Electronic – Using a PIC to perform switch on and timed shutdowns in automotive environment

automotivepictransistors

I would like to build a circuit to perform a timed shutdown, based on logic inputs from headlamp wires and ignition wires.

I have decided on PIC10F200 (datasheet) to perform the logic.

I also have many spare 7805 linear regulators, so thought I'd use those to perform 12V->5V regulation before connecting directly to PIC pins configured as inputs. Is this a terrible idea? I have added pulldown resistors so that the PIC input pins can read defined values.

Using Fritzing, i've created a circuit:
enter image description here

I'm still tinkering at the moment, and this being my first PIC circuit, I want to make sure I can avoid all the possible traps when working in an environment like this.

So basically, is the circuit okay, or have I overlooked things?

All and any help appreciated, please go easy on me. I'm new!

Update:

Following advice from Olin and Wouter, I now have this:

enter image description here
Is this ok?

Olin says making use of the weak pullups on the PIC could save on component count, yhis is how i've interpreted him:
enter image description here
This works in the simulator, but I've no idea if my pile of BC547 are suitable!

I also don't know which to choose, or why?

Thanks again guys.

Best Answer

You can use 7805's this way, but as they only replace one resistor (and require 2 additional decoupling capacitors each!) I would never do it this way. My suggestions:

  • replace the two 'sensor' 7805's by resistor / zener-diode combinations. Use 4v7 zeners.
  • add two 100nF capacitors at the input and utput of the remaining 7805
  • add a suppressor diode over the relays coil!!
  • I am not sure a jellybean 7805 is a good idea in an automotive environment, there can be nasty spikes on the power line.
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