Digital Loggers

IoT Relay FAQs

Find answers to FAQs below and on this spec sheet.

Arduino Example
A microcontroller is a perfect mate for the IoT relay.  Here's how they connect:



This code is just like the Arduino BLINK example. There are three parts: 1. Specify the pin, 2. Initialize as output, 3. Toggle a data bit to control the relay.

int relay = 13;              // Tells Arduino the relay is connected to pin 13

void setup()
pinMode(relay, OUTPUT);      // Initialize the Atmel GPIO pin as an output

void loop()                  // Loops forever
digitalWrite(relay, HIGH);   // Turn the relay on (HIGH is the voltage level = 1)
delay(5000);                 // Stay ON for 5 seconds
digitalWrite(relay, LOW);    // Turn the relay off by making the voltage LOW = 0
delay(5000);                 // Stay OFF for 5 seconds

GPIOs on PIC chips, Raspberry Pi, Intel Gallieo, Edison and others are very similar.  It's just too easy :) 


PC USB Power Connection Example
Here's a very simple example showing how "sniffing" the power from a USB port can be used to trigger a relay. No programming is required. The relay will activate when USB power is present,  and de-activate when USB power is turned off.  Turning the relay OFF (no power) sets the normally ON outlets ON and normally OFF outlets OFF.  Powering up the relay via the control input inverts this.

You can disable USB power with a command like this:

devcon.exe disable @USB\*

or by setting



Parallel Port Example
Let's say you'd like to connect a number of IoT relays to a parallel port. Here's a video showing how:

The computer side of a parallel port is a 25 pin D-shaped female connector. The mating connector is called a "male DB-25". Low cost USB->Parallel adapters do the job.

 Printers and other devices (such as the IoT relay) can be wired directly. The 8 output pins are listed below:


Pin Function Hex Value
2 D0 01
3 D1 02
4 D2 04
5 D3 08
6 D4 10
7 D5 20
8 D6 40
9 D7 80

Pins 18,19,20,21,22,23,24 and 25 are all ground pins. Grounds connect to the negative pins on the relay. In the video, we connected the positive wires to the data pins 2 and 3, and the negative wires to pin 24 and 24.  When the port outputs a zero on but 1, ie 0x00, the relay is off. When the port outputs a 1 bit, ie 0x01, the first relay turns on. When the port outputs a 2 bit, 0x02, the second relay goes on. To turn both relays on, output a 3. You can see how this extends easily to 8 relays. Communicating with the port is easy.  If the port is on the motherboard, you can directly write to an I/O port.  If you're using a USB to Parallel adapter, you'll need to "bit bang" through the driver. Here's an example of direct control from older versions of Windows:

void main (void)
outportb(0x378,0xff); /* turn all relays on - 0x378 is the IO address AKA LPT1*/
getch(); /* wait for keypress before switching off */
outportb(0x378,0x0); /* turn all relays off*/

Newer versions of Windows restrict direct access to I/O ports for security.  But you can still "bit bang" most of them. 

A library function can simplify things:

parallel.setData(0)   # sets all pins low (all relays off)
parallel.setData(255) # sets all pins high (all relays on)


PC USB -> Serial Port Example
It's easy to use a USB to serial adapter with a couple IoT relays. 

Just connect the handshaking lines and a ground pin:


DB-9 Pin Function DB-25 Pin
4 DTR (wire this pin to the IoT relay #1 + pin) 20
7 RTS (wire this pin to the IoT relay #2 + pin) 4
5 GND (wire to both relay - pins) 7
3 DATA OUT (another relay can go here*) 2

If you have an FTDI based adapter, there's a nice internal bit-bang mode.  There several ways of controlling these bits from Windows, including the EscapeCommFunction   Here's the Linux serial "howto".   This is often easier than using a USB-> parallel adapter, but there is a limitation: There are only 2 or 3 bits, so you can use at most 2-3 relays per adapter.  *The third relay on the TX data will work only if you can send a persistent "break" from your environment..


Linux Direct I/O Example
Here's the code to directly control a parallel port in Linux:

#define port 0x378 /* For "LPT1", the printer port base address is 0x378 */
#define Both_ON 5     /* This is the numeric value to send to printer port, for example 5 (4+1) turns D0 and D2 on */

outb(port, Both_ON);  /* Turns both relays 1 and 4 on */
delay (1000);
outb(port, 0);        /* Turns all relays off */


Here's a command line example.


Using the Relay Without a Micro
Thanks to the universal input, it's easy to use the relay without a microprocessor.  You can use most any AC or DC signal to turn things on (or off) ff at the same time.

Here are a few examples from real customers:


Alarm System - Flashes 120VAC lights when alarm is activated:


Attic Fan Control Control - Turns attic fan circuit on when air-conditioner is running:



Turn on worklights and stereo on, turn off shop-vac when power-saw is active: