Parking Meter
A relatively simple project that my lab partner and I designed. We created a parking meter prototype. The objective of this project was to use a finite state machine to model the behaviors of any device in our daily life. The project was intended to familiarize us with the process of applying logic and states to produce a desired output using a finite state machine.
Circuit Videos
The circuit simulation can be seen in operation with the output being the red LED. The functioning circuit can be seen with the inputs on the left and the LED’s on the right representing the states and the red LED as the output.
Circuit Simulation in Logisim
Functioning Circuit
State Diagram
For our state machine we needed four states so we settled on the four states idle, on, ready, and time. The machine will need to be turned on, for efficiency, to go from idle to on. While in the on state the customer would be using the parking meters point of sale which will send a $ input to the machine, after the sale, moving the state machine to the ready state. The ready state will require the enter input, which moves the machine to the time state. In the time state the machine outputs a 1. This machine was conceptualized to allow the user to add more time with a $ input returning the machine to ready state, otherwise the machine will return to idle. The state diagram can be seen below.
State Transition Table
The state transition table is a direct relation of the encoded states and the inputs to the finite state machine and the state diagram. The inputs are labeled 00(Reset), 01($), 10(Enter), and 11(On). The states are labeled 00(Idle), 01(On), 10(Ready), and 11(Time). This is what we used to create the output and next state truth table.
Encoded States, Inputs, and Outputs
The tables below represent the arbitrary encoding of our states and inputs.
Output and Next-State Truth Tables
This is our complete next-state truth table, which is what we used below to create the three k-maps needed to finish this circuit.
Output and Next-State Logic Equations
The logic equations were created below in three separate images of k-maps from the previous truth table. The first image below is the S1’ which is the input to the first flip-flop S1. The second image is the S0’ which is the input to the second flip-flop S0. The third image is the k-map for the output of the two flip-flops, which is the final output of the finite state machine.
Designed Circuit
The circuit was initially designed in Logisim, but we quickly built it into the breadboard which is shown below. There are four switches shown to the left which are from the top to the bottom are the inputs Reset, $, Enter, and On. The switch to the right is a clock input switch we used to help simulate this design. The three LED’s on the bottom right are the output (red), S1 output (blue), and S0 output (yellow). The blue and yellow lights were mostly for troubleshooting, but they illustrated the state's transfer pretty well so we kept them in the prototype.
