🥤 6-Choice Vending Machine — Pure TTL Hardware Implementation

A fully functional, hardware-implemented juice vending machine designed and built from scratch using pure TTL discrete logic ICs (74-series). This project contains no microcontroller, no microprocessor, and no programmable logic devices (such as FPGAs, CPLDs, or EEPROMs). All state, timing, arithmetic, validation, and display decoding are driven entirely by a hardwired network of 50+ discrete logic integrated circuits.

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📖 Table of Contents

• What This Project Does
• System Schematic
• Technical Specifications
• System Architecture
• Deep-Dive Logic Implementations
  • 1. Stock Storage & Inventory Management
  • 2. Input Currency Register
  • 3. Change Calculation & Discount ALU
  • 4. Binary to BCD Conversion & Displays
• Simulation & Validation
• License

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💡 What This Project Does

In modern system design, complex state control and arithmetic are offloaded to microcontrollers. This project steps back to classical hardware engineering:

• Designing a multi-state machine entirely out of logic gates.
• Managing stock and inventory using discrete D Flip-Flops.
• Calculating dynamic transaction variables (accumulated money, price comparisons, discounts, and change) using hardwired full-adder arrays and magnitude comparators.
• Converting binary numbers to Binary Coded Decimal (BCD) for real-time output on dual 7-segment displays.

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🗺️ System Schematic

Below is the complete engineering schematic of the vending machine's digital logic design, built and simulated in CircuitMaker:

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📊 Technical Specifications

Feature / Metric	Specification	Engineering Implementation
Product Selection	6 Unique Combinations	3 Flavors (Apple, Orange, Kiwi) × 2 Sizes (Large, Small)
Inventory Storage	6-bit Discrete Latches	6 × D Flip-Flops (74LS74) for per-item out-of-stock storage
Cash Input Channels	4 Currency Inputs	Pushbuttons representing 1 AED, 2 AED, 5 AED, and 10 AED
Input Capacity Cap	15 AED Maximum	Hardwired 4-bit binary comparator block
Arithmetic Precision	8-bit Two's Complement	5 × 4-bit Binary Full Adders (74LS283) + 74LS86 XOR gates
Conditional Coupon	2 AED Automatic Discount	Triggers when the transaction purchase total exceeds 10 AED
Display Subsystem	Dual BCD 7-Segment Displays	Real-time transaction change display using dual BCD decoders (74LS248)
User Controls	Global Reset & Buy Button	Edge-triggered debounce logic with discrete sequential flags

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⚙️ System Architecture

The entire machine operates as a hardware-level data path managed by combinational pricing networks and sequential control logic. The block diagram below illustrates how signals propagate through the system:

graph TD
    %% Inputs
    SubGraph_Inputs["User Interface / Inputs"]
    Selection["Flavor & Size Switches"] -->|Comb. Logic| PriceGen[Price Generation Logic]
    CashInput["Cash Input Buttons: 1, 2, 5, 10 AED"] -->|Shift Reg / Register| CashReg[Cash Accumulator Register]
    BuyBtn[Buy Button] -->|Edge Trigger| Sequencer["Timing & Sequencing Control"]
    ResetBtn[Global Reset] -->|Asynchronous| Inventory["Stock Storage: 7474 D Flip-Flops"]

    %% Price and Validation
    PriceGen -->|Selected Price| CompEnough[7485 Magnitude Comparator]
    CashReg -->|Total Cash| CompEnough
    CompEnough -->|Cash >= Price| Sequencer
    CompEnough -->|Cash < Price| LedNotEnough["Not Enough $ LED"]

    %% Arithmetic & Discount
    CashReg -->|Total Cash| ALU["Arithmetic & Change ALU: 74283 & 7486"]
    PriceGen -->|Selected Price| ALU
    CashReg -->|Spend Check| CompDiscount[7485 Coupon Comparator]
    CompDiscount -->|Spend > 10 AED| ALU

    %% BCD Conversion & Displays
    ALU -->|Binary Change| ChangeComp[7485 BCD Comparator]
    ChangeComp -->|Change >= 10| BCDSub["Sub-10 Arithmetic Block"]
    BCDSub -->|Units: 0-9| Dec1["BCD Decoder 1: 74LS248"]
    ChangeComp -->|Tens Digit: 1| Dec2["BCD Decoder 2: 74LS248"]
    Dec1 --> Disp1["7-Segment Display: Units"]
    Dec2 --> Disp2["7-Segment Display: Tens"]

    %% Dispensing & Inventory
    Sequencer -->|Decrement Stock| Inventory
    Inventory -->|Stock Status LEDs| LedDispense["Vended LED / Item LEDs"]

    classDef inputs fill:#1a1a2e,stroke:#3b5360,stroke-width:2px,color:#fff;
    classDef logic fill:#0f3460,stroke:#16c79a,stroke-width:2px,color:#fff;
    classDef outputs fill:#162447,stroke:#e94560,stroke-width:2px,color:#fff;
    class Selection,CashInput,BuyBtn,ResetBtn inputs;
    class PriceGen,CashReg,Sequencer,CompEnough,ALU,CompDiscount,ChangeComp,BCDSub,Inventory logic;
    class LedNotEnough,Dec1,Dec2,Disp1,Disp2,LedDispense outputs;

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🔬 Deep-Dive Logic Implementations

1. Stock Storage & Inventory Management

• Hardware: Implemented using six 74LS74 D-Type Flip-Flops (one for each product flavor and size).
• Operation:
  • The flip-flops act as 1-bit stock registers where a logical high (1) represents "in stock" and logical low (0) represents "out of stock".
  • When an item is vended, a clock pulse derived from the sequential control sequencer triggers the selected flip-flop, latching a 0 to indicate out-of-stock.
  • The status is output directly to individual product LEDs on the front panel.

2. Input Currency Register

• Hardware: Built using a cascaded array of 4-bit bidirectional registers.
• Operation:
  • Pushbuttons on the user interface capture bill insertion.
  • An edge-triggered debounce network filters mechanical button noise.
  • Once validated, the corresponding value (1, 2, 5, or 10) is added to the total accumulated cash register.
  • An over-capacity comparator blocks input if the total exceeds 15 AED.

3. Change Calculation & Discount ALU

• Hardware: Implemented using 74LS283 4-bit Binary Adders and 74LS86 Quad XOR gates configured for subtraction.
• Operation:
  • Performs 8-bit Two's Complement subtraction: $\text{Change} = \text{Total Cash} - \text{Price}$.
  • A parallel magnitude comparator (74LS85) constantly checks if $\text{Total Cash} \ge \text{Selected Price}$.
  • Discount Logic: If the selected purchase exceeds 10 AED, a separate comparator routes the subtraction path through a discount offset, automatically subtracting 2 AED from the final price.

4. Binary to BCD Conversion & Displays

• Hardware: Custom logic gates combined with dual 74LS248 BCD-to-7-segment decoders.
• Operation:
  • Converts binary change values (0–15) to BCD.
  • A threshold comparator checks if $\text{Change} \ge 10$. If true, it subtracts 10 from the units digit and outputs a 1 to the tens-digit 7-segment display.
  • Otherwise, the tens digit remains blank, and the unit digit is routed directly to the unit display.

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⚡ Simulation & Validation

The design has been validated for logical correctness in CircuitMaker under multiple simulated use cases:

1. Underpayment Block: Selecting an item and pressing buy with insufficient funds correctly activates the Not Enough Money LED without dispensing.
2. Dynamic Stock Depletion: Buying the last available juice correctly transitions the storage flip-flop, lighting up the Out of Stock LED and blocking future selections.
3. Discount Calculations: Buying a combo costing 12 AED with a 10 AED and 5 AED bill correctly displays 5 AED change (applying the 2 AED coupon discount).

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📄 License

This repository is licensed under the CC0 1.0 Universal (CC0 1.0) Public Domain Dedication.