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First Sch Review

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Makaram

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View attachment 67204

Hey guys,
was hoping for a little professional input into my schematic design.

Basically the function of the schematic is a very simple bluetooth unlocking system for a car.
So 12V power supply with a buck to 3v3.
Pic12f1840 is my mcu and TL2575 is my buck.


Am I missing anything or could be doing something better?

Looking forward to some feedback as I always learn a lot.
Thanks

Matt
 
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A schematic would be nice.
 
A schematic would be nice.
No, it really isn't :S

There are a few general rules to drafting a schematic... a few of them might be:
1) don't draw lines/wires through components
2) use 90 degree lines only
3) label the components e.g. part numbers or description
4) use appropriate symbols where possible e.g. an unlabeled square is not a good symbol for an inductor
5) text shouldn't be placed over lines/wires or components
 
Is it Eagle? Have you run ERC on it? That will tell you whether any of the nets and pins are not connected or are just overlapped.

As for appearance, I rarely use 45° bends and connections, unless the intention is a point ground. I also try to line things up and don't run nets under devices. Doing as you have done may make the schematic a bit harder to read, but may not affect its functionality. The ERC will help determine whether any functions are affected.

John

Oops, I forgot to ask whether you can attach the .sch file.
 
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You REALLY need to label all the parts names/values!

Is your buck device rated for automotive use? You might need some filtering where the 12v comes in to fix the nasty spikes etc that happen on vehicle 12v power.

Also your two outputs have no protection, do these drive solenoids or relays or motors? Either way you should have some diodes there across the load (diode to ground, reversed) as your output FETs are pull ups.

And the BIG PROBLEM is that you are using PIC output pins which are either 0v or 3.3v, to drive the gates of PFETs whose source pin is at automotive +12v! Best case, the FETs will be always on, worst case this will damage the PIC as you have 12v-14.5v connected to the PIC pins via the FETs and resistors.

To fix this you will need two more FETs or NPN transistors, to go between the PIC output pin and the live PFET gate pin.
 
Thanks everyone!
Should have been more specific as my question was more with regards to my actual circuit so thanks for clearing up those issues Mr Rb. I have already created this but used a 9v battery as my supply, also with a relay rather than a fet.

I'm very green with automotive stuff, (in fact most stuff), so failing to find a wiring diagram I simply used trial and error to discover connecting what locks and unlocks my doors. When connecting using a 10A max volt meter I didnt register any current? ?!
 
A Mosfet turns off when its gate voltage is the same as its source voltage.
Since your sources are at +12V and your highest gate voltages are +3.3V then the Mosfets ARE ALWAYS TURNED ON and never turn off.

You need to use transistors to level-adjust the 0V to +3.3V signal to 0V to +12V.
 
Thanks audio I was wondering if that would be an issue. Though I am confused as how a transistor would connect to the mcu with out once again being 12v. Sorry for my ignorance I have only ever worked with logic level
 
You need additional transistors (or open-collector logic gates and two resistors) to convert the 0V to 3.3V to 0V to 12V. Then these new transistors would be inverters so the MCU logic must be reversed or two more transistor inverters must be added.
 
A 12V relay needs about 9V to turn on which you do not have. It turns off at about 3V which you also do not have.
You could have the MCU turn a transistor on and off to drive a relay.
 
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Oh wow you've lost me there. Would it be possible to get a point in the right direction of where to find a circuit like this, or perhaps that other method you previously mentioned.
 
Hi Makaram, it is simple enough to do without relays, all you need are two more small NFETs, wired like this;

PIC output pin -> NFET gate.
NFET source -> ground.
NFET drain -> PFET gate
12v -> 4k7 pullup resistor -> PFET gate
PFET source -> 12v
PFET drain -> to motor + terminal
ground -> to motor - terminal

You already have the two PFETs and two pullup resistors. All it requires is two small NFETs and some re-wiring. Easy. :)
 
If not having sufficient voltage to switch my PFET was my problem in the first place, how does adding a NFET change that? Are transistors a possibility?
 
Makaram;1086965Hows this one looking? said:
Couple of pnp transistors doing the switching
It is completely wrong.
You want the MCU to turn the PNP transistors on and off so they can turn the Mosfets on and off.
But your PNP transistors (and the Mosfets they drive) are never turned off.

I copied part of your schematic. Do you see that the PNP transistor is never turned off?
 
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