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

My वायरलेस प्रेसेंटर

Wow! Hindi transliteration on blogspot! That's an interesting feature! :-) Useless, but interesting, nonetheless.

Just a small point to note: I'm typing these first few lines from 15 feet away, using my wireless presenter. (Big smile :-) )

Naturally, you scoff, "How can you do that? Oh! You have a wireless keyboard! Umm, hang on a sec! You said वायरलेस प्रेसेंटर . How are you typing text?" (Bigger, mysterious smile :-) )

Well, I made my own Wireless Presenter-cum-Keyboard-cum-Mouse. (Biggest smile yet :-) And, in Sheldon's words - TBBTS02E07 - Muaa-hah-hah). Here are the details.

This project originally started off beacuse I used to get exceedingly irritated with the abysmal lack of dexterity afforded by standard keyboards. This lack was / is felt, most painfully, when getting fragged in a fast-paced game of CoD. "I need my grenade! Where the hell is the 'G' key. Dratted keyboard." My rifle! My rifle! '1'! '1'! '1'. Shots fired! Duck! 'Q'! 'Q'! 'Qqqqqqq....!'. I'm sure you get the point. I also do a lot of 3D modelling in AutoCAD, and image editing in Photoshop. My mouse, which would ordinarily be considered reasonably good, fares pathetically at this zoom-in-zoom-out-click-to-do-this-tap-to-do-that kind of stuff. Wouldn't just be great to have an additional fully customizeable keyboard? It would have a zoom slider, a wonderfully sexy scroll wheel (much like the iPod's), multiple buttons, and special keys which have several keyboard buttons bound to one (so to press 'Ctrl-Alt-Del', I'd just need to press one key on this keyboard). The mouse would be integrated into this device, and would have similar useful features.

I can, of course, buy such a device, but that would mean shelling out absurd quantities of cash - something that I don't have much of! Making one would mean I'd save potloads of money. I have been reasonably successful; hence this post.

So far, I've only made a Wireless Presenter, but this same device can be adapted to do all the stuff I mentioned above. A little effort, some pain, and a couple of milk-and-cookies-filled all-nighters are all that are necessary.

Without going into the overly-technical details of the device I'll tell you what the system consists of:
- a full speed USB PICmicro (the 18F2550) (free from Microchip);
- a TSOP1738 IR detector (Rs.10);
- a cheap roadside SONY TV remote (Rs. 60);
- a USB cable (free from an old mouse), crystal (Rs. 4), capacitors (Rs. 4).

The SONY remote sends IR data to the TSOP, which is connected to the '2550. The PIC decodes the IR signals (which are in SONY's SIRC format). The '2550 then sends this data through USB to the PC. Now, I've programmed the PIC to enumerate as a USB keyboard-mouse combo device. So my PC, for all practical purposes, thinks that it has a second keyboard and mouse. I have mapped certain keys of my laptop keyboard to keys on the remote. So pressing the 'Program +' key on the remote, is equivalent to pressing the 'Right Arrow' key on my keyboard. Pressing the 'Power' button on the remote is like pressing 'Esc' on the keyboard.

This is the nice thing about putting software (firmware) onto a microcontroller - you can fiddle around, and map virtually any key combination to any remote key!

I use this device to control MS Powerpoint slides from upto 30 feet away. I've mapped most of the Powerpoint keyboard shortcuts to buttons on the remote. The 'Contrast' button is 'F5', the 'Program Jump' button is 'Enter', and so on.

For a full listing of the source code go here .

I've made this using a PIC, but if you get the logic and code right, I'm pretty sure you could use any processor.

Future modifications include adding an accelerometer to provide intuitive mouse support (right now I'm pressing 'Mute' and the 'Volume / Program +/-' buttons for mouse-like action). Maybe I'll even port the whole thing to RF (Zigbee, or MiWi); it's running IR now - the great thing about IR is that its laughably cheap and easy! RF would involve additional cost and effort (protocols, and other crazinesses). For now, this is good. Anyway, in the beginning you had asked (well, alright, I had made you ask) how I was typing out text using my वायरलेस प्रेसेंटर.....I think you have the answer now.

I will be, at some point in time, making the gaming device I talked about earlier, but until then, I'll have to amuse myself with this :-) If anyone is planning on building something similar drop me a mail - I'll be glad to help.