Sunday, January 22, 2012

Dangerous Prototypes superprobe...

Not a lot of posts lately (been working on learning some new things - which means lots and lots of failures along the way - combined with the holiday season means little time to make posts).

A while ago I got a free pcb from dangerous prototypes but didn't get around to populating it for a while (one of those cases where I had all the parts on hand - but in the wrong sizes -> so it got delayed and then temporarily forgotten).  Icy day here yesterday so I was looking for something different to do -> opened up the loose parts box -> built a crystal radio -> built an amplifier -> played with voltage biasing some transistors to increase the sensitivity (all by the seat of your pants, no schematics, no plans, just simple fun playing with parts)... then I found the superprobe pcb and since I ordered the missing parts before Christmas I had everything I  needed on hand (almost) so I decided to build it.

Board quality is quite good, layout of parts is reasonably well thought out (biased towards a defined board size I suspect) - a few things I would have done differently (especially the cap locations and placement relative to the pic, also there is no power led... I really like to have a simple power led near regulators) but there is nothing wrong with the way it was done here.  (not the most beautiful soldering job on my part - 0603 is annoying by hand).

I didn't have the buttons intended, but I found that if I turned the ones I had on hand diagonally I could solder them in (works perfectly and more durable than I expected).

On the backside we have the 5V regulator on the far right, some resistor arrays that act as pullups for the buttons and current limiters for the display, the pic (Microchip 16F870 classic) with a tantalum cap below it, then to the left is the crystal (with caps) under the display (convenient) and then farther left is a line of resistors that can be connected into the probe line as needed by the pic.  It's an interesting layout (lots of compromises, but it works).  One thing I would recommend is to label the component values and the connector pinouts on the board.

Powered it up with 4 AA batteries (rechargeable - more than enough to make the regulator happy to output 5V).  No magic smoke escaped (yay)...  The superprobe is an implementation of a mondo tech. project - I actually remember seeing this a long time ago - thought it was neat but didn't use pic a lot back then.  It's a neat little project to make a very useful little tool from a limited number of parts...

Of course in addition to measuring voltage it can measure capacitance (here is a 4.7 uF elec.Cap) - same result as the BK LCR meter but it takes much longer to get to the final result.  I was surprised it was as accurate as it turned out to be (I looked at a few values between 0.1 uF and 300 uF - all were sufficiently accurate).

In logic probe mode it gives L for close to ground and H for close to 5V and a - for floating... I didn't have time to see exactly what the limits are...

It also has a mode for sending ir 38kHz pulses (neat)...

There isn't an led on board, but the signals in this mode are sufficient that attaching one to the probe leads ought to make a good ir receiver tester.

There's a mode for servos (radio control planes) that provides an easy way to test (and move) standard hobby servos... cool. 

There's also a PWM mode - what I like about this is how easy it is to adjust the signal (one button increases, one decreases) -> one neat thing is how intuitive the device becomes with only 2 buttons -> the button timing behavior was very well thought out.

The square wave generator is quite useful - it's more accurate in the middle of the range and a bit less towards the extremes but it's a very useful little tool.  I'm not certain how much it can drive might want to make a buffer and a isolate it with a cap - not certain what will happen to the pic if the circuit attached can backdrive current.

There's a number of other modes including but not limited to an ntsc signal, a serial test mode, a midi test mode, a coil mode (which I didn't test because I am not certain where the pic has sufficient protection), the event counter mode is a very easy way to look at button debouncing control (probably will only catch simple situations, I don't know how fast it samples).

Overall it's a neat little device (I like projects like this) - I think that with the newer generations of pics (lots of capacitance measuring and charge control integrated elements and bigger counters and more interrupts) even more could be built into these types of devices...