiting resistor (R15) between VCC and the gate - but as this one isn’t being driven by a micro controller pin, is it actually required? My thinking to include it anyway was that the speed of switch on isn’t critical and the resistor might help smooth the current surge on startup slightly. If it’ll make no difference though I’ll happily remove it. In theory VMOT could pull up to 2A but in reality it’s likely to be half that or less.
If anything looks horribly wrong with the above I’d be very grateful to be put right on my mistakes