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Build the machine around a gravity-fed hopper and a mechanically constrained one-item dispenser. For a first version, use a pushbutton to trigger a positional servo and sliding pusher; add tokens, coins, displays, or multiple candy types only after the mechanism dispenses reliably.
This is a tabletop prototype, not a commercial vending machine. Its reliability depends more on candy dimensions, hopper geometry, and power delivery than on complex Arduino code.
How the machine works
Button / token / coin
↓
Arduino
↓
Servo or stepper → dispenser mechanism
↓
Candy chute
↓
Optional delivery sensor → ArduinoCandy rests in a hopper. Gravity moves the stack toward a retaining ledge, while a servo-driven gate or pusher releases the bottom item. The item falls into a chute, and the actuator returns to its home position.
A sliding pusher is the most practical general-purpose design. SparkFun documented a similar gravity-fed chocolate dispenser with a servo-driven sliding mechanism and first prototyped it in cardboard: see the project documentation.
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Choose the candy first
Do not design for “any candy.” Measure a particular item and define its maximum width, thickness, length, weight, wrapper friction, and tendency to bridge or rotate.
| Candy | Suitability | Main problem |
|---|---|---|
| Gumballs | Excellent | Needs a compatible pocket wheel or chute |
| Uniformly wrapped bars | Good | Wrapper friction and hopper clearance |
| Wrapped hard candy | Moderate | Irregular shapes can jam |
| Loose small candy | Poor | Several pieces may fall together |
| Sticky or heat-sensitive candy | Poor | Adhesion and deformation |
| Large candy bars | Possible | Needs a strong pusher or spiral |
Make the dispensing channel only slightly wider than the chosen item and prevent two items from passing side by side. A “one piece” claim is meaningful only for the dimensions and mechanism you have tested.
Recommended parts
Required
- Arduino UNO-compatible board
- 180-degree positional hobby servo
- Separate regulated 5 V supply sized for the servo
- Pushbutton, jumper wires, and a breadboard or prototyping board
- Hopper, retaining ledge, pusher channel, and chute
- Rigid construction material such as acrylic, plywood, foam board, cardboard for a prototype, or suitable 3D-printed parts
The Arduino UNO R4 Minima is a suitable current choice: Arduino lists its UNO form factor and 5 V operation, with broad hardware compatibility with UNO R3 accessories. AVR-specific libraries or code may still require changes. An UNO R3 also works with conventional Servo-library sketches. Prices and availability change, so check the official store before buying.
Optional
- IR break-beam sensor for delivery confirmation
- Status LED and buzzer
- Token or coin acceptor
- Keypad and LCD or OLED display
- Second actuator for another candy column
Select the dispensing mechanism
Sliding pusher: best first build
- The bottom candy rests on a retaining ledge.
- The servo drives a flat paddle forward.
- The paddle pushes one item into the exit chute.
- The servo retracts and the next item settles into position.
This works best with small packets or bars of consistent size. Use a short, guided pusher rather than relying on the servo horn alone.
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Rotating pocket wheel
A wheel with a pocket captures one gumball or similarly uniform item and rotates it to the outlet. It provides positive metering, but the pocket must match the candy closely. SparkFun’s gumball project used a continuous-rotation servo to turn the original machine’s dispensing cog.
Spiral coil
A rotating helical coil gives a familiar vending-machine appearance and suits uniform packages, especially in multiple columns. It is harder to fabricate accurately and can leave an item partly supported at the edge. Use a stepper or geared DC motor with the appropriate driver and position or end-stop feedback; a standard positional servo is usually not the right actuator.
Choose the actuator
| Actuator | Best use | Trade-off |
|---|---|---|
| 180° positional servo | Pusher, flap, or gate | Simple, but limited travel |
| Continuous-rotation servo | Pocket wheel or cog | Controls speed and direction, not exact angle |
| Stepper motor | Indexed wheel or coil | Repeatable, but needs a driver and homing |
| Geared DC motor | Coil or conveyor | Powerful and inexpensive, but needs feedback |
As SparkFun explains, a positional servo moves to a commanded angle, while a continuous-rotation servo responds mainly with speed and direction. A specific high-torque continuous-rotation servo, for example, is specified by SparkFun for 4.8–6.0 V operation and up to 66.7 oz-in torque at 6 V; those figures do not apply to every servo.
