Published October 11, 2023 © CERN-OHL

Secret Payphone STEM Library

What if an unassuming payphone housed a tiny STEM library? Dial STEM to learn!

IntermediateShowcase (no instructions)20 hours1,344

Things used in this project

Hardware components

Arduino Nano R3

Solenoid Lock

DC-DC converter 12V to 5V

Mosfet switch module

Rotary potentiometer (generic)

SparkFun Snappable Protoboard

Solder Sleeve, Heat Shrinkable

Hand tools and fabrication machines

Soldering iron (generic)

Woodworking tools

Story

If you've been checking out Hackster recently, you probably know about the Little Free STEM Library contest. It's Avnet communities, Hackster and element14, coming together to invite you to build a Little Free Library for STEM resources! Well, Hackster challenged me to make an over the top tech enabled submission, and this project is the result!

Payphones occupy a special place in the societal consciousness. They represent nostalgia for simpler communication, and in the end, that’s what libraries represent as well. Communication between an author and the reader.

In the speakeasy library, the reader needs only one word of power to initiate an hours long conversation. A conversation that could be life changing. The word is STEM.

So I made the Payphone Secret STEM library! To access it, you just need to lift the phone and dial STEM (7836). The door will unlock and open up magically.

I started this project by getting hold of an old payphone from a laundromat! Although you can buy payphones on Amazon for pretty cheap as well, if you want to replicate the project.

It took a lot of elbow grease, sweat, and broken bits to get the payphone unlocked. Ended up drilling out the locks completely, and I was rewarded with about $3.25 in change and the guts of the electronics.

Routing out the payphone locks

Success!

The electronics exposed quite a few interesting things, most importantly the handset speaker(regular 32 ohm speaker), a switch that opens when the phone is taken off the hook, and a 4x3 numberpad in the regular matrix configuration so that I could input data from that into the Arduino Nano.

Reverse engineering payphone electronics

I proceeded to connect the electronics on a perf board to the Arduino Nano and programmed it to power a solenoid lock if someone dials in STEM(7836) on the numberpad. The code and schematics are attached.

Assembling custom electronics

Next came the mechanical build, which is essentially a box with the payphone on a hinged door. Inside the hinged door are three shelves to house books and electronics. The box is on a pedestal with screw on feet to adjust to uneven flooring.

PayPhone box

Everything is painted in chalkboard black paint so that the booth can be decorated easily. A fun classroom take on urban graffiti.

Painting!

The door has a latch that locks into the solenoid lock on the box. A small spring pushes on the door so that upon unlocking the door is automatically opened slightly.

Putting on the secret door

In case the electronics fail, there's a mechanical failsafe to open the door. A small hole is drilled into the solenoid lock, attached to a braided steel wire out of the box. Pulling on this wire unlocks the door so the electronics can be accesses and repaired.

Locking mechanism

The code has a cheeky addon, the Arduino Talkie library! It allows you to output pretty decent quality voice/music out of even the most underpowered chip like the ATmega328 on the Arduino Nano R3. The handset speaker asks you to input the password and welcomes you to the library on opening in my voice. And of course, there's also the dial and number pad tones.

Finally, everything was boxed into a crate for shipping. Next time you visit the Avnet HQ, look for the payphone for some free electronics goodies and books. Learn on!

Inspired? Be sure to submit your own design by October 31st, 2023!

Schematics

Code

#include <Keypad.h>

#include "Talkie.h"
#include "Vocab_Tinkrmind.h"
const int speaker1 = 3;
const int speaker2 = 11;

const byte ROWS = 4;  // Four rows
const byte COLS = 3;  // Three columns

char keys[ROWS][COLS] = {
  { '1', '2', '3' },
  { '4', '5', '6' },
  { '7', '8', '9' },
  { '*', '0', '#' }
};

byte rowPins[ROWS] = { 8, 10, A1, 7 };  // Connect to the row pinouts of the keypad
byte colPins[COLS] = { 5, 6, 12 };      // Connect to the column pinouts of the keypad

Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);

const int receiver = 4;    // Connect button 1 to this pin
const int doorSwitch = 2;  // Connect button 2 to this pin
const int solenoid = A0;

boolean button1State = false;
boolean button2State = false;
boolean lastButton1State = false;
boolean lastButton2State = false;

unsigned long lastDebounceTime = 0;
unsigned long debounceDelay = 50;

boolean sendValue = false;
unsigned long sendValueStartTime = 0;
const unsigned long sendValueDuration = 30000;  // 30 seconds

char last_key;
char password[] = { '7', '8', '3', '6' };
int password_success = 0;
int alertedListener = 0;
int offTheHook = 0;

void (*resetFunc)(void) = 0;  //declare reset function @ address 0

void keyPress() {
  digitalWrite(speaker2, LOW);
  tone(speaker1, 1300);
  delay(100);
  tone(speaker1, 440);
}

void dialTone(){
  digitalWrite(speaker2, LOW);
  tone(speaker1, 440);
}

void stopSpeaker(){
  digitalWrite(speaker2, LOW);
  noTone(speaker1);
}

void setup() {
  Serial.begin(115200);
  pinMode(receiver, INPUT_PULLUP);
  pinMode(doorSwitch, INPUT_PULLUP);
  pinMode(solenoid, OUTPUT);
  digitalWrite(solenoid, LOW);
}

// void loop(){
//   if(digitalRead(doorSwitch) == 0){
//     Serial.println("door switch alert");
//   }
//   if(digitalRead(receiver) == 1){
//     Serial.println("receiver alert");
//   }
// }

unsigned long offTheHookTime = millis();
int enterPasswordPrompt = 0;

void loop() {
  //check if receiver is on hook
  if (digitalRead(receiver) == 0 && offTheHook == 0) {
    delay(250);
    if (digitalRead(receiver) == 0) {
      // phone is off the hook
      Serial.println("Receiver off the hook");
      offTheHook = 1;
      offTheHookTime = millis();
      enterPasswordPrompt = 0;
      dialTone();
      alertedListener = 0;
      password_success = 0;
    }
  }

  if (digitalRead(receiver) == 1 && offTheHook == 1) {
    // receiver put back on
    Serial.println("Receiver back on");
    offTheHook = 0;
    stopSpeaker();
  }

  if (offTheHook) {
    if (millis() - offTheHookTime > 2000 && enterPasswordPrompt == 0) {

      enterPasswordPrompt = 1;
    }

    // Read keypad
    char key = keypad.getKey();
    if (key) {
      keyPress();
      if (key == password[password_success]) {
        Serial.println(key);
        password_success++;
      } else {
        password_success = 0;
      }
      if (password_success == 4) {
        Serial.println("Password correct");
        openDoor();
        password_success = 0;
      }
    }

    if (alertedListener == 0) {
      Serial.println("#");
      alertedListener = 1;
    }
    if (Serial.available()) {
      char c = Serial.read();
      if (c == '*') {
        Serial.println("User said Secret Stem");
        openDoor();
      }
    }
  }
}

void openDoor() {
  Serial.println("Unlocking door");
  digitalWrite(solenoid, HIGH);
  unsigned long start = millis();
  Talkie voice;
  voice.say(spWELCOMETOTHESTEMLIBRARY);
  voice.terminate();
  while (millis() - start < 3000 && digitalRead(doorSwitch) == 1) {
    //door not opened and time has been less than 10 sec
  }
  Serial.println("Re-locking door");
  digitalWrite(solenoid, LOW);
  dialTone();

  // while(digitalRead(receiver) == 0){
  // }
  // Serial.println("On the hook");
  // offTheHook = 0;
  // noTone(speaker);
}

// Talkie library
// Copyright 2011 Peter Knight
// This code is released under GPLv2 license.
//
// Amitabh Shrivastava(@tinkrmind) sounds for Hackster.io STEM Library Aug 2023
//

#include <Arduino.h>

extern const uint8_t spWHATSTHEPASSWORD[]    PROGMEM = {0xc0,0x3a,0x91,0x76,0xd0,0x2c,0x8a,0xe9,0x4d,0xd8,0x05,0xeb,0x31,0xa9,0x0d,0x7e,0x06,0xa3,0xc3,0x94,0x3a,0xf9,0x4c,0x9c,0xb4,0x6c,0xea,0x18,0xc3,0x69,0xcc,0x94,0xb8,0x42,0xc8,0xd2,0xb2,0x48,0x62,0x4b,0x26,0x4a,0x53,0x22,0x8b,0xb5,0x19,0x4b,0x2d,0x4b,0x22,0xbe,0x1e,0x23,0xb5,0x34,0x2e,0x7b,0x46,0x88,0x90,0x10,0x3b,0xe4,0x6f,0x56,0x5c,0x53,0xe2,0x84,0x3f,0x7a,0x73,0xa9,0x8a,0xad,0x26,0x1f,0xcc,0xb4,0x3a,0x34,0x7a,0x8c,0x37,0xe7,0x2a,0xd3,0xe8,0x71,0xc6,0x9c,0x33,0x23,0x8b,0x28,0x86,0x48,0x51,0x8b,0x5c,0x86,0x54,0xa2,0x54,0xb3,0x72,0x19,0x52,0x8f,0xa6,0x2e,0xd3,0xa9,0x49,0xc3,0x9b,0x26,0x2d,0xa7,0x22,0x76,0x4b,0x1a,0x17,0x6d,0xf2,0x98,0x3d,0x39,0xdd,0x94,0x2a,0x53,0xf0,0x14,0xd3,0xd8,0x22,0x2b,0x26,0xd2,0xc2,0x62,0xab,0xbb,0xa6,0x6a,0x77,0xb6,0x6d,0x8a,0x9a,0x27,0xcb,0xc4,0xae,0x2a,0x75,0xf0,0x0c,0xcf,0x4a,0xae,0xd1,0x31,0xca,0x3c,0x27,0xb9,0x21,0x04,0x4b,0xf5,0xba,0xec,0x86,0xe8,0x35,0x25,0x6a,0xb6,0xab,0xa3,0x93,0xd4,0xac,0x51,0xa6,0xc8,0x96,0x53,0x27,0x26,0xb3,0x3c,0x5b,0x4e,0xe9,0x6a};
// extern const uint8_t spWELCOMETOTHESTEMLIBRARY[]    PROGMEM = {0x80,0x1a,0xae,0x34,0xad,0xac,0x36,0x69,0xb4,0xd6,0x90,0x8c,0x3a,0xa1,0x88,0xd6,0x43,0x23,0x47,0x8f,0x2c,0xba,0x48,0xf5,0x3e,0x3d,0x92,0x14,0x22,0xd9,0xa7,0x4a,0x0b,0x63,0x8a,0xe4,0x9a,0xc8,0xc9,0xb7,0x25,0x93,0xb6,0x4d,0x85,0xc0,0x84,0x5e,0xcc,0x08,0xa5,0x02,0x6f,0x73,0xd8,0x3c,0x32,0x0b,0x42,0xf4,0xd0,0x0e,0xb1,0xc2,0x2f,0x8d,0x4d,0xd3,0xe2,0x04,0xaf,0x6c,0x0b,0x4d,0x8e,0x9b,0xd2,0xe0,0x25,0x2d,0xbb,0x72,0x2a,0x7c,0xb4,0xf0,0xec,0x4a,0xa6,0xb1,0x5e,0xd3,0xac,0x22,0xab,0xd6,0x1b,0x29,0xf5,0x4a,0xac,0x3a,0x6f,0xa5,0xd4,0