Detect amplifier keying line (PTT) with Arduino

Use an Arduino to detect transmit/receive state.

Using an Arduino Uno to detect the transmit state of a radio. The amplifier keying line (PTT) is grounded on transmit. The Arduino sees it as a switch being pulled low. The keying line is connected to digital input 2 (D2) and ground. The digital input uses the builtin pullup resistor. The sketch is the Arduino digital | input pullup example. Max polls the serial data from the Arduino and displays the state of the keying line.

Remote controlled shortwave radio system

Under construction…

The first in a series describing a system for internet remote control of a shortwave radio station. Its not something new. There are commercial products that provide remote operation of amateur radio transceivers. The purpose of this project is to make it possible to use shortwave radio sounds in musical performance, without the need of an antenna system.


  • Max/MSP for USB serial control of radio, OSC remote interface, user interface, Midi device handling, and an SQLITE database of preset frequencies.
  • Low latency, good quality audio using Soundjack by Alex Carot.
  • Hardware control of radio using Midi controllers (CDJ-101 and Launchpad)
  • Bi-directional OSC and VOIP using Logmein Hamachi VPN
  • Additional hardware control of AC power and antenna selection using Arduino and a WeMo switch.
  • TouchOSC Ipad audio mixer control using MOTU Cuemix
  • TeamViewer remote desktop software for logging into to base station compuer
  • Optional radio user interface control with Ipod TouchOSC, Griffin Powermate dial, and Korg Nano-kontrol.
  • Optional VOIP backup using Mumble.

System diagram

base station:


remote control:








Convert photocell data to MIDI

With Arduino and Max.

Screen Shot 2015-04-26 at 7.27.20 PM

An update to the basic Arduino/Max patches:

Replace the potentiometer in the “Arduino Serial Read” project with a photocell (LDR: Light dependent resistor)  and a 10K pulldown resistor, wired as shown in the image above and explained here: (from Adadfruit)

The “Analog” lead represents the center terminal of a potentiometer and connects to A0.

The Arduino sketch is the example sketch: Analog | analogInOutSerial


Screen Shot 2015-04-26 at 7.50.42 PM

folder: arduino-basics

patch: arduino-serial-read-midi.maxpat

For instructions and circuit, refer to “Arduino Serial Read” project:

Arduino electric eye and musical stairs

Summary of experiments with IR beam detection.

Initial testing with Radio Shack sensors and IRremote library:

musical stairs project

Built by students at Gould Academy, Bethel, Maine 2013.

Detects movement on stairs using individual IR sensor pairs on each step. When an IR beam is broken, a note is triggered and status LED lights up. Using an Ethernet shield, the data is tracked in a feed at


Adafruit IR emitters and receivers

construction of sensor units

IR transmitters and receivers wired into terminal strips (no soldering):

transmitting unit:


receiving unit:


layout of stairs

Arduino connections and code


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(local file: musicalStairsVersion3tz2)


AM audio transmitter using Arduino

Transmit AM audio on 727 KHz. using a voltage divider and an Arduino.

By Markus Gritsch

Note: with the actual circuit the signal is closer to 760 KHz.

Samples incoming audio at 34 KHz. and rebroadcasts as RF using the Arduino clock.

This is a simplification of Markus’ original circuit. It eliminates the tuned output circuit. Probably at the expense of increased harmonic distortion.

The voltage divider is uses 2 47K Ohm resistors and a 1 uF electrolytic capacitor. It is the input half of the original circuit.

(I substituted 2 100 K Ohm resistors in parallel for each of the 47 K’s.)

A voltage divider is useful as a general purpose coupler, for sampling analog signals using  Arduino PWM input.

The Arduino sketch is Markus’ original – reprinted here:

(note) Local file is AM_audio_transmitter.

// Simple AM Radio Signal Generator :: Markus Gritsch
// /|\ +5V ANT
// | \ | /
// | −−−−−−−−−−−−−−−− \|/
// | | R1 | Arduino 16 MHz | C2 |
// | | 47k | | || | about
// audio C1 | | | TIMER_PIN >−−−−−||−−−−−+ 40Vpp
 // input || | | | || |
 // o−−−−−||−−−−−+−−−−−−−> INPUT_PIN | 1nF |
 // +|| | | | )
 // 1uF | | R2 | ATmega328P | ) L1
 // | | 47k −−−−−−−−−−−−−−−− fres = ) 47uH
 // fg < 7 Hz | | 734 kHz )
 // | |
 // | |
 // −−− GND −−− GND
 // fg = 1 / ( 2 * pi * ( R1 || R2 ) * C1 ) < 7 Hz
 // fres = 1 / ( 2 * pi * sqrt( L1 * C2 ) ) = 734 kHz

#define INPUT_PIN 0 // ADC input pin
#define TIMER_PIN 3 // PWM output pin, OC2B (PD3)
#define DEBUG_PIN 2 // to measure the sampling frequency
#define LED_PIN 13 // displays input overdrive

#define SHIFT_BY 3 // 2 ... 7 input attenuator
#define TIMER_TOP 20 // determines the carrier frequency
#define A_MAX TIMER_TOP / 4

void setup() {
pinMode( LED_PIN, OUTPUT );

// set ADC prescaler to 16 to decrease conversion time (0b100)
ADCSRA = ( ADCSRA | _BV( ADPS2 ) ) & ~( _BV( ADPS1 ) | _BV( ADPS0 ) );

// non−inverting; fast PWM with TOP; no prescaling
TCCR2A = 0b10100011; // COM2A1 COM2A0 COM2B1 COM2B0 − − WGM21 WGM20
TCCR2B = 0b00001001; // FOC2A FOC2B − − WGM22 CS22 CS21 CS20

// 16E6 / ( OCR2A + 1 ) = 762 kHz @ TIMER_TOP = 20
OCR2A = TIMER_TOP; // = 727 kHz @ TIMER_TOP = 21
OCR2B = TIMER_TOP / 2; // maximum carrier amplitude at 50% duty cycle

void loop() {
// about 34 kHz sampling frequency
digitalWrite( DEBUG_PIN, HIGH );
int8_t value = (analogRead( INPUT_PIN ) >> SHIFT_BY ) - (1 << (9 - SHIFT_BY ));

digitalWrite( DEBUG_PIN, LOW );

// clipping

if( value < -A_MAX) {

  value = -A_MAX;
  digitalWrite( LED_PIN, HIGH );
  } else if ( value > A_MAX ) {
  value = A_MAX;
  digitalWrite( LED_PIN, HIGH );
  } else {
  digitalWrite( LED_PIN, LOW );

OCR2B = A_MAX + value;







RTTY with Arduino and NTX2

A circuit from the UK High Altitude Balloon sight, that sends RTTY, from Arduino using the Radiometrix NTX2 transmitter on 434.650 Mhz.

circuit notes

substituted  2 100k resistors in parallel for the 47k connected to TX output in the circuit

RTTY settings

We used rtl-sdr with Max as the receiver, in SSB mode, sending audio via Soundcloud to dl-fldigi to decode the RTTY. The params that worked:

  • 50 baud
  • carrier shift 400-480 hz (varies)
  • 7 bits per char
  • no parity
  • 2 stop bits
  • reverse mode on
audio input monitor in dl-fldigi

Unfortunately there was no way to hear the signal while decoding because dl-fldigi doesn’t feed through the input audio. A workaround is to run  Audacity in the background. In audacity:

  • set input to soundflower,
  • set output to built-in output
  •  press pause
  •  press record – now you should hear the radio.

pwm circuit

An updated version of the system described above.

This circuit features no additional components. Just the NTX2 and Arduino. The voltages are generated using PWM (pulse width modulation).

Described in a 3 part series.

by Anthony Stirk at

Example 1 – testing

The first example generates a carrier shift of 310 Hz. It took several minutes for the transmit frequency to stop drifting. The AFC setting in dl-fldigi helps to keep things locked in – as long as their is a constant carrier.

Example 2 – RTTY

Reducing the filter bandwidth to less than 150 Hz helps, due to the narrow carrier shift.

Example 3 – dominoEX MFSK

Requires a 175 ohm resistor for precise timing. Didn’t have one, so I just ran the sketch without decoding. Here’s what it sounds like:

what’s next?
  • decoding RTTY in Max feed with Arduino

[Note: is gone. This system doesn’t work. Post is here for historical reasons only]

Bi-directional communication from Arduino to a feed using an ethernet shield.

  • Initializes an internet connection (DHCP)
  • Connects to servers every minute
  • Stores random value in the feed using HTTP PUT
  • Retrieves current feed value using HTTP GET
  • Lights up LED when transmitting
By the way, xively used to be cosm used to be pachube… 
Arduino circuit
  • Use an ethernet shield.
  • Connect ethernet cable. (I am using a Netgear WNCE2001 ethernet to wiFi adapter)
  • LED is connected to pin 5 and ground. The shorter lead connects to ground.


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  • xively_test1 (Arduino sketch)
Arduino files and libraries

Copy the xively_test1/ folder to Documents/Arduino. This puts it in the Arduino sketchbook.