Build the hopper and chute
- Measure the candy envelope. Record the largest item dimensions and allow enough clearance for wrappers without allowing double feeds.
- Prototype in cardboard. Build only the hopper, retaining ledge, pusher channel, and chute. Test 10–20 pieces manually before making the finished enclosure.
- Guide the pusher. It should travel straight and expose only the bottom item.
- Provide a real home position. The pusher must retract far enough for the next item to settle. A physical guide or stop is preferable to software timing alone.
- Mount the servo rigidly. A flexible mount changes the travel distance and wastes torque. Hot glue is acceptable for an early prototype, but use brackets or screws for repeated operation.
- Make refilling and clearing easy. Include a removable lid or access panel and avoid hidden cavities where wrappers can accumulate.
Wire the Arduino safely
| Function | Pin |
|---|---|
| Servo signal | D9 |
| Dispense button | D2 |
| Optional delivery sensor | D3 |
| Status LED | D13 |
| Optional buzzer | D8 |
5 V regulated supply positive ─── Servo V+
5 V regulated supply ground ─── Servo GND
Arduino GND ─── Common ground
Arduino D9 ─── Servo signal
Arduino D2 ─── Button terminal 1
Arduino GND ─── Button terminal 2Use INPUT_PULLUP, so the button reads HIGH when idle and LOW when pressed. Do not make the Arduino’s 5 V pin the default power source for a mechanically loaded servo. Use a regulated external supply, connect its ground to Arduino ground, and use short, suitably thick motor wiring. If motion causes resets, add a large electrolytic capacitor near the servo supply and correct the power system rather than simply increasing software delays.
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A bare DC motor or stepper motor must use a suitable driver. Never connect either directly to an Arduino GPIO pin.
Upload a first servo test
#include <Servo.h>
Servo dispenser;
const byte SERVO_PIN = 9;
const int HOME_ANGLE = 10;
const int PUSH_ANGLE = 80;
void setup() {
dispenser.attach(SERVO_PIN);
dispenser.write(HOME_ANGLE);
}
void loop() {
dispenser.write(PUSH_ANGLE);
delay(450);
dispenser.write(HOME_ANGLE);
delay(1000);
}Run this before attaching the pusher. The angles are starting values, not universal settings. Move the servo through safe positions, then adjust the linkage in small increments. Never command the servo to fight a hard stop for an extended period.
Add the dispense button
#include <Servo.h>
Servo dispenser;
const byte SERVO_PIN = 9;
const byte BUTTON_PIN = 2;
const int HOME_ANGLE = 10;
const int PUSH_ANGLE = 80;
bool busy = false;
void setup() {
dispenser.attach(SERVO_PIN);
dispenser.write(HOME_ANGLE);
pinMode(BUTTON_PIN, INPUT_PULLUP);
}
void loop() {
if (!busy && digitalRead(BUTTON_PIN) == LOW) {
busy = true;
dispenseOne();
delay(250);
busy = false;
}
}
void dispenseOne() {
dispenser.write(PUSH_ANGLE);
delay(450);
dispenser.write(HOME_ANGLE);
delay(650);
}The busy flag prevents a second request during the cycle, while the final delay provides basic debounce and settling time. Tune both delays to the mechanism. A production-quality prototype should replace the blocking delays with a non-blocking state machine, but this guarded sketch is adequate for initial mechanical testing.
Tune one-item dispensing
Test in stages:
- Run 20–50 empty cycles. Check alignment, binding, servo temperature, loose screws, and Arduino resets.
- Test one candy at a time.
- Test a partially filled hopper.
- Only then test a completely loaded hopper.
