The Motor Control Board

The motor control board that comes with issue 3 of the magazine essentially contains two circuits.  One for the motors, and one that checks the voltage of the 9V PP3 battery, and reduces this voltage down to +5V.

 

The Motor Circuit

The circuit for the motors is based on a common motor control circuit, called an 'H Bridge' circuit.  Before we look at the circuit, I will explain what an H-Bridges circuit does, which will then help us examine the circuit itself.

The H-Bridge Circuit

Look at the diagram below.  It consists of a motor, two supply rails and four switches.  We can close any of the switches at any time, giving us a total of sixteen different combinations.  However, only three of the combinations are useful.  The rest will either not work, or cause the motor to burn out.

Now, if we turn switches A1 and A2 on, current will flow in one direction through the motor, causing the motor to turn.  If we turn switches B1 and B2 on, current will flow in the opposite direction through the motor, causing it to spin in the opposite direction.  If all the switches are off, then no current will flow through the motor, and it is free to spin.  The simplicity of this type of circuit makes it ideal for robotics.

OK, time to look at the circuit.

Circuit Diagram of The Motor Circuit

Below is the circuit diagram of the Motor control board.  You may prefer to save the graphic by right clicking the mouse on the circuit, and then viewing it while you read the description.  I couldn't make it much smaller, as you would not be able to read the values etc.

Four connectors come onto the board.  Two are for the motors.  The other two are for power and the main processor board (which comes later).

The +6V power comes in on SK2 pin 4, and is also routed to SK1 on pin 6.   The 0V of the PP3 and the four alkaline batteries come in on SK2 pins 1 and 2 respectively.  They are combined, and also routed to SK1 pin7.

As you may have noticed, the circuitry around motor M1 is exactly the same round M2.  This is because there are two H-Bridge circuits - one for each motor.  I will explain the operation of the circuit around M1.

Q11, Q12, Q7 and Q8 form our H-Bridge circuit.  If you compare this to the diagram above, these correspond to switches A1, B1, B2 and A2 respectively.  If Pin 3 of J1 is taken high, this voltage is passed to Q4 via resistor R29.  As Q4 is an NPN transistor, with +6V on the base, the voltage at the collector will be low, and that at the emitter will be high.  This will turn on transistors Q12 and Q7 (Q12 is a PNP transistor whose collector voltage goes low when its base is low).  This will cause the motor to turn, as current now passes through the windings.  In the way the motor is mounted on Cybot, this will be in the forward direction.

If a positive voltage is applied to SK1 pin 4, this will turn Q3 on, making the collector low and the emitter high.  This will turn on Q8 and Q11.  Current will flow through the transistors, and the motor will now spin in the opposite direction i.e. backwards.

If we put both pin3 and pin 4 high, then the motor will stop dead, acting as a brake to Cybot. 

R33 is there to limit the current into Q11, likewise with resistor R32.

C15 across the motor is to stop any interference coming back onto the electronics boards.  All motors produce interference, as a result of the commutator making and breaking the electrical supply when the motor turns.

And that is about it.  By the way, you may have already guessed, that the motor test board is simply connecting the AA batteries to SK1 pin 3 and pin 1, which allows the motors to turn  forward all the time.

 

The 9V Circuitry

This part of the board has two functions.  First, it reduces the voltage of our 9V PP3 battery to +5V.  This is because the microcontroller used (either the PIC in the trial version or the Elan device in the current version) will not take more than +6V.  +5V is used to give a safety margin.  Its second function is to tell us the state of the +9V supply.  The green LED tells us it is on, and the red LED tells us that the voltage on our 9V battery is low.

Let us look at the circuit:

 

 

The +9V from our PP3 battery is fed into SK2 pin 3.  U1 is a 78L05 +5V regulator.  The letter L in its number tells us that it can only deliver a maximum of 100mA.  This IC ensures that no matter what the input voltage is (within reason), the output from this regulator is always +5V.  This regulator will be used to power the other boards from the 9V PP3 battery, by the way.  C14 and C13 are there to reduce any noise that is produced by the voltage regulator.  Voltage regulators inherently produce noise, which is due to their design.  The +5V is fed off the board to J1 pin 5, via R23, which will limit our current to just 50mA (5/100 = 0.05A).

Q15, is biased by resistors R26 and R27.  When our supply voltage is +9V, VBE is about 0.96V.  This is enough voltage to turn Q15 on.  As a result, the collector of Q15 goes low, and the red LED does not light up.  If our supply rail drops to below +6V, Q15 will turn off, as VBE will be less than 0.65V.  This will cause the collector to go high, and hence turn the red LED on.  The green LED will always be on while there is a supply voltage.  R25 and R24 limit the current going through our LEDs to a maximum of 3mA.

So, as you can see, the green LED tells us that Cybot is on, and the red LED tells us when to change the PP3 battery.  But why tell us at +6V?  Well, the +5V regulator (U1) will not work below +6V - simple, really.