Projects/Nav Bot II

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When I was a small boy, I wanted a BigTrak.

I didn't get one.

This is probably a good thing, because I would have just dismantled it, like most things I got my hands on at that age.

A few months ago the BigTrak was relaunched, I still wanted one.

Then this week I noticed that Amazon were selling them for £15.99 - only one thing for it, I bought one.

When it arrives, I'm going to dismantle it.

Bot II - BigTrak Evolution

When I've dismantled it it's going to be rebuilt, and it's going to have features that the kids of the 70's could only dream of.

Stay tuned, it's gonna be fun.


A shiny new Big Trak

Well, it's arrived, after fitting it with some batteries it's had a quick test just to make sure it all worked (you can't really return it as faulty once it's in bits).

Screw positions

Once you know it works flip it over, and remove the battery cover. Then remove the screws through the white plastic down both sides and around the front. There's no need to remove the screws in the black plastic - they just hold the gearbox to the lower case. Don't try to open the case at this stage - there are 2 more screws that need to be removed first. They can be seen right at the back if you carefully open the front of the body shell and look right to the back.

The two remaining screws are hidden by the grey plastic section on the rear. This can be removed by squeezing one side towards the centre until you hear the latch release. A small screwdriver can then be used to apply downward pressure to stop it re-engaging while you press to release the two rear latches. Then push to release the remaining side latch and the whole piece should slide downwards giving you access to the screws hidden beneath.

Rear cover removed

Keypad ribbon cable

You can now carefully lift the top part of the body away from the base, but don't life it too far as the two are joined by a couple of cables. You should be able to move the top sideways to give you access to the keypad ribbon cable connector which is the shorter of the connecting cables. Remove the two screws of the clamp and the end of the cable just lifts from the board. The remaining cable is long enough to allow you to open the case completely.

Inside Big Trak

The Hardware

Main Big Trak PCB

So what do we have?

Main PCB
Not very interesting, since it's very limited. We'll be replacing this in order to build a far more capable bot.
Thankfully this hasn't really changed from the original. It's a nicely designed gearbox with a magnetic clutch that ensures the bot moves in a straight line when we want it to, and perforated gears with opto sensors for measuring distance travelled.
Mounted in the front, it's not really in the way, so we'll probably retain this.
There are two LEDs, a blue LED mounted in the front for the phaser (it's very tempting to fit a laser), and one mounted below the attachment point for the trailer (I'm sure this connector was a 3.5mm jack on the original, can anyone confirm?)
Power switch
A 3 way switch which provides 2 outputs - ON-OFF-ON from the red/black pair (power) and OFF-OFF-ON from the yellow/orange pair (used to select carpet/hard floor mode).
Used for programming, may come in handy for user interaction in the rebuilt version.
Naked Big Trak


Opto sensors rebuilt on veroboard
Opto Sensors
Removed from the original PCB and reassembled on some stripboard - luckily the pinouts and mounting holes are a close enough fit.
Testing the pulse output from the gearbox
The LED and phototransistor in the sensor both look the same, the only difference is that the pin shape inside the transparent housings are left and right handed. A multimeter in diode test mode will allow you to identify the emitter LED, then with that connected to a suitable supply you can do the same to identify the pins of the phototransistor.
The prototype motor controller on breadboard
Prototype motor controller fitted for testing
Motor Controller
The new Big Trak revision uses full H bridges for motor control rather than a split power supply and half bridges used in the old version, but since we're removing the main PCB it made sense to build a nice beefy controller - using TIP120/125 transistors.