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I finally obtained a vexplorer. (I hung around ebay for about 3 weeks.) The day I picked it up Amazon had them for $99 and then $60! (It seems like the $60 price was a mistake). It came well packed and I spent about 90 minutes putting the pieces together with my daughter.

After driving it around for about 10 minutes I decided to test the camera. The camera works immediately with my 2.4 Ghz Wireless to USB box. The channels that Vex assigned are non standard 1=4 etc! However with a bit of fiddling it works. Unfortunately the camera included on the Vexplorer is about the worst quality I have ever seen. Terrible low light performance, lots of interference etc. I have bought inexpensive cameras, but never had a NTSC camera this bad. Still it should be workable in Roborealm.

Here are some snaps that I took from Roborealm:

Now on to hacking the remote. Here is a picture of the backside of the remote. There are two structures that have circles in the middle. These are the backsides of the joysticks. The large surface mount chip is the microprocessor.

The remote operates at 27 Mhz. There are four analog channels and two digital channels. Each pair of analog channels is fed by a very nice joystick which controls a two ~1K pots. When the joystick is moved, the resistance between the central terminal and one of the outlying terminals decreases. This is hooked up with a voltage divider network. The microprocessor uses an ADC to measure this voltage and encode the appropriate signal to transmit. I removed one of the joysticks and wired into it. I am not sure what I did (I think I just shorted power to ground) but the remote stopped working for about 20 minutes. It did eventually recover after I soldered the pot back in (Phew!) It seems like interfacing to the remote this way will be tricky!

The microprocessor outputs some kind of modulated pulse on one of its PWM outputs. I left my oscilloscope probes at work, and all I have are a pair of alligator clips soldered to a BNC connector. Needless to say all I see on the PWM output is noise right now. When I go into work next, I will bring the probes home so I can look at generating the appropriate PWM output.

The microprocessor on both the transmitter and RX is an ELAN 78P458. Here is the data sheet for this microprocessor. elan78p458.pdf  Unfortunately, it doesn’t support rewritable memory, so we can’t get at the toy that way!

For now I will breadboard up a Picaxe controlling a set of 4 hbridges to drive the robot. I will use an XBee module to provide serial communication with the PC. This is an expensive way to do this, but it will work for now. I would prefer to just use a Picaxe on the transmitter side and use all the vex electronics on the bot.