Circuit Notebook 108 - A 3 Channel Push-button Switcher using Latching Relays

Latching relays

Conventional relays have an operating coil which, when energised by a current, produces a magnetic field that moves a spring loaded armature and closes a pair of contacts. The contacts open when the coil is no longer energised.

A latching relay (RS Components 369-602), is shown in Fig. 1.. It operates in the same way except the armature has a toggle action and two opposing coils, one to set it and the other to reset it. Only a pulse of current is required to set or reset it and the relay retains its setting when the energising current is removed. The latching relay can be used to store a circuit condition. GB3TM TX uses latching relays to store and display fault information after the power has been removed.

Push-button, Latching Relay, Control

This push-button switcher uses latching relays and could be used to switch audio, video, D.C. and even mains. The 3 way circuit was developed for a specific project but could easily be extended to 4 or more circuits. Its main feature is that the connections are mutually exclusive and that the settings and connectivity are retained when power is removed.

The circuit is shown in Fig. 2.. It employs three latching relays, one for each channel. See ref.2. As mentioned, the latching relays have two coils. When the 'S' (set) coil is energised, the relay latches in the 'on' condition with COM connected to NO. When the 'R' (reset) coil is energised the relay latches in the 'off' condition with COM connected to NC. The bi-stable latching action is provided by an internal permanent magnet. (NO = normally open, NC = normally closed, COM = common). See Ref.1.

The setting and resetting of the relays is performed by a number of diodes. For example, if push-button PB1 is closed the 'S' coil of RLA is energised and the 'R' coils of RLB and RLC are energised through D7 and D8 respectively. This ensures that RLA is energised and RLB and RLC are not. An equivalent arrangement applies to button PB2 and RLB. Also, PB3 and RLC.

Each set of COM, NO and NC connections are used to connect to different parts of the external circuit being controlled. A combination of connections can be used for special control functions. The other set of COM, NO and NC connections are used to power LEDs to indicate which circuit is active.

Veroboard Layout

The prototype circuit was built on Veroboard and the layout is shown in Fig. 3.. The completed prototype is shown in Fig 4. Three LEDs were fitted directly to the Veroboard for testing purposes. Typical connections to the Veroboard are shown in Fig. 5..



Ref.1. Wikipedia. 'Latching Relay with Permanent Magnet'

Ref.2. RS Components 369-602