Adjust servo angles, pusher thickness, hopper slope, chute width, travel distance, and settling pauses. If two items emerge, narrow the throat or add a retaining ledge. If nothing emerges, do not keep increasing servo force: check for bridging, insufficient slope, an undersized opening, sticky wrappers, or a slipping horn.
Add delivery confirmation
Mount an IR break-beam sensor across the delivery chute, after the item leaves the hopper. The emitter and receiver must be aligned so the candy interrupts the beam. Adafruit describes this arrangement in its break-beam sensor documentation.
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A robust cycle is:
Request received
→ actuator moves
→ actuator returns home
→ item detected
→ vend completeIf the beam is not interrupted within a timeout, stop and report a jam or empty hopper. Do not automatically dispense another item, particularly if the machine has accepted a coin or token. Detection can vary with alignment, distance, ambient light, and wrapper material; shield the sensor and test it with the actual candy.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Add tokens or coins
A token input preserves the vending-machine experience without the accounting and safety problems of real money. A coin-operated version should implement a transaction state:
IDLE
→ coin detected
→ credit increased
→ selection accepted
→ credit checked
→ dispense
→ delivery confirmed
→ credit deducted
→ IDLEMany coin acceptors output pulses, but pulse count, timing, voltage level, accepted-coin programming, and wiring vary by model. The Arduino Forum example illustrates pulse counting; it is not a universal wiring specification.
Handle coins inserted during a vend, multiple pulses from one coin, invalid coins, stuck acceptors, remaining credit, power loss, and no-change operation. Counting pulses is not the same as validating payment, issuing refunds, or protecting a customer from a failed vend. For an unattended public machine, payment, electrical, accessibility, consumer-protection, and food-handling requirements need separate review.
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Expand to multiple candy types
For four selections, the simplest arrangement is four separate hoppers and four servos. It is straightforward to debug but increases wiring, power requirements, and software states.
Other options include four helical coils with motor drivers, a mechanically indexed selector, or a rotating carousel with position sensing. The larger Arduino-documented DIY vending machine used multiple storage spaces, continuous-rotation servos, a stepper-driven product mechanism, and an elevator, demonstrating how quickly a multi-product build becomes more complex: Arduino’s project overview.
Troubleshooting
Two candies dispense
- Narrow the chute and opening.
- Add a retaining ledge or pocket wheel.
- Reduce pusher travel.
- Restrict the machine to a more uniform candy.
No candy dispenses
- Look for hopper bridging.
- Check pusher travel and servo-horn attachment.
- Increase the hopper slope modestly.
- Remove sticky or oversized items.
- Redesign the throat instead of forcing the actuator.
The Arduino resets
- Use a separate regulated servo supply.
- Connect grounds together.
- Inspect connectors and wiring.
- Add local supply capacitance.
- Size the supply for startup and stall current, not average current.
The servo chatters
The mechanism may be forcing the servo against a stop, the supply may be unstable, or the commanded angle may be too close to its limit. Reduce the angle, improve alignment, add compliance, and move the physical stop.
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Continuous-rotation servos differ in their neutral setting. Calibrate the actual stop command for your unit rather than assuming a nominal center value.
Useful upgrades
- OLED or LCD selection display
- Buzzer and status LEDs
- RFID or supervised token input
- Low-stock sensor
- Stepper-driven indexed carousel
- Wi-Fi monitoring with an UNO R4 WiFi
- Removable, cleanable chute
The UNO R4 WiFi is useful when remote status or web control is genuinely needed, but connectivity adds software scope and is unnecessary for a standalone dispenser. A basic button-operated machine is easier to test and safer to demonstrate.
Quick Recap
Safety and construction limits
- Guard moving linkages and keep fingers away from pinch points.
- Insulate electrical connections and provide strain relief.
- Use a fuse or current-limited supply for larger installations.
- Remove wrapper debris and make refill and cleaning access easy.
- Do not describe unfinished plywood, hot glue, or an arbitrary 3D-printing filament as food-safe.
- Do not operate an unattended real-money machine without appropriate compliance review.
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