This article was first published in 2009.
Last week in How to Electronically
Modify Your Car, Part 6 we looked at a special way in which pots can be used
to alter the output of sensors used in electronic car systems. It’s a very
tricky technique that allows you to achieve lots of effective outcomes at
nearly zero cost. This week we’re also looking at simple and cheap electronic
components that are again often overlooked as ways of achieving excellent car
modifications – relays.
What’s a Relay?
A relay is just a small switch whose movement is
caused by the action of an electromagnet inside the box. When power is applied
to the relay’s coil, the electromagnet comes alive and pulls across the
The simplest relay is a single pole, single throw
(SPST) design. This designation refers to the switching part of the relay where
when it's activated, one wire (a "single pole") can be connected only one way (a
"single throw"). Just like an on/off switch, when you power up the relay's coil,
the connection is made; when you un-power the coil, the connection is broken.
In this diagram the relay's coil is yellow. Near
to the coil you can see a switch, which is open. This is called a Normally Open
contact - it's open when there's no power being applied to the relay. When power
is applied to the relay’s coil, the single contact closes. This is a Single Pole
Single Throw relay - SPST.
SPST relays have four terminals - two are to power
the coil and the other two are the connections for the internal switch. Look at
the diagram and identify which terminals are which. As you can see, there
is no electrical connection between the pair of contacts for the coil and the
pair of contacts for the switching part of the relay.
Car Modification – Auto-Dimming LED Indicator
In How to Electronically Modify Your Car, Part 3 we talked about using
LEDs in a car. As this diagram shows, if the LED is to survive, you need to place a series resistor in the
we’ve added a pot (wired as a variable resistor) into the circuit. By turning
the pot, the brightness of the LED will able to be adjusted. Because the
resistor is still there, you can’t blow-up the LED – even you turn the pot fully
in the ‘bright’ direction.
we’ve added a switch in parallel with the pot. By closing the switch, the pot is
bypassed – as far as the circuit is concerned, it ceases to exist. So we can
have two switched light levels for the LED – one that’s bright (switch closed)
and one that is dim (switch open). The ‘dim’ setting can be adjusted by the pot.
now we’ve replaced the switch with a relay. This SPST relay is a normally
closed design – it opens when power is applied to its coil. Here we’ve wired the coil
so it is activated whenever the parking lights are switched on. So what have we
ended up with? A LED dashboard indicator that is bright in daytime and dims
itself whenever you turn on the lights! You can even fine-tune the ‘night’ level
with the pot.
LED could show turbo boost (just add a boost switch, as shown here), indicate
that a fan is running – or a whole host of things.
here is the built circuit – from left to right: resistor, pot and relay (the LED
and switch are remote mounted).
In the above example, the relay’s contacts had to
take only a very small current – just enough to run the LED. Therefore, a quite
light duty relay could be used (more on relay ratings in a moment). But many
relays used in cars are heavy duty designs.
In fact, the most common application for a SPST
relay is to use a small electrical current to control a large electrical
For example, a radiator fan might be triggered by
a temperature switch. The temp switch is capable of flowing only 2 amps, but the
radiator fan at switch-on takes 15 amps (and then settles back to 8 amps
If you wire the radiator fan to the switch like
this, after a few weeks the temp switch will fail – its contacts are being
The solution is to add a SPST relay that is wired
into the circuit like this. Now the temp switch only has to pass enough current
to turn on the relay’s coil – a much easier job than directly running the fan!
Neither the relay coil nor the switching part of
the relay has a polarity – both can be connected either way around to 12V and
Ground. As we said earlier, relays are very hard to blow-up!
On automotive SPST relays, the pins are given
standardised numbers. The coil connections are 85 and 86, while the two
connections for the internal switch are 30 and 87. However, most general purpose
relays don’t have any numbers on the pins – instead the functions of the pins
are shown on a little diagram on the body of the relay.
But wouldn't it be good if we had two contacts
inside the relay – one that was opened at the same time as another one was
closed? That's what happens in the Single Pole, Double Throw (SPDT) design.
(Can’t think of much use for that type of relay? There is – and I’ll show you in
This is what a SPDT relay looks like inside. When
the relay is energised, one contact is opened and the other one (the Normally
Open contact) is closed. We still have only a single pole to be switched, but
now it can be connected two ways - a double throw design. As you can see, it has
both Normally Open (NO) and Normally Closed (NC) contacts. (Some people call
this a changeover relay.)
A SPDT relay allows you to control two devices,
switching one off as the other is switched on. SPDT automotive relays use the
following codes for their pins: the coil connections are again 85 and 86, the
normally closed output is 87a, the normally open output is 87 and the input is
Car Modification – Switchable Fuel Pressure
example of where I used a SPDT relay was in a fuel system that needed to be
switched between two different fuel pressures. To raise the fuel pressure, a
solenoid valve had to be turned off and at the same time, a fuel pump needed to
be switched on. Both devices draw a fair amount of current so a heavy duty
automotive relay was used – one of the pictured relays did this function.
circuit diagram for the fuel system relay looked like this. Power was normally
supplied to the solenoid through the Normally Closed (NC) relay contact,
energising the solenoid. But when the relay’s coil was activated (by closing the
High/Low Fuel Pressure switch), the relay pulled the contact across, switching
off the solenoid and switching on the fuel pump.
you get lost, follow the circuit in two parts, starting off from either of the
‘12V’ power supplies.)
