A latching relay is used in this power bar. With this special relay you can switch on and off a lamp, your PC, your Hard-disk or any other devise with a little python program on your Raspberry Pi
Advantage and disadvantage of the Latching Relay
Why using a latching relay instead of a normal relay.
A relay is a switch controlled by an electro magnet. When an electric current is flowing through the electro magnet (coil) the contacts of the switch will be closed and the devise attached to it will switch on. In most cases a transistor is used to switch on and off the current trough the coil of the relay. The General Purpose Input and Output (GPIO) of the Raspberry Pi can drive the transistor base (input).
Advantage of this “normal” single side stable relay:
• Needs only one GPIO pin
• When the control voltage on the GPIO pin disappears due to program end or powering off the RPi, the relay will switch off.
• In the on-state of the relay current is flowing through the coil all the time
A latching relay has two relaxed states (bi-stable). These are also called “impulse”, “keep”, or “stay” relays. A short pulse is used to alter the state of the switch. After the pulse, the relay remains in this state. This relay can be seen as a “one bit memory”. Even when the power is off, the relay remains in its last state.
Advantage of the “latching” bi-stable relay:
• The relay stays in its last state when you close your python program. Even if you reboot or switch off your Raspberry Pi the device attached to the relay will keep working.
• You can read out the current state of the relay (See further in this article)
• Needs three GPIO pins
• More complex electronic circuit to control the relay
The schematic diagram
Latching relays are available with two types of coils.
1-coil latching relays: The direction of the current pulse through the coil determines the state of the relay.
For more detail: Raspberry Pi controlled power bar.