The approach we took to the electrical design component of our robot emphasized simplicity and ease of use. We minimized the number of different types of circuits on the robot and made these easily replaceable. 



The most difficult circuits to create were the H-bridges. Our TINAH (custom shield using a Wiring microcontroller) being unable to directly provide the power necessary to run our motors, we used three of these to power the two rear wheels as well as the shovel lifter. H-bridges are circuits used to deliver large amounts of current to motors, while using relatively weak signals to control the flow of this current. An example of the H-bridges we created is shown on the right. We made these modular so that we could quickly and easily replace broken H-bridges. The circuit diagram for the H-bridges we used is shown below.



The TINAH board can simulate analog output signals by using what is called Pulse-Width-Modulation (PWM). With the PWM outputs, we could control the speed of our motors with a resolution of 1/1024. In other words, we had 1024 different speeds, around 512 in each direction. 

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PWM functions by sending binary signal pulses (high or low) at different rates and for different amounts of time through a component. The effect of this is shown in the picture on the bottom right.



The TINAH did not actually output a signal fluctuating between ground and five volts as depicted to the right. Rather, it would fluctuate between ~4 to 9 volts during the 'on' periods, and ground to ~4 volts during the 'off' periods. This shift up forced us to use a comparing circuit to analyze the signals input to the H-bridges in order to determine which way to run the current. We did this by using an LM393N dual comparator integrated circuit (seen on the right in the H-bridge picture). We linked each of two corresponding TINAH PWM outputs to the non-inverting inputs of the comparator and a 5 volt reference from the TINAH to its inverting-inputs. The result was an output signal that would pulse between 0 and 15 volts (the chip was powered by a 15 volt supply) in accordance with the PWM signals.



The remainder of our circuits are trivial, consisting mainly of switches and phototransistors (QRD). In making these, we also planned for ease of wiring and replacing broken components. We collected all of the similar signal lines (eg: push buttons or QRDs) to nodes in the front of the robot from which we ran ribbon cables to the TINAH board inputs. Should the TINAH or a single component break, we would never need to remove more than a few cables.

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The picture to the right shows the wiring of our robot. The TINAH is the board in the centre with the LCD. Its inputs and outputs are on the far side, underneath the servos. From these ports, ribbon cable runs to the front of the robot where our sensing circuit cables are grouped together. The three H-bridges are grouped together on the near side of the robot.