These two programs allow you to turn a relay on/off from a remote location using a switch. Two raspberry pi's (tested on a PI-2 but any flavor should work fine) are the controlling computers. The control signals are sent over a TCP/IP network, so it is necessary for the server (relay end) to be network accessable. The IP address must be known to the client computer (the switch end), and is currently a #define statement in the source code. If both computers are on the same network then the server does not need to be publically accessable. These two computers can be used for other purposes while running these two programs. More than one switch/relay combination can be defined with a copy/paste. Notes are in the software. Using I/O expanders the number of switch/relays can be very large.
The program relayServer.c runs on the PI with the relay attached. The program relayClient.c runs on the PI with the switch to turn on/off the remote relay.
Starting out: The PI with the relay runs the program relayServer. When a command is sent over the network from the client computer, the command is decoded, the relay is energized, and a response is sent back to the client computer. When the client sees the response, it lights up a LED giving feedback to the user. Over a reasonably fast connection the response is almost immediate. When the switch on the client computer is turned off, a command is sent to the server and the relay is de-energized. A response is sent back to the client, and the LED is turned off.
While the client is connected to the server, the server will send a keep-alive packet to the client. If the client fails to respond the server will close the connection. This happens if the connection is broken by hardware failure or if the client computer stops working.
Currently there is provision for one relay/switch, but is should be a copy/paste change to add as many relay/switch/LED's as needed. As written, the programs use about 0.2% of the CPU's resources.
On the server end, the top of the relay coil connects to VCC (+5v, +12V, whatever). The bottom of the relay coil connects to the drain of a 2N7000 fet. The source goes to ground, and the gate goes to the output pin of the raspberry pi (currently defined as hardware pin 16 (GPIO 23).
On the client end, one end of the switch goes to ground. The other end of the switch goes to hardware pin 18 (GPIO 24). There needs to be a pull-up resistor (4.7k ohm is a good value) on the switch (the internal pull-up resistors are not reliably controlled due to changes in the rasbian image changing). One end goes to +3.3v, obtained in the PI's hardware pin 1 of the PI's 40 pin expansion port. The other end of the resistor connects to hardware pin 18. This puts the hardware at 3.3 volts until the switch is closed, at which point the pin goes to ground. Make sure you use the right pin or your PI can be quickly destroyed.
Also on the client PI connect hardware pin 35 to a 680 ohm resistor. The other end of the resistor connects to the positive lead of a low-voltage LED (red/yellow). The PI doesn't have enough voltage to directly drive a higher voltage (blue, green, white) LED. The negative lead of the LED goes to ground.
Start the server program first (relayServer).
Next, start the client program (relayClient). When the switch on the client side is ON, the relay will energize and the LED on the client will light. When the switch on the client side is turned off, the relay will turn off and the LED on the client will also turn off.
Building the programs:
The hardware on the PI's are controlled with the wiringPi library. If it is not already installed run:
sudo apt-get install wiringpi
On the server end (relayServer.c) the TCP/IP port is defined as 9000. Change this if needed. If you do, don't forget to make the same change in the relayClient.c file. Then build the server program:
cc -o relayServer relayServer.c -lwiringPi
On the client computer, set the SERVER_ADDR to the IP address of the server computer. This is defined just after the comments. If you changed the port number of the server, also change the port number here.
Now build the client:
cc -o relayClient relayClient.c -lwiringPi
Start the client after the server. (Make sure both computers are connected to a network.) The server can be left running, and the client turn on/off as need be.