Warnings

The RELAYplate is designed to switch potentially lethal voltage levels. When handling the wires and loads that carry these voltages always keep this warning in mind:

Please read and remember the following warnings before using the RELAYplate:

1. Every RELAYplate has a small shield on the bottom that protects the user from potentially high voltages. This shield is held on by five screws. DO NOT REMOVE THIS SHIELD . Doing so will expose you, and the rest of your circuitry to potentially lethal voltage levels. Tighten the screws if they appear to be loose.
2. The RELAYplate has been designed to keep both the user and the user's electronics safe from high voltages in the event of a single point failure. DO NOT ATTEMPT TO MODIFY ANY OF THE CIRCUITRY ON THE RELAYplate . Doing so will expose you, and the rest of your circuitry to potentially lethal voltage levels.
3. Whenever attaching wires and loads to the RELAYplate, ENSURE THAT ALL POWER IS OFF. Do not attempt to attach live wires to the RELAYplate. Doing so will expose you, and the rest of your circuitry to potentially lethal voltage levels.
4. When preparing a wire for use with a terminal block on the RELAYplate:
   • Never use a wire gauge greater than 14 or less than 22. Exceeding these limits can expose you, and the rest of your circuitry to potentially lethal voltage levels.
   • Strip off no more than 6 to 8mm (1/4" to 1/3") of insulation off the end of the wire that will attach to the RELAYplate.
     
   • If the wire is stranded, "tin" it with solder after you have stripped it to ensure that no strands can become exposed after insertion
   • Whenever pushing wires into the terminal blocks on the REALYplate, make sure that the insulation completely covers the wire and that no wire is exposed after insertion.
     
   • If you are using stranded or a small gauge of wire it will be necessary to push the button on the terminal block as you slide the stripped end of the wire end. Once it is completely inserted, release the button to lock it in place. Then, do a pull test to ensure that the connection is secure.
   • Whenever you are about to power your circuit, inspect your connections and ensure that no wire is exposed on the terminal blocks. Also make sure that the wires and terminals on your load are also protected.
5. Do not exceed an AC voltage of 120VAC or a current greater than 1 amp.
6. Do not exceed a DC voltage of 30VDC or a current greater than 1 amps.
7. Be smart and keep children away from your work area while using or setting up your RELAYplate project
8. Be even smarter and don't let a child use or touch a RELAYplate once you have installed it in your project.

OK, if all those warnings didn't scare you off then let's learn about how to use your new RELAYplate.

What is a Relay?

Simply put, a relay is an electromechanical device that allows a high power load to be controlled with a low power circuit. The images below show a cross section of a relay very similar to what is on the RELAYplate. The first image shows the relay when the coil is deactivated. In this situation, the armature is pulled up by the spring placing the common terminal in contact with the normally closed terminal. Due to space constraints, the normally closed terminal is not used on the RELAYplate.

When a small voltage is applied to the coil, the current produces a magnetic field in the core which pulls down the armature. When this happens, the common terminal comes in contact with the normally open terminal.

Below is a photograph that shows these components in an actual relay.

Basic Features of the RELAYplate

Shown below are the major features of the RELAYplate. The two most obvious sets of features are the seven relays and the seven screwless terminal blocks. There are also seven LEDs labeled K1 through K7 which indicate to the user which relays are in the "on" state.

Unique to the relay plate is a combination of features that were added for safety. These include:

1. The slots in the board. These provide an isolation barrier between the low voltage side of the board and the high energy side of the board.
2. A small power supply that provides isolated power for the relay coils
3. The small "shield" on the bottom of the RELAYplate that prevents the user from accidentally coming into contact with potentially hazardous voltages.
4. A small area covered with yellow electrical tape. Under normal circumstances the traces under this tape are safe. However, in the event of a single point failure, these traces could become hazardous. So, DO NOT REMOVE THIS TAPE.

Relay coils require a substantial amount of power and if you only have a single RELAYplate connected to your Raspberry Pi then there should be enough power available to drive your board. However, your RPi power supply may not have enough power to drive more than a single RELAYplate. In the event that you need two or more RELAYplates on your stack of Pi-Plates, there are two different auxiliary power inputs available. Selecting and using these is discussed below.

Finally there are the features common to every Pi-Plate:

1. The address header which allows you to stack up to eight RELAYplates on your stack
2. A single programmable LED. A turn on this LED is turned on but its state can be controlled in software.
3. An onboard microprocessor that controls each relay while interfacing to the Raspberry Pi.

Attaching a Load to the RELAYplate

Think of a relay as a programmable switch. As you know, a switch is a device that controls the power reaching a load. The load can be something like a heater, a light bulb, a motor, or an alarm. The power can be the AC coming from your wall socket or DC from a lead acid battery. By convention, the "hot" wire is the side of the power connection that is usually switched. For AC the hot wire is referred to as "line" while for DC this would be the positive terminal of a battery or power supply.Traditionally, relays have reference designators that start with the letter K. This can be seen in how we labelled the terminal blocks associated with K1 through K7 in the image below:

Below is a simple schematic of a lightbulb connected to an AC plug through relay K3 on the RELAYplate. Note that the line or "hot" wire is routed from the plug to one side of the terminal block for the K3 relay. Connecting loads this way reduces the exposure of potentially lethal voltages to the users.

Once you have made these connections and verified that the wires are securely and safely attached to the terminal blocks, plug the power connector into a wall socket and enter the following commands: