Nuts and Volts - December 2008

PICAXE PRIMER

to adjust the size of the stripboard to fit your project box.

You may have noticed that I haven’t included a circuit diagram for either the IR transmitter or the receiver. That’s because all the necessary circuitry is already contained on the IRMB boards. All you need to do is make sure that both IRMB circuits are properly connected to +5V and ground. On the transmitter circuit, you may also want to connect an LED and current-limiting resistor from the 08M output1 to ground. I used it to blink after each transmitted digit just so I knew the circuit was functioning properly. To test your SIRC IR link, type the SIRC transmission program (listed in Figure 5) into the Programming Editor, connect a programming adapter and cable from your PC to the IRMB, and use the Programming Editor to load the program into your battery-powered IRMB transmitter. For the receiver circuit, you can again use the “IRMB_ Remote.bas” program we tested last time (available at ww.nutsvolts.com). Don’t forget to leave the receiver’s cable connected to your PC because the received data will be sent to the Terminal Window. The transmitter program repetitively sends the digits 0 through 8 to the receiver, which serially sends the data on to the Terminal Window. Don’t forget, the “IRMB_Remote” program adds 1 to each received signal to correspond to the digits on the TV remote, so you should see the digits 1 through 9 being received in the Terminal Window.

It’s a simple matter to test the range of your system. Just move the portable transmitter a few feet from the receiver (making sure the IR LED is pointed directly at the receiver) and walk back to the PC to see if the correct data is being received. (Of course, if you walk between the two systems you may get some garbled data, but the transmission should correct itself as soon as you get out of the way.) I was easily able to get reliable operation at a distance of 20 feet, which is as far away as I could get in my basement work area.

USING IR FOR OBJECT DETECTION

; === IRMB_SIRC_Tx.bas =====

; === Declarations =========

The next PICAXE IR function we'll investigate is obstacle detection. In this case, the same IRMB circuit will implement both the transmission of the IR carrier wave and the detection of an echo that indicates the presence of an obstacle in front of the IRMB. The software ( IRMB_Echo.bas) is too long to include here — you can download it from the Nuts & Volts website and open it with the Programming Editor. You can use either a horizontal or a vertical IRMB setup to test the software. Also, be sure you include an LED and current-limiting resistor on output1, because it's used to indicate the presence of an obstacle.

There are a couple of points about the "IRMB_Echo" program that may require explanation. To begin with, let's examine the process involved. Essentially, the IRMB emits five short bursts of a 42 kHz IR signal and immediately after each burst listens for the presence of an echo. Since the signal is traveling at the speed of light, it's necessary for the 08M to be very fast in making the transition between transmitting and listening.

This is why the program is configured to run at 8 MHz; 4 MHz is just too slow to detect the echo signal. Figure 6 presents a graph of the sensitivity of the PNA4602 verses the frequency of the incoming IR signal. As you can see, the sensitivity decreases as the frequency varies either up or down from the 4602's central frequency of 38 kHz. Considering this, you're probably wondering why I chose 42 kHz rather than 38 kHz for the PWM frequency.

When I first

symbol index = b0
symbol LED =  1

; === Directives ===========

#com1
#picaxe 08M

; === Begin Main Program ===

high 2

; enable IRMB
; transistor

do
  for index = 0 to 8
 high LED
 infraout 1, index
 pause 250
 low LED
 pause 250
  next index
loop

■ FIGURE 5. SIRC Tx program.

began development of the program (using only single transmissions), I naturally did choose 38 kHz but my results were plagued with sporadic false positives. The IRMB would frequently report an echo when there was no obstacle anywhere near it. My first thought was that the heat-shrink tubing that shields the sides of the IR LED might be "leaking," so I used a toothpick to apply a small amount of black silicone sealant wherever I thought it would help, especially around the base of the LED on the horizontal version of the IRMB. This did reduce the false echoes somewhat, but it did not eliminate them entirely. At that