This article was first published in 2009.
Last week in How to Electronically Modify Your Car, Part 7 we looked at how relays can be used for a lot more
than just operating driving lights and similar uses. This week, we’re going to
look at how to use an off-the-shelf pre-built electronics module, the
eLabtronics Voltage Switch.
Two points need to be stated at the outset.
So far in this series, we’ve used components that
are very cheap indeed – resistors, pots and relays. The electronic
module covered here (available from the eLabtronics) is much more expensive – although
in terms of most car modifications, it’s still very cheap.
Secondly, this module looks a lot more complex
than the modifications we’ve so far covered – there are lots of complicated
electronic parts on the printed circuit board! However, working with a module
like this is actually very simple - if you have been reading each part of this
series, you’ll have no problem.
So what does the module do? The Voltage Switch
monitors a signal voltage and when an adjustable trip-point is reached, it
lights the on-board LED and turns on its output. Let’s look at that in more
Voltage Switch Module
As can be seen here, there are only four wiring
connections. These are:
+12V (ie power)
There is also a multi-position option switch and
two adjustable pots. One pot sets the trip-point and the other pot sets the
difference in voltage between the turn-on and turn-off values.
Here’s a wiring diagram showing one way that the
module can be used. In this case, we want to turn on a fan when the engine load
is low. For example, that could be an intercooler fan that switches on when the
car is at idle, reducing heat soak at traffic lights. Engine load is sensed from the engine management's system airlow meter, and the fan is directly driven by the module.
Starting from the bottom of the diagram, the
module is connected to 12V and ground.
The next connection is to the airflow meter output
signal. This connection is made in parallel with the existing connection between
the airflow meter and the engine management ECU – we just tap into this wire.
Because the module takes almost no current on this input, the extra monitoring
of the signal doesn’t change the signal that the ECU sees. In other words, not
only does the airflow meter feed information to the ECU, it also now feeds
information to our Voltage Switch.
Finally, here the output of the Voltage Switch is
connected to a fan, with the other side of the fan grounded. This fan wiring is
the same as we’ve shown previously in this series - one side of the component is
grounded (and so connected through the car body to the negative terminal of the
battery) and the other side is fed +12V.
Now that’s the wiring finished! As you can see,
despite is looking like a complex module, with only four connections (and two of
those just power and ground), it’s actually very easy to wire into place.
Let’s now look at the adjustments, starting with
the multi-position switch.
DIP Switch Positions
The Voltage Switch has a four-position DIP option
switch. When setting switch positions, the board is positioned so that the terminal strip is on the right.
So what are the different switch positions
If you think about the example we gave above of
turning on an intercooler fan when the airflow meter signal was low, you’ll
realise that in that case we want the Voltage Switch to trip when the signal
falls below a certain point. For example, if the airflow meter output is
1.2V at idle and 4.6 volts at full power, we might want the intercooler fan to
come on when the airflow meter signal drops below 1.3V.
But a much more common requirement is to trip the
switch when the monitored voltage rises above a certain level. For,
example, you might want to turn on an intercooler water spray pump when the
engine load (and so airflow meter voltage) are high. Therefore, you might want
the Voltage Switch to trip when the monitored voltage rises above 3.8 volts.
Clearly then, two different modes need to be
provided – one that trips when voltage falls below a certain level, and one that
trips when voltage rises above a certain level.
With the DIP switches set in the pattern shown
above, the Voltage Switch trips as the monitored voltage rises above the
set-point, causing the on-board LED to illuminate and output to turn on. The LED and output then switch off when input voltage falls below
With the switches set in this pattern, the Voltage
Switch trips as the monitored voltage falls below the set-point, causing
the on-board LED to illuminate and the output to turn on. The LED
and output switch off when input voltage rises above set-point.
(There are other DIP switch positions that allow
the output to flash just a few pulses when the trip-point is met, or
alternatively to continuously pulse the output.
The eLabtronics Voltage Switch, Part 1 describes all the
Two adjustable pots are provided on the
The first is Set-Point. I’ve been using
this term without having first defined it – but that’s because it’s pretty
obvious. The set-point is the signal level at which the switch trips or
switches. You adjust this pot to set the voltage level at which you want the
output to turn on. Rotating this pot clockwise increases the input
voltage level at which the module trips.
The other pot has a name that looks a lot more
daunting – Hysteresis. Hysteresis is the difference between the switch-on
and switch-off values.
In one of the above examples, we had an
intercooler water spray turn on when the airflow meter signal rose to 3.8V. But
at what voltage does the water spray switch back off again? If it switches
on at 3.8V, and switches off at 3.8V, there’s clearly a problem.
