A Guide To Relays

A Guide To Relays

In this article i will explain the differences in between the two main types of relays, when used properly and creatively can be used a very wide variety of applications. Ranging from the addition of driving lights to adding a circuit to allow your car stereo to stay on with the key off until the drivers door is opened.

There are two main types of relays, the SPST (Single Pole Single Throw) and SPDT (Single Pole Double Throw). The basic internal difference in between the two is that the SPST only has a N/O (Normally Open) switch. While the SPDT has both N/O and N/C (Normally Closed) switches. You can easily distinguish the two by looking at the external terminals. The SPST relay will only have 4 terminals, the SPDT relay has 5.

These are marked as follows:

 

 

Now, thats the basics over and done with. Heres a few examples of simple applications using a SPST relay:

(Note, never install a relay using power derived directly from the positive battery terminal without using an appropriate fuse, all my wiring diagrams include the appropriately - sized fuse/s.)

 

DRIVING / FOG LIGHTS

Here i have provided a simple wiring diagram and information about wiring up aftermarket driving / fog lights:

 

 

CONNECTION OF ADDITIONAL COMPONENTS TO AMP REMOTE WIRE

This application is useful for connecting a range of components to the amplifier remote wire, without running any extra wiring from the front of the car. This is ideal for amplifier cooling fans and neon / LED lights:

 

OK, thats about it using a standard SPST relay. Now we'll get into some applications using a SPDT relay:

 

HEADLIGHT REMINDER

This is especially useful if your car dosent have this option, the standard buzzer has failed and you cant find it or if you want to have a light indication. This works by not giving power to the buzzer / LED light when the ignition is on and the headlights are on. When the headlights stay on and the ignition is turned off the relay gives power to the buzzer / LED, activating it. Turning the headlights off will cause the relay to disconnect power to the buzzer / LED:

 

 

 

 

THERES MORE TO COME - STAY TUNED!