x2b,0x96,0xe8,0x9d,0xd3,0x92,0xca,0x98,0xac,0x77,0x4e,0x4b,0x3a,0x2a,0x91,0x41,0x5b,0x1d,0x1d,0x0b,0x85,0x46,0x6e,0xb4,0xad,0xcc,0x12,0x9a,0xa2,0x60,0xd7,0xd0,0x24,0x00,0xe4,0xef,0xd9,0x8c,0xdc,0xed,0xb8,0xab,0x65,0x33,0xca,0x48,0x6c,0xea,0x60,0xa4,0xbc,0xdb,0x94,0x6a,0xbc,0xd1,0xd4,0x9a,0x50,0x64,0x14,0xd6,0xd2,0xaa,0x4d,0x8b,0x31,0x49,0x09,0xa9,0xac,0xec,0xfa,0x68,0x2c,0x65,0x26,0x92,0x19,0xa2,0xb3,0x90,0x0b,0xcb,0x6a,0x2c,0x4a,0x5d,0xd2,0x6b,0xab,0x7b,0x04,0x0b,0x09,0x89,0xa3,0x9e,0x91,0x32,0x35,0xd5,0x0a,0xf3,0x87,0x8f,0x54,0x93,0x38,0xc8,0x9b,0x52,0x8c,0x5d,0xed,0xb2,0x67,0xa6,0x70,0x71,0x8d,0xeb,0xca,0xee,0x55,0xa5,0xb2,0x6a,0xe9,0x5a,0xe0,0xd0,0x89,0xca,0x69,0x68,0x99,0x53,0xda,0xab,0x84,0xbe,0x44,0x29,0x6e,0xaf,0xac,0xea,0xe0,0xa5,0xb8,0x3c,0x8e,0x2a,0xad,0xd7,0xe2,0xb2,0xda,0x2e,0x8f,0x4a,0x5b,0x42,0x93,0x9a,0x2c,0x68,0x6b,0x71,0x4d,0x6b,0x72,0x13,0xa3,0xc5,0xbd,0x89,0x2b,0x4c,0xec,0x36,0xd3,0x39,0xa1,0x0c,0xd9,0x43,0xaa,0xc6,0x94,0x32,0x0d,0x4d,0x99,0x32,0xdb,0x8a,0x5c,0xb4,0xac,0x32,0x4c,0xcb,0x4b,0xe2,0xf4,0xac,0x32,0x29,0x8f,0x81,0x27,0x33,0xcb,0xa8,0xd2,0x05,0x19,0x37,0x15,0xcb,0x2a,0x1f,0x38,0xd3,0xc4,0xac,0xa8,0xab,0x15,0x97,0x94,0xda,0xa6,0x6f,0xd1,0x82,0x9d,0x92,0x99,0x21,0x45,0x0a,0xe9,0x68,0x6c,0xc6,0x94,0x28,0x65,0x75,0xb6,0x19,0x53,0xe2,0x90,0xd5,0xd9,0x66,0xca,0x99,0x9d,0x57,0x17,0x8b,0x37,0x27,0x76,0x5e,0x5b,0x2c,0x9e,0x9a,0x30,0x64,0x7d,0xb1,0x29,0x7b,0x25,0xa7,0xf5,0x27,0xee,0xed,0x45,0x58,0x26,0x6e,0xbb,0x67,0x44,0x61,0x9e,0xbc,0x6d,0xb2,0x11,0x99,0x65,0x6a,0x92,0x1a,0x7a,0x40,0xcd,0xa9,0x82,0xac,0xcf,0x8a,0x72,0x4a,0x4d,0x93,0x36,0x29,0x52,0x73,0xab,0x42,0xea,0xe1,0x54,0x35,0x2c,0x31,0xfa,0x47,0x50,0xd3,0x50,0xbb};