In fact, at 3.8V the pump would chatter on and off.
But what if we have the pump switch on at 3.8V and
switch off when the voltage drops back to 3.4V? That way, the pump will be
decisively on or off – not chattering on and off. In this case, the hysteresis
has been set to 0.4V (3.8 – 3.4V).
With the module, hysteresis is adjustable over a
wide range - rotating the hysteresis pot clockwise increases the hysteresis.
we look at an on-car modification using the Voltage Switch, consider some of its
can monitor any sensor that uses a changing voltage output – not just an airflow
meter but also a MAP sensor, coolant temperature sensor, intake air temp sensor,
fuel gauge, oil pressure gauge, yaw sensor, throttle position sensor and so on.
So you can switch devices on and off on the basis of temperature, engine
airflow, intake manifold pressure, oil pressure, fuel level – whatever is being
measured by the sensor.
can even monitor battery voltage itself, eg to sound an alarm if the voltage
gets too low, or to disconnect a battery charger if the voltage gets too high.
Let’s take a look in more detail at one
application of the Voltage Switch module.
Car Modification – Auto Air Conditioning Controller
you have a small engine car, or one where you want to chase every possible ounce
of power and fuel economy, automatically switching off the air conditioner at
high throttle angles is very effective. You get more power when you put your
foot down, and – because when accelerating and climbing hills, you can hold
taller gears and so use less engine revs – better fuel economy as well.
achieve this outcome, the Voltage Switch monitors the output of the throttle
position sensor. These sensors invariably have a voltage output that increases
as the throttle is opened wider. By monitoring this voltage, the Voltage Switch
can be set to trip when a certain throttle angle is exceeded. And, by setting an
appropriate hysteresis, the point at which air con turns back on can also be
how do you turn off the air con when the Voltage Switch trips?
conditioner compressors use an electro-magnetic clutch. This means that when
power is fed to the clutch, the compressor is able to be driven by the engine.
When power is cut off, the compressor drive wheel just spins freely. This
compressor clutch power feed is the single wire that you can see going to the
front of the compressor. To disable the compressor, all that we need to do is to
put a relay in this circuit, so that when the Voltage Switch trips, the power
feed to the compressor clutch is turned off. (See How to Electronically Modify Your Car, Part 7 for coverage of relays.)
is the wiring diagram. When looking at a diagram like this, mentally break it
down into sections.
know from earlier that the Voltage Switch needs power and ground connections. At
the bottom, we can see that the power connection is fed from the air conditioner
switch – that is, the switch on the dash that turns on the air con. Wiring the
12V supply in this way means the Voltage Switch is powered-up only when the air
con is running. The negative (-) connection of the module connects to Ground.
OK, so that’s power and ground connections.
input of the module connects to the throttle position sensor.
output of the module connects to a relay. From Part 7 in this series we know
that connection must be to one side of the coil of the relay (ie pin 85) and the
other side of the coil must connect to ground (pin 86 – and yes, it does go to
Voltage Switch is set to trip when the throttle position sensor voltage rises
above a certain level, at which point the relay is activated. Now we want the
relay to break the circuit to the air con compressor (ie turn it off) so we
connect the module to the relay contacts that open when power is applied (87A
is the modification wired into place. The green arrow points to the Voltage
Switch module mounted inside a box, and the red arrow points to the relay
here’s the view with the box open and the mounting bracket not yet in place.
fitted the Auto Air Conditioner Controller to my Honda Insight and the results
have exceeded my expectations. On the small engine Honda, the ability to hold
higher gears up hills and accelerate more easily at small throttle angles is
unexpected benefit – and probably the thing I notice the most – is the absence
of the air con cutting-in when climbing a long hill. Previously, you’d have the
car set to work with a fairly large throttle angle at low-ish revs in a high
gear (the best approach for fuel economy) and then part way up the hill, the air
con would cut-in and all the balance would be lost.
the Auto Air Conditioner Controller in place, time after time it’s like you have
a really attentive passenger with their finger hovering over the air con button,
ready to turn the air con off at the first sign you can benefit from their doing
economy has also clearly improved – previously, running the air con would
increase fuel consumption by about 20 per cent. With the air con controller in
operation, this is reduced to about 10 per cent.
The availability of prebuilt electronic modules
like the eLabtronics Voltage Switch allows very effective and easy modifications
to be made, utilising the sensors that are already present in the car. This
reduces cost (no need for new sensors), makes it easier to wire into place (no
need for new wires from the sensor), and better integrates the modification into
Next week, we’ll look at building your own
electronic modification module – that is, constructing a kit.
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