Notes on installing xively/cosm/pachube libraries for arduino:


  1. Connect Arduino to Macbook via USB.
  2. Open the Arduino serial monitor to initialize the ethernet connection and display the IP address.
  3. Every minute data gets send to the feed
  4. Monitor feed data here:
Arduino sketch

5/20/2014 - Arduino/xively feed interaction
Uses Ethernet Shield and and LED connected between pin D5 and ground
Sends a random value to a feed every minute
The LED lights up during data transmissions
HTTP PUT - send data to xiveyly feed and store
HTTP GET - read xively feed value
#include <SPI.h>
#include <Ethernet.h>
#include <HttpClient.h>
#include <Cosm.h>
int ledPin = 5;
int upCount = 0; // counters for number of times going up and down
#define API_KEY "96PqSh4rj7HzNif3WtTpN7GjX96SAKxrWms3SUhwaDFGUT0g" // your Cosm API key
#define FEED_ID 98281 // your Cosm feed ID
// MAC address for your Ethernet shield
byte mac[] = { 0x90, 0xA2, 0xDA, 0x0D, 0x0B, 0xCE };
// note that pins 0 and 1 are used by the Ethernet shield
unsigned long lastConnectionTime = 0; // last time we connected to Cosm
const unsigned long connectionInterval = 60000; // delay between connecting to Cosm in milliseconds
// Initialize the Cosm library
// Define the string for our datastream ID
char sensorId[] = "count";
CosmDatastream datastreams[] = {
 CosmDatastream(sensorId, strlen(sensorId), DATASTREAM_FLOAT),
// Wrap the datastream into a feed
CosmFeed feed(FEED_ID, datastreams, 1 /* number of datastreams */);
EthernetClient client;
CosmClient cosmclient(client);
void setup() {

 // initialize the detector pins 

 pinMode(ledPin, OUTPUT ); // internet transmitting indicator
 // start the Monitor (console) serial port

// display happy messages 

 Serial.println("Xively test");
// Keep trying to initialize the Internet connection
 // Note - we should eventually timeout of this and just run the stairs independently

 Serial.println("Initializing network");
 while (Ethernet.begin(mac) != 1) {
 Serial.println("Error getting IP address via DHCP, trying again...");
Serial.println("Network initialized");
 // print your local IP address:
 Serial.print("Arduino IP address: ");
 for (byte thisByte = 0; thisByte < 4; thisByte++) {
 // print the value of each byte of the IP address:
 Serial.print(Ethernet.localIP()[thisByte], DEC);

} // end of setup function
//////////////////////////// control loop ///////////////////////////
void loop() {
 // main program loop

 ////////////////////////////// Internet sending/receiving code ////////////////////////////////

 if (millis() - lastConnectionTime > connectionInterval) {

 // uncomment this to just send a random value...
 upCount = random(256);

 digitalWrite(ledPin, HIGH ); // turn on transmitter light
 // read the datastream back from Cosm - comment out to save time
 digitalWrite(ledPin, LOW );
 // update connection time so we wait before connecting again
 lastConnectionTime = millis();


 ///////////////////// end of internet send/receive code /////////////////

} // end of main loop code
/////////////////// additional functions //////////////////////////
// send the supplied value to Cosm, printing some debug information as we go
void sendData(int sensorValue) {
Serial.print("Read sensor value ");
Serial.println("Uploading to Cosm");
 int ret = cosmclient.put(feed, API_KEY);
 Serial.print("PUT return code: ");
// get the value of the datastream from Cosm, printing out the value we received
void getData() {
 Serial.println("Reading data from Cosm");
int ret = cosmclient.get(feed, API_KEY);
 Serial.print("GET return code: ");
if (ret > 0) {
 Serial.print("Datastream is: ");
Serial.print("Sensor value is: ");


basic Arduino connections to Max

  • Dim an LED from Max.
  • Read the value of a potentiometer in Max.


folder: arduino-basics


  • arduino-dimmer.maxpat
  • arduino-serial-read.maxpat

Arduino dimmer

dim an LED from Max

From the Arduino playground


Connect an LED to pin 9 and ground. The shorter lead goes to ground.


Load the Arduino example sketch: communications | dimmer

  1. open arduino-dimmer.maxat
  2. Click the “print” message to print the list of ports to the Max window. Then set the port in the serial object, using the port message. For example: “port c”.
  3. Move the slider to dim the LED

Arduino serial read

Read the value of a potentiometer

This patch is a bit more involved. Refer to the Max Communications Tutorial 2: Serial Communication. In fact, much of this code was lifted from the tutorial.

  • Connect an LED to pin 9 and ground. The shorter lead goes to ground.
  • Connect a potentiometer (center lead) to Analog pin 0. The side leads connect to +5v and ground.

Load the Arduino example sketch: Analog | analogInOutSerial

  1. Open arduino-serial-read.maxpat
  2. Click the “print” message to print the list of ports to the Max window. Then set the port in the serial object, using the port message. For example: “port c”.
  3. Activate toggle to start polling serial port
  4. Activate toggle number 3 to view formatted output in the Max window
  5. Turn the potentiometer to send data to Max

general suggestions:

  • Try not to get discouraged.
  • If weird things happen, close Max, reconnect Arduino, the re-open Max