The Super Probe

I remember mentioning in one of my earlier posts that an Oscilloscope was very high on my wish-list. The other day I entered into a bidding war on eBay-USA for a no-name DSO. It was pretty inexpensive, and the features it had seemed reasonably good. Sadly, I was outdone by some chap from Germany, who was willing to pay more than four times the amount I was.

After the eBay misadventure, my electronics toolkit continued to remain woefully incomplete. Apart from a simple multimeter and the PICKit2-Logic Analyser, I lacked the resources to take essential circuit parameter measurements – frequency, event counting, capacitance, inductance. I would also like a function/signal generator which would be able to output a PWM signal, pulses, pseudo-random number sequences and such like. Was there a multipurpose, do-it-all out there that would enable me to do all of this without breaking a bank?

About two years ago I had chanced upon the Super Probe while googling for a low cost capacitance meter. The Super Probe, designed by Luhan Monat, is a DIY low-cost and very low parts-count tool. As its name suggests, the Super Probe is capable of taking a variety of measurements with a fair degree of accuracy. It can also be used to output some useful waveforms and signals.

When I first saw Luhan’s site, I built a solderless-breadboard-prototype for myself. It lasted all of three days before the rats-nest of wires and components was deemed unsightly and cumbersome, and was relegated to the corner of my room, before being cannibalised for parts. I’ve finally gotten around to building my self a ‘proper’ Super Probe – the ensuing photos are ‘proof’' :-)

 

My 'Probe is enclosed in an old Orpat ODM-100 Multimeter case. I ripped out the insides (naturally keeping some of the components – the LCD, and some precision resistors – for future projects :-D ), replacing them with my own circuits. The entire 'Probe is built on proto board using DIP components. I plan to design my own PCB later with SMD parts.

Using my Dremel XPR400, I cut a slot in the side of the old multimeter to accommodate an ON-OFF slide switch. A circular piece of acrylic was glued in place of the rotary switch of the multimeter. Openings were also cut for buttons, which were ‘borrowed’  from an old non-functional calculator. Both, the button holes, and the switch slot, were made using the etching bit of the Dremel. To make the circular acrylic piece, I started by trimming the sides of a square piece. I drilled a small hole in the circular-ish piece and mounted it on a mandrel. This setup was spun at about 20,000RPM on a rough file till a circular shape was achieved, and then on a smooth file for finishing. You could just as well use a lathe to do this, but I had to make do with what tools I have :-( The circular acrylic piece was super-glued into place, as was the slide-switch.

The images below, from left to right, are of the slot cut for the switch; the switch glued from the inside; and, the modifications made to accommodate the buttons.

There are four PCBs inside the multimeter case – the display board, the actual 'Probe, a button mount, and a 7805 regulator. The reason I used a ‘modular’ approach is for ‘upgradeability’. Yeah, yeah, don’t laugh! I’ll tell you how ‘upgradeable’ this device is – I plan to replace the just PCBs with just one, which integrates everything. But until the time I actually design and etch the integrated PCB (I use the toner-FeCl3 method, and I no longer have access to a good laser printer) I would like to tinker and test the individual circuits. Each time I fiddle with a certain sub-circuit, and I wouldn’t want to build a completely new 'Probe.

The PCB wrapped up in red electrical tape is the 7805. The tape was necessary to prevent shorting. In the second pic, the crystal, wrapped in black electrical tape (also to prevent shorting) is visible.

For example, tomorrow I’m going to swap out the power-hungry and inefficient 7805 regulator with the much nicer, and comparatively power-thrifty MAX1836 buck converter from Maxim. I’m currently running off a 9v PP3 battery. With the 7805, I’d get 5v on the output as long as the PP3 voltage is above 7v. Current draw by the circuit is 20mA peak and about 10mA average, so with 9v on the PP3, I’d lose across the 7805. With 7v, I’d lose 20mW.

Now, although this doesn’t seem like much, I could improve battery life tremendously by using a buck regulator which is near about 85-95% efficient. So I’d reduce the power lost across the regulator to about 8mW – a reduction in power loss of 80%. Another advantage of Maxim’s 1836 is that it allows me to run the PP3 down to 5.5v, and it will still give me a nominal 5v on its output.

Back to the construction. For the display, I used individual 7 segment LED displays. The original 'Probe uses a 4-digit multiplexed display, but individual display blocks can be used as well, though soldering them is one hell of a pain in the butt (literally! Hehe :-D ). I put a sheet of cellophane paper (plastic) on top of the display to improve contrast.

The 'Probe uses a PIC16F870 as its brain. Luhan has been kind enough to provide a pre-complied HEX file which only needs to be burnt onto the PIC. He’s also provided the source code for download so you can modify the code as per your needs.

The Super Probe does have its limitations. It certainly is very inexpensive; the caveat is accuracy. The device’s readings are only as reliable as its components. Measurements should not be taken as God’s Word and whenever possible, must be checked against standard tools.

Anyway, the 'Probe has rapidly become one of my most useful tools. Of its 17 functions, I use the Frequency Counter, Event Counter, Servo Pulse Generator, Square Wave Generator, PWM Generator, and 38kHz IR Pulse Generator most often. For a list and description of all its functions look at the list below or visit the Super Probe website.

 

Super Probe Functions
Logic Probe – High, Low, Tristate
Logic Pulser with variable pulse rate
Autoranging Frequency Counter
Event Counter
Voltmeter 5v max.
Diode Junction Drop
Capacitance Meter
Inductance Meter
Signal Generator – 0.5v, 500Hz square wave
NTSC Video Signal Generator
Serial ASCII Generator – selectable baud rates: 1.2/2.4/4.8/9.6K
MIDI Note Generator
Hobby Servo Pulse Generator
Square Wave Generator – 1 to 9999 Hz
10kHz Pseudorandom Number Generator
38kHz Generator – useful for testing IR receivers
PWM Generator – 3 to 97% duty of a 6kHz square wave