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 detail.
Voltage Switch Module
As can be seen here, there are only four wiring connections. These are:
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 for?
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 set-point.
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 switch functions.)
Two adjustable pots are provided on the module.
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.
Let’s take a look in more detail at one application of the Voltage Switch module.
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 the car.
Next week, we’ll look at building your own electronic modification module – that is, constructing a kit.