Big Trak Circuits
This page documents the circuit inside the new Big Trak.
There are two know revisions of Big Traks. The main features of both designs remain the same but Rev 2.0 has a different circuit card assembly and a 3 pole not 2 pole switch.
Both Big Trak circuits are powered by 3 x 1.5v batteries. This is regulated to 3.3v by IC1 on the circuit board with all the board running off this 3.3v supply. The only exception are the motors which are powered by the un-regulated 4.5volts.
Rev 1.0 IC Pinouts
|Pin Number||Input / Output||Description|
|7||INPUT||CTRL (from IC3)|
|8||INPUT||CTRL (from IC3)|
|9||INPUT||CTRL (from IC3)|
|10||INPUT||CTRL (from IC3)|
Rev 2.0 IC Pinouts
If you can safely remove the main IC, then then the pads remaining are a good point to jack in your RPi control lines.
|Pin Number||Input / Output||Description|
|1||Input||Motor Left -ve PWM|
|17||Output||Left Encoder Feedback|
|18||Output||Right Encoder Feedback|
|28||Input||Motor Right +ve PWM|
|29||Input||Motor Right -ve PWM|
|30||Input||Motor Left +ve PWM|
Big Trak contains a H-bridge Motor Control circuit that makes it possible to rotate the motor in both direction from a single power source. In this case it is 4.5volts. The circuit diagram below shows the left hand side motor control circuit. The circuit is the same for the right hand side.
To make the motor rotate in a forward direction a positive voltage is applied to the forward input whilst the reverse input is pulled low. With these inputs transistors Q5 and Q13 will conduct allowing current to flow through the motor. To reverse the motor direction a positive voltage is applied to the reverse input while pulling the forward input low. This will cause Q4 and Q12 to conduct and hence will reverse the current flow through the motor.
Note Do not apply a positive signal to both inputs at the same time.
Using this circuit it is possible to control the speed of the motors by pulsing the inputs (PWM Control).
From the Big Trak circuit it is not possible to determine the identities of transistors Q8 and Q10 as they are very small surface mount devices.
Big Trak uses PWM control to accelerate the motors up to speed and also to slow them down when stopping. The scope plots below show the output from IC2 when Big Trak is programmed to move in a straight line then stop. The first plot shows the acceleratiom, the second the deceleration. The period of the pulses are 8ms.
As the Big Trak circuit will eventually be removed it will be necessary to make a replacement motor control circuit. This web site  has some useful information and also suggests an H-Biridge motor control IC that could be used instead of discrete components. This is the National Semiconductors LMD18200. This is probably a bit of an overkill but it's a start.
Big Trak has a simple encoder system for measuring how far it has travelled. They monitor the second stage gears which have a cut out in them. The circuit diagram for this is shown below. Not 100% sure this is correct (Needs checking).
To conserve power the circuit is only enabled when required. This is done using transistor Q1. The control line is connected to IC3.
Connecting the Raspberry Pi to a Rev 1.0 BT Main board
Before the Big Trak circuit can be used by the Raspberry Pi it is necessary to remove IC2 and IC3 from the board. These are tricky to remove but so far the best approach is to use a heat gun to remove them. If you don't wan't to remove the IC's then the tracks could always be cut.
Big Trak main board Rev 2.0 has a slightly different component idents and layout, however the locations to hack into are the same. Alternatively, after removing the main IC daughter board it possible to use the pads in accordance with the pinout.
The image below has been annotated to show the points on the Big Trak board that need connecting to the Raspberry Pi.
|1||Motor Left -ve PWM|
|2||Motor Left +ve PWM|
|3||Motor Right -ve PWM|
|4||Motor Right +ve PWM|
|5||Left Encoder Feedback|
|6||Right Encoder Feedback|
|8||Encoder Enable Over-ride|
If required the encoders on the Big Trak can be permanently enabled by connecting point 8 to the 3V3 supply at point 9.
The flat cable to the keyboard can be removed. The cable restraint can be reused for a ribbon that connects the BigTrak board to your Raspberry Pi. Reusing the cable restraint prevents strain on the solder joints, particularly when reassembling the completed project.