Last week in
eLabtronics Voltage Switch,
Part 1 we introduced the eLabtronics Voltage Switch, a cheap and easy to
connect pre-built electronic module that allows you to switch devices by
monitoring existing engine management sensors.
This week we look at how the Voltage Switch can be
used as a standalone device, monitoring temperature or the intensity of light.
Example uses include automatically switching on headlights when it gets dark,
turning on pumps or fans when the temperature rises, or monitoring two temps and
switching when the difference is greater than a preset value.
Some soldering and component recognition skills
are required when configuring the Voltage Switch for automatic switch-on, so the
electronic skills needed are a little higher than when wiring-in the Voltage
Switch for normal manual use. However, the wiring is still very
straightforward.
5V supply
To automatically trigger the Voltage Switch on the
basis of temperature or light intensity, use is made of a regulated 5V supply
sourced from the module. This is available on the pin shown here.
Note that while a regulated 5V is available on
this pin, the amount of current that can be drawn is strictly limited. There is
sufficient current available to operate the temperature and light sensor
circuits described here, but there is not enough current available to run
other sensors (eg automotive MAP sensors). In fact, the output current rating of
this source is only 2 milliamps.
Effectively, the 5V pin supplies a fixed voltage
that is them modified by the action of the specific sensor (temp or light) and
adjustment pot before being fed to the Input.
All the extra components needed for sensing
temperature and light intensity are available as a single pack – see Performance Modules Accessory Pack.
So how is the Voltage Switch wired for auto
operation? Let’s look at temperature first.
Temperature
By using one or two low cost temperature sensors
(thermistors) and a few other components, the Voltage Switch can be configured
to switch itself on the basis of temperature. The switching point and hysteresis
(difference between turn on and turn off points) are fully adjustable and a
number of different output configurations can be used.
This is the approach to go for when things need to
be switched once the temperature rises above a certain point.
Here is the wiring diagram. Note that for the sake
of simplicity, the power and load connections for the Voltage Switch are not
shown here (or in most of the wiring diagrams in this story).
However, as a special once-off, here is a full
working system, complete with ground, +12V and the load connections.
To trigger the Voltage Switch on the basis of
temperature, the required additional components are:
- 200 kilo-ohm resistor
- 100 kilo-ohm thermistor
The circuit is wired as shown here. The thermistor
and resistor have no polarity so they can go into the circuit either way around.
Adjustment of the on-board trip-point pot sets the
temp at which the switch triggers. The hysteresis (difference between on and off
temps) is adjustable with the hysteresis pot. With the depicted components, the
selectable temp range is from about 0 degrees C to about 100 degrees C.
This is the approach to go for when things need to
be pulsed once the temperature falls below a certain point.
For example, to be warned of the potential
presence of black ice on the road, you’d set the system to pulse a dashboard
light when the outside temp falls below about 3 degrees C.
Another way of looking at this is to say that the
output will be on when it is cold, and off when it is hot. An additional example
use is a warning light that stays on when the engine oil temp is still cold.
Place the sensor so that it can detect engine oil temp and as soon as the cold
car is started, the light will flash. Once the oil temp reaches your pre-set
value, the light will stop flashing.
Again adjustment of the on-board trip-point pot
sets the temp at which the switch triggers. The hysteresis (difference between
on and off temps) is adjustable with the hysteresis pot. With the depicted
components, the selectable temp range is from about 0 degrees C to about 100
degrees C.
This is the approach to go for when things need to
be pulsed once the temperature difference between two sensors
increases above a certain point.
The benefit of using two sensors is that it takes
into account different ambient temp levels that might exist. An example makes it
clearer.
Suppose you want to turn on an intercooler water
spray when the core temp exceeds 50 degrees C. Trouble is, if the intercooler is
under the bonnet, that will happen most times you’re stopped in traffic and the
day is hot! The result is an empty water tank.
Now change that set-up to using two sensors – one
positioned in the general area of the intercooler core and the other actually
buried in the fins of the intercooler core. If the spray is set to trip when the
intercooler core sensor is (say) 15 degrees C hotter than the other sensor, the
spray will come on only when the core is not working sufficiently well – in
fact, when it is working as a pre-heater! Tricky, eh?
The wiring is carried out as shown here. The ‘hot’
sensor is placed so that it will be the hotter of the two sensors. When the
‘hot’ sensor is (say) 15 degrees C higher in temp than the ‘cold’ sensor, the
Voltage Switch will be switched on.
The required components are:
Again adjustment of the on-board trip-point pot
sets the temp at which the switch triggers. The hysteresis (difference between
on and off temps) is adjustable with the hysteresis pot. With the depicted
components, the selectable temp range is from about 0 degrees C to about 100
degrees C.
Light Intensity
Wired in this form, the Voltage Switch switches
itself on when it gets dark.
An example use is to automatically turn on an
in-cabin ‘alarm armed’ flasher LED whenever it’s dark. (If the sensor is placed
near the dashboard lights, the flashing LED will stay off until it is dark
and the dash lights are off!)
The wiring requires a Light Dependent Resistor
(LDR) with a nominal 48 – 140 kilo-ohm response and a 200 kilo-ohm resistor.
Mounting
the Sensors
Both
the thermistors and LDR come as bare electronic components. To wire them into
place, you’ll need to do two things: solder them to extension wiring and mount
them.
Here’s
a bare thermistor
Shorten
the leads and then solder two insulted wires to the leads.
Use
insulation tape (when working with relatively low temps) or good quality
heatshrink (high temp sensing) to insulate the connections.
The
Light Dependent Resistor (LDR) can be handled in the same way.
If
the sensor is detecting just ambient conditions (eg in-cabin temperature or
light intensity), the sensor can simply be positioned appropriately and held in
place with a cable tie. However, if the sensor is working in a much tougher
environment, use high-temp epoxy to mount the sensor in a threaded brass fitting
so that it can be securely mounted.
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If
you’re triggering the Voltage Switch on the basis of temperature or light
intensity, you’ll need the
Accessories Pack.
It comprises:
1
x 1 meg-ohm pot
1
x 500 kilo-ohm pot
1
x 200 kilo-ohm resistor
2
x 100 kilo-ohm thermistors
1
light dependent resistor (LDR)
A
box is also available for the Voltage Switch – see
Performance Modules Box - Bulkhead - Black
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Conclusion
The automatic turn-on facility of the Voltage
Switch increases its versatility, allowing it to react to changing temperature
and light levels. By selecting the correct output mode, the switching can occur
when levels rise or fall, and the output can be a warning double pulse, a
continuous pulsing or a continuously switched on output. The system is also
completely independent of the car’s existing electronics. But best of all, it’s
cheap and the wiring and set-up are easy!
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The
Voltage Switch is available fully built and tested from the AutoSpeed Shop
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The
eLabtronics modules are engineered and manufactured by eLabtronics. The modules
are based on concepts and specifications developed by Julian Edgar, with the aim
being to provide cost-effective and useful modules for car modification (and
also industrial and educational uses).
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