I am a mechanical engineer, not an electronics expert; so I am a bit out of my depth here.
I have a classic car with an analogue speedo, but now with an all electronic gearbox. After much research I did find some usefull resource from Guy Carpenter amongst others about driving switec X-25 stepper motors. Thses are the auto industries standard instrument steppers. I also need 2 small OLEDs to register the mileage and a trip counter.
I would like to be able to house the whole lot inside the standard instrument casing so selected an arduino pro mini. Once I have a satisfactorily working model I will make a pcb to fit.
I have designed the following circuit from various sources of information across the web.
I have installed only one stepper motor, I may need to install another for the tachometer, but this will require remote wiring.
My son is an IT specialist so when I start with the sketch he will be able to (hopefully) create some tidy code for me. That will come later.
Now please be gentle with me. I have just had to learn the basics of Eagle. I have been struggling with finding components in the library, so when I have the circuit right I will create the correct components for this.
QUESTIONS:-
Do I have to pull down/up the unused arduino pins?
does this circuit look right?
Have i missed something?
Have I god the Vdd, Vss, & Vcc pins right?
This has been done before, but without the VID66-06. This is the reccomended driver for the stepper motor and should avoid some of the problems others have reported.
Do I have to pull down/up the unused arduino pins?
As long as your code does not read those pins and perform any function based on the result, then no.
does this circuit look right? Have i missed something?
Is it a 5V 16MHz Pro Mini or a 3.3V 8MHz one? If the former, don't connect the 5V from your 7805 regulator to the RAW pin, connect it to the Vcc pin. The RAW pin connects to the on-board regulator, which needs 6~6.5V input. Connecting to Vcc bypasses the on-board regulator.
If you have a 3.3V Pro Mini, that's a mistake, given the rest of the circuit is 5V. The output signals from.a 3.3V chip may not work correctly when connected to another chip expecting a 5V signal.
Wow! I didn'texpect such a great rteaponse. Thank you all.
Clearly I have forgotten loads of things.
I will be using the input pin on the arduino. The gearbox has a hall sensor in it which outputs 8 pulses per revolution. The original speedo read 940 Pulses per mile, but will change with the new diff ratio. It was my intention to set a variable in the code for the number of pulses per revolution, and also the number of revolutions per mile. it will make life easier if anything changes.
I will be using a 5V arduino as I beleive the engine ECU has a 5V output and I may need this to drive a tachometer, but not sure yet.
Whilst I can clamp the 7805 to the metal casing as a heat sink perhaps I should be looking at a different regulaor, perhaps a UA7805CKCS voltage regulator which can supply up to 1.5A. This has just prompted another thought though. Whilst the nominal battery voltage is 12V, under cranking it can drop as low as 7V or 8V.
Whilst charging it should be running at about 14.2-14.7V. However if the alternator regulator fails the voltage can go much higher. perhaps I should be considering some form of protection? I think I need to research this.
Gnd3, that was an error. I thought that those pins at the top were for programming the device and that nothing should be connected there. Thanks.
The RAW pin, I think I must have misunderstood something somewhere. I have now been back and looked that up and disconnected the pin. Thanks again.
I have been unable to find a datasheet for the pro mini which details it's current consumption, only it's maximum load per pin (40mA)
One last question. First atempt at a schematic, and Eagle, so a learning curve. Have I got this about right? or is is a bit scruffy?
Whilst I can clamp the 7805 to the metal casing as a heat sink perhaps I should be looking at a different regulaor, perhaps a UA7805CKCS voltage regulator which can supply up to 1.5A. This has just prompted another thought though. Whilst the nominal battery voltage is 12V, under cranking it can drop as low as 7V or 8V.
You will still need a heatsink, the metal casing should be fine, even though the regulator is rated higher in current, you will still have to dissipate the same amount of heat from the IC.
As you don't need the speedo to start, you could run your project off the accessories 12V power.
You can make schematic look a lot neater and easier to follow by using the "VCC" & "GND" symbols. Add as many as you need around the schematic, rather than drawing lines connecting them together.
