Moderator: pebe
pebe wrote:Hi xenix,
Your circuit looks over-engineered for what you want.
pebe wrote:1. Assuming one side of the fan is grounded and the other is connected to the relay contacts, C6 appears to be blocking the DC path for the fan.
2. What are the connections at the bottom left corner (fan, ground etc) meant to be?
3. No critical temperature is given for switch-off so the thermistor cannot be specified.
pebe wrote:The 7805 can power the ATmega all the times and its program can sit waiting for the ‘ignition’ pin to go low when ignition is turned off. But the controller you are using is an overkill, bearing in mind you only need 4 ports, ie.
An A/D input for the thermistor,
An input from the 12V 'ignition' via a potential divider to give 5V,
An output to the relay driver,
An output for the LED indicator.
So a PIC 12F675 PIC could do the job easily.
pebe wrote:Let me know if I can help further.
Thanks for your feedback. Before I redraw, are there any obvious advantages of using a Microchip MCU over a PICAXE?
I have some PICAXE chips and programming cable already as well as a TI MSP430 LaunchPad with 14-pin MCUs, but after spending a couple hours on the Microchip website, I could certainly go that route though I would need to invest in a PICkit for programming it, correct?
pebe wrote:I still think your circuit is more complex than needed to do the job. Would you be interested in a circuit showing how I would do it?
pebe wrote:OK. Here is an alternative circuit. It uses a PIC permanently powered from the vehicle battery via a 5V regulator.
pebe wrote:You will need a value for R4 so that at 85ºC GP0 will read somewhere around 2.5V; the actual value you can the set up in your program to compare GP0 with. Alternatively, if you cannot find the value for 85º from the data sheet of the thermistor, you could make R4 a 10k variable, and adjust it on test.
pebe wrote:You could also use the PIC’s inbuilt comparator instead of measuring the A/D voltage. In that case you could use a 12F629 in place of the 12F675.
pebe wrote:I hope that may help.
The thermistor is simply creating a voltage divider, so it makes no difference in functionality sending 5V to it versus 12V as in the original design?
I'll have to read up on this but this would mean that the PIC is comparing the voltage into GP0 with a Vref (for example GP3)? The circuit would stay the same but the program would, of course, change?
I've never been able to find a data sheet. I suppose I could construct a resistance/temp curve by measuring both while idling in the driveway?
A couple thoughts/questions...
I could replace the 7805 with a 2931 (using appropriate resistors per the data sheet)?
Any automotive (12V) relay with a 15A or greater load capacity should work? For example: http://media.digikey.com/pdf/Data%20She ... Relays.pdf
pebe wrote:You have shown the LED wired backwards. Its anode should go to Vdd.
Other than that it's great. Congratulations.
BTW, GP5 is not used. So you can either set it up as an output when it will automatically take up a 5V or 0V position (so don't connect it), or you can set it as an input and hard-wire it high or low (as an input it must not be allowed it to float).
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