Edward Isaac Bot: Sketchbook 5

The weird third eye thing on this alternate sketch of EIB is supposed to be a Polaroid sonar transducer. But much has changed in the last 15 years in the way of robotic rangefinders.

Pokey uses Sharp GPD12 infrared rangers (right) which provide surprisingly accurate results. They determine distance based on the angle of reflection. They're cheap, only $12 each, and they're small, maybe 1.5" x 1/2".

There is a whole family of IR rangefinders, each with a unique (but somewhat limited) distance range. The GPD12's 4"-30" range seemed ideal for measuring distances within the firefighting maze in which Pokey competed.

Modern sonar systems are significantly smaller and also inexpensive, about $30 for something like the Devantech SRF04 Ranger (right). The device is about 2" x 3/4" and includes the driver circuitry on the back of the board.

This particular model uses a separate transducer and receiver. However, you can also get single transducer/receiver models. This device can measure distances from 3cm to 3m!

Of course in the 1990's, tiny cameras with any kind of image processing fell into the realm of impossible or unaffordable.

The CMUcam2+ (left) is able to do high contrast color tracking, all in a 2.25" square package. How times have changed!

There are a number of sources for this gear. I got a sonar ranger from Radio Shack when they were having a sale. I got my Sharp rangers from Acroname (the source of the pictures and info above; no advertising affiliation with the company, I just like 'em).

This flying robot from MIT is just ... wow.

I wonder what laser scanner they use??

Slowing Down

I have a lot of irons in the fire and up to various stuff that I can squeeze in between the time when baby girl goes to sleep and when I do. Or occasionally during a lunch hour.

I'm in the midst of several electronics projects: IR repeater, oscilloscope calibrator, ESR test harness, ideas for a mini robot made of audio parts. You'll see posts on these in the ensuing weeks and months.

Meanwhile I've been fixing a vintage receiver with some help. I'm helping out someone in South Africa diagnose and repair an oscilloscope. And I've got a stack of non-functioning audio gear I want to work on, like an all-tube Fisher 400, a Yamaha CR-2020, and much more.

I've also got some prep work to do for hunting season, some work to do on the Jeep (both stories for my other blog). And my hobby fund needs to fatten up a bit...

So I need to slow the pace of posting here to give myself a break. Most likely you'll see articles every other week instead of weekly, at least for awhile.

IR Repeater: Part 1

Introduction

Our Tivo, satellite receiver, and DVD player are on a shelf at the front of the living room, partially obscured by a sofa making it difficult to control them with remotes. I could go and buy an infrared (IR) remote extender (aka IR repeater), but what fun is that when I could build one? Plus, I want to use IR signaling for robotic communications so here's a perfect learning opportunity.

Requirements: The repeater will have to allow for very reliable control of these three devices, it should be aesthetically pleasing or at least unobtrusive, fairly small, and run for months off a pair of AA batteries.

How Infrared Remote Works

The oscilloscope trace at the right shows the digital signal from the remote. There are multiple coding schemes for remote control like pulse, space, and phase encoding.

Some systems represent binary data in long and short pulses, or pulse coding. Others represent binary data as long and short spaces between fixed length pulses, or space coding. Phase coding represents data by way of signal transitions. The beauty of this IR repeater is that it doesn't really care what coding scheme is used; it simply passes the signal along*.

To avoid interference with ambient light, IR remote controls modulate the light signal at somewhere in the vicinity of 38kHz. That way the receiver isn't fooled into thinking some random flash of light is coming from the remote.

So we need a way to detect pulsed 38kHz IR light and convert that into logic pulses then repeat the signal as pulses of 38kHz modulated infrared light. Detecting IR remote signals and converting them to logic signals is exactly what IR detection modules are for, like the Vishay TSOP32138. An astable multivibrator and an infrared LED handle the rebroadcast.

The Circuit

After searching the web, I settled on a circuit from this website as a starting point. After some tweaking and experimenting I arrived at this:

The circuit, as you can imagine, breaks down into three sections, the receiver, the transmitter and, not pictured above, the power supply.

The receiver consists of an IR receiver module with an active low signal. This signal is inverted by a simple small signal BJT (bipolar junction transistor) as an input signal to trigger the transmitter.

The transmitter section consists of, no surprise, an IR LED, which is driven by another transistor. The LED is pulsed on and off at approximately 40kHz by a pulse generator circuit with a 555 timer IC at its heart. I'll fine tune the modulation in the near future.

The power supply will eventually consist of 2 AA batteries, for their compact size and energy density, and a voltage regulator to provide 5V to the parts that need it. For now I am using a 9V battery and a good old-fashioned 7805 linear regulator. I'll address power reduction in a later article.

The good news is the prototype circuit (see pic at the beginning of this post) works pretty well with our DVD player and satellite receiver. But we also have a TiVo and the TiVo is not happy.

TiVo Problems

When in operation, the repeater renders the TiVo remote utterly non-functional. I found an explanation at this website by Edwin Olson.

*Apparently the IR module is distorting the signal, so it isn't truly repeating the signal verbatim as I had originally hoped. After dragging out my trusty, massive Hitachi V1050F oscilloscope and after fiddling with knobs awhile, I finally duplicated Mr. Olson's results.

You can clearly see that the module is adding extra delay on the falling edge of the signal and virtually none on the rising edge. Hm, well, this is a bit of a problem. Stay tuned to see how I end up solving it. I can't wait to find out! :)