You can buy relay boards (search for Arduino relay board).

Or just use the TIP122 circuit which will basically act like a switch. The Vcc can be regulated to a lower voltage if needed. If a different supply is needed have the TIP switch a relay on and off.

TIP122 Tutorial

Here's URL to ebay

I use something similar to the one posted in the above link to turn something on and off inside the Roomba. I like it because it can be operated with either a high (+5v) or low (gnd) signal from a digital port. Another nice feature is that the relay is optically isolated so there is no interference between the relay operation and the power supply used to power the EZB.

This relay is rated at 30 Amp. 250VAC/30VDC single pole double throw optocoupled relay module that can be controlled by a high or low output from a micro controller. Trigger current is 3 milliamperes (mA) (connected across VREF and CH1) to hold the coil closed. If you are not familiar with these components read on.

One thing to be aware of is that the ratings printed on the relays are CCC ratings. The UL/CUL ratings are lower. The spec. sheet for the SLA series relays states NO: 20A/240VAC 28VDC NC: 10A/240VAC 28VDC (NO normally open NC normally closed)(UL is Underwriters Laboratories, cUL is Underwriters Laboratories tested to Canadian standards).

This relay is designed so that the controller is electrically isolated from the relay power. All the controller has to do is illuminate the LED in the optocoupler. It also has the advantage of having screw terminals for all connections, no easily dislodged jumper connections.

An unusual feature of this module is the TLP290 photocoupler. It has two LEDs back to back so it doesn't matter which of the input connections is positive and which is negative. With 5V across the VREF and CH1 terminals an LED will illuminate and cause the relay to close.

The connections on the control side would be 5V DC from a power supply to DC+ and DC- and a digital high or low output from the micro controller to the CH1 terminal. If a high output is used to energize the relay then the VREF is connected to the controller GRD terminal, and if a low output from the controller will energize the relay the VREF is connected to VCC (+5V).

On the contact side the load can be connected to the normally open (NO) or normally closed (NC) connections.

Most micro controllers digital control pins can source an absolute maximum of 40 milliamperes (mA). And the VCC (positive voltage) and GRD (negative voltage) pins can source (positive voltage) or sink (negative voltage) 200 mA. So since 40 mA is not enough to operate most relay coils a transistor is often used to switch the current for the coil. In this case the transistor is controlled by an optocoupler. It is best to supply the relay coil power independent of the micro controller and that is what this relay is designed for.

I use these 5V Relay because they can be connected directly to the EZB4 signal pin (only draws 20mA@5V)

Wait. does the vcc and ground of the ez-b turn on and off? or just the signal?

The relay has its own power supply of 5 volts dc. The EZB uses a digital port to turn the relay on and off by using just the white signal wire.

I found a relay that uses robot-doc's wiring diagram. Thanks dude! relay of choice

Thanks Rich and Richard for the help. I appreciate it.

Since robot-doc had an inexpensive link and gave some help I'm giving it to him.

Thanks guys!