Feature creep has been very real since I've started putting this together and I've been thinking of using a microcontroller to handle the logic instead of discrete components. Going this route the initial circuit wouldn't work anymore. Well it would, but it would cut power to the AVR or whatever I end up using on a fault.
I suppose it may help to tell what this thing is supposed to do. The basic features of the ideal tool would be to have 3 states for the probe:
1) at 12v to test components out of circuit that have their ground side connected through an auxiliary ground probe on the device itself, i.e. bench test a headlight before putting it in the car
2) at 0v (ground) to test components in circuit replacing it's ground wire with the probe, i.e. circuit is suspect of a faulty ground, replace the ground wire at the connector with the probe. If the component works your suspicions were correct.
3) a floating state to be used as a multimeter.
In the "instructable" in the OP, #3 is where that project stopped and mine begins. That probe simply turns on a green LED if the probe is close to ground and a red LED if the probe is close to positive battery supply. I would like to display and use that data the probe is picking up. Showing current, min, and max voltage would be very nice and doable I think . If I touch on a signal wire it would be nice to display info about it (this is where the project starts getting way over my head, basically a mini oscilloscope) but just displaying that it is a signal wire should be good enough.
And one extra feature which is to load test a control circuit. A switch to short the probe to ground through a hefty resistor and display the voltage drop if any.
This is where I was at when I decided it was getting too complicated and I should just use a micro. With an AVR all I would really need is to drop the voltage to power the ic (plus other low level stuff), protected high side and low side drivers for giving battery supply to the probe, and to read whatever is going on at the probe tip with the AVR. The rest would be all software and simple lights and buttons
Jeroen, could you elaborate on the last part of your reply? I'm new to electronics outside of relatively simple projects. So it's a circuit that would measure the resistance from the probe to ground, and then allow full battery voltage if the resistance was an acceptable level, i.e. not shorted to ground? This is a slick solution and I feel like it would be a good feature for the probe to measure resistance and I could use an ADC on the AVR to read/display it on a screen of some sort.
Ian, those seem like the perfect solution except they are all smd that I've found (at least for high side switching). SMD is fine for the final version but makes prototyping much more difficult. It would tie in well with a microcontroller, most from what I've seen have an output pin that sends data about the current draw which could be useful. Maybe I will pick up some smd adapter boards and give these a shot if I can't come up with anything. Only thing I've noticed about these is the current they can source seems to be around 16 or so amps. This is above the expected constant load but some motors will have a large spike of current to get them going which may make the ic think it's shorted.
Sequoia, a fuse is a good idea. I may use a traditional one as sort of a back up failure mode. Those PPTC fuses I don't think will work for this application though. I was looking through the data sheet and it lists "time-to-trip" in seconds! One of them was rated to trip in 10 seconds, way way too long I think to be safe for this tool. Plus they reset on there own, on this tool I would like the user to know that the probe was shorted and have to manually reset it somehow.
Thanks to all of you for the suggestions and info.
I came across this circuit involving the power switching:
http://electronicdesign.com/power/add-short-circuit-protection-diagnostics-automotive-high-sidelow-side-driverThis seems like a good solution to me, all of these components are super cheap. But that circuit seems to be only for switching low-side, do you think it could be adapted for high-side? I think I should be able to use just that circuit but have the NPN transistor driving a PNP mosfet instead of the load?
Thanks again for the help