Friday 16 October 2015

transistors - Using a PIC to perform switch on and timed shutdowns in automotive environment


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.



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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