extern const uint8_t spWELCOMETOTHESTEMLIBRARY[]    PROGMEM = {0x80,0x1a,0xae,0x34,0xad,0xac,0x36,0x69,0xb4,0xd6,0x90,0x8c,0x3a,0xa1,0x88,0xd6,0x43,0x23,0x47,0x8f,0x2c,0xba,0x48,0xf5,0x3e,0x3d,0x92,0x14,0x22,0xd9,0xa7,0x4a,0x0b,0x63,0x8a,0xe4,0x9a,0xc8,0xc9,0xb7,0x25,0x93,0xb6,0x4d,0x85,0xc0,0x84,0x5e,0xcc,0x08,0xa5,0x02,0x6f,0x73,0xd8,0x3c,0x32,0x0b,0x42,0xf4,0xd0,0x0e,0xb1,0xc2,0x2f,0x8d,0x4d,0xd3,0xe2,0x04,0xaf,0x6c,0x0b,0x4d,0x8e,0x9b,0xd2,0xe0,0x25,0x2d,0xbb,0x72,0x2a,0x7c,0xb4,0xf0,0xec,0x4a,0xa6,0xb1,0x5e,0xd3,0xac,0x22,0xab,0xd6,0x1b,0x29,0xf5,0x4a,0xac,0x3a,0x6f,0xa5,0xd4,0x2b,0x96,0xe8,0x9d,0xd3,0x92,0xca,0x98,0xac,0x77,0x4e,0x4b,0x3a,0x2a,0x91,0x41,0x5b,0x1d,0x1d,0x0b,0x85,0x46,0x6e,0xb4,0xad,0xcc,0x12,0x9a,0xa2,0x60,0xd7,0xd0,0x24,0x00,0xe4,0xef,0xd9,0x8c,0xdc,0xed,0xb8,0xab,0x65,0x33,0xca,0x48,0x6c,0xea,0x60,0xa4,0xbc,0xdb,0x94,0x6a,0xbc,0xd1,0xd4,0x9a,0x50,0x64,0x14,0xd6,0xd2,0xaa,0x4d,0x8b,0x31,0x49,0x09,0xa9,0xac,0xec,0xfa,0x68,0x2c,0x65,0x26,0x92,0x19,0xa2,0xb3,0x90,0x0b,0xcb,0x6a,0x2c,0x4a,0x5d,0xd2,0x6b,0xab,0x7b,0x04,0x0b,0x09,0x89,0xa3,0x9e,0x91,0x32,0x35,0xd5,0x0a,0xf3,0x87,0x8f,0x54,0x93,0x38,0xc8,0x9b,0x52,0x8c,0x5d,0xed,0xb2,0x67,0xa6,0x70,0x71,0x8d,0xeb,0xca,0xee,0x55,0xa5,0xb2,0x6a,0xe9,0x5a,0xe0,0xd0,0x89,0xca,0x69,0x68,0x99,0x53,0xda,0xab,0x84,0xbe,0x44,0x29,0x6e,0xaf,0xac,0xea,0xe0,0xa5,0xb8,0x3c,0x8e,0x2a,0xad,0xd7,0xe2,0xb2,0xda,0x2e,0x8f,0x4a,0x5b,0x42,0x93,0x9a,0x2c,0x68,0x6b,0x71,0x4d,0x6b,0x72,0x13,0xa3,0xc5,0xbd,0x89,0x2b,0x4c,0xec,0x36,0xd3,0x39,0xa1,0x0c,0xd9,0x43,0xaa,0xc6,0x94,0x32,0x0d,0x4d,0x99,0x32,0xdb,0x8a,0x5c,0xb4,0xac,0x32,0x4c,0xcb,0x4b,0xe2,0xf4,0xac,0x32,0x29,0x8f,0x81,0x27,0x33,0xcb,0xa8,0xd2,0x05,0x19,0x37,0x15,0xcb,0x2a,0x1f,0x38,0xd3,0xc4,0xac,0xa8,0xab,0x15,0x97,0x94,0xda,0xa6,0x6f,0xd1,0x82,0x9d,0x92,0x99,0x21,0x45,0x0a,0xe9,0x68,0x6c,0xc6,0x94,0x28,0x65,0x75,0xb6,0x19,0x53,0xe2,0x90,0xd5,0xd9,0x66,0xca,0x99,0x9d,0x57,0x17,0x8b,0x37,0x27,0x76,0x5e,0x5b,0x2c,0x9e,0x9a,0x30,0x64,0x7d,0xb1,0x29,0x7b,0x25,0xa7,0xf5,0x27,0xee,0xed,0x45,0x58,0x26,0x6e,0xbb,0x67,0x44,0x61,0x9e,0xbc,0x6d,0xb2,0x11,0x99,0x65,0x6a,0x92,0x1a,0x7a,0x40,0xcd,0xa9,0x82,0xac,0xcf,0x8a,0x72,0x4a,0x4d,0x93,0x36,0x29,0x52,0x73,0xab,0x42,0xea,0xe1,0x54,0x35,0x2c,0x31,0xfa,0x47,0x50,0xd3,0x50,0xbb,0xf0};

Credits

Amitabh Shrivastava

2 projects • 4 followers

Physicist turned Inventor

Secret Payphone STEM Library

Things used in this project

Hardware components

Hand tools and fabrication machines

Story

Custom parts and enclosures

CAD

Schematics

Schematic

Code

Code

Vocab_Tinkrmind.h

Vocab_Tinkrmind.cpp

Credits

Amitabh Shrivastava

Comments

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Secret Payphone STEM Library

Secret Payphone STEM Library

Things used in this project

Hardware components

Hand tools and fabrication machines

Story

Custom parts and enclosures

CAD

Schematics

Schematic

Code

Code

Vocab_Tinkrmind.h

Vocab_Tinkrmind.cpp

Credits

Amitabh Shrivastava

Comments

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