High/Low Fuel Pressure switch had to handle only enough current to switch the
relay’s coil, so this could have been be a light-duty switch (eg a boost
pressure switch or a microswitch).
A Double Pole, Double Throw relay allows you to
switch two different circuits simultaneously. The 'Double Pole' bit just means
that it has two separate inputs that can be switched - and we now know the
'double throw' means that one contact gets opened as the other is closed. With
this type of relay you can:
These relays are less common in automotive
aftermarket use and so don’t have coded numbers for the pins.
Car Modification – Switching Out Oxy Sensors
what use is a DPDT relay, then?
I’ll use an example from a car modification I’ve done. What was needed was the
on-demand disconnection of two oxygen sensor input signals from the ECU.
two signal wires from the oxy sensors to the ECU needed to be kept completely
separate; this meant they couldn’t be joined together and a SPST relay used.
Instead a DPDT relay was used. (It didn’t actually have to be a double throw
design, but DPDT relays are more common than SPDT designs.)
the relay’s coil was energised, both oxy sensors were simultaneously
disconnected from the ECU.
Using a relay is made a lot simpler if you follow
Draw a circuit diagram. The first step is to draw a simple circuit diagram showing where the wires
go. Which wires go to the relay coil, which to the Normally Open and Normally
Closed contacts of the relay?
Decide what type of relay is
needed. If just one connection needs to be switched on and off, you’ll use a
SPST design. If two connections need to be switched, a DPST or (more commonly) a
DPDT design will be the one to use. A changeover (where one device is switched
off and the other switched on) can use a SPDT or a DPDT design.
Work out the functions of
each pin. If it’s a standard automotive relay, read the numbers. If it’s a
general purpose relay, look for the diagram on the relay body. If neither of
these apply, by careful use of a short-circuit protected power supply and a
multimeter, you can work out the functions of each pin. (Unless you use too high
a test voltage, you can’t damage the relay!)
Wire the relay coil
first. If you wire the relay’s coil first, you’ll be able to check that the
relay is working by listening to its click.
Car Modification – Disabling Traction Control
easy to think of relays as being suitable for just simple car modifications, but
that’s not always the case.
circuit shows the use of two relays that deactivate traction control
without affecting ABS or stability control.
system works by connecting the un-driven wheel ABS sensor outputs to the driven
wheel ECU inputs, so that the ECU cannot see a speed difference between the
undriven and driven wheels. The modification is automatically switched off
whenever the brakes are applied, or by a manual on/off switch. This diagram
shows only half of the system - the complete the system mirror-images the wiring
for the other side of the car.
total cost of the modification was well under AUD$30 – relays are cheap! For
more on this approach, see Modifying Electronic Car Handling Systems, Part 3.
In addition to its contact configuration (SPST,
DPDT, etc) there are at least three other specifications that are important.
This refers to the voltage which the relay is
designed to have its coil triggered by. A nominally 12V relay is fine on car
voltages, even though they can extend as high as 13.8V. However, you shouldn’t
use a 5V coil relay on a 12V system.
This is the amount of current the relay coil will
draw when energised. This can be expressed directly in milliamps, or indirectly
as a coil resistance. A very sensitive relay might have a coil resistance of 360
ohms. 13.8 volts divided by 360 ohms gives a coil current of 0.038 amps, or 38
milliamps. In other words, the switch that you’re using to operate the relay has
to handle just 38 milliamps. That is a very low value of required current.
A typical automotive relay is more likely to have
a coil resistance of 80 ohms, giving a coil current flow of 170 milliamps.
(13.8/80 = 0.17 amps). That’s still low – most switches will handle this without
This spec refers to the max current that a relay’s
contacts can handle. To avoid arcing, you should use a factor of safety where
the max current of your switched circuit is less than the relay’s spec.
Automotive relays are available with current
ratings like 25, 30 and even 60 amps. Be careful when checking max current specs
that the listing is for the DC at or above the voltage you’ll be using – ie, in
cars, 13.8V. For example, a relay rated at 10 amps at 240V AC is not the same as
one rated at 10 amps at 12V DC.
Relays can be utilised in nearly every electrical
or electronic car modification. Get your head around their use and you’ll never
regret having spent the time to find out how they work.
Next week we’ll look at using an off-the-shelf
The parts in this series:
Part 1 - background and tools
Part 2 - understanding electrical circuits.
Part 3 - volts, amps and ohms
Part 4 - using a multimeter
Part 5 - modifying car systems with resistors and pots
Part 6 - shifting input signals using pots
Part 7 - using relays
Part 8 - using pre-built electronic modules
Part 9 - building electronic kits
Part 10 - understanding analog and digital signals
Part 11 - measuring analog and digital signals
Part 12 - intercepting analog and digital signals
Part 13 - the best approaches to modifying car electronics ? and the series conclusion