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The eLabtronics Voltage Switch, Part 2

Turning it into a standalone temperature or light switch

by Julian Edgar

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At a glance...

  • Turn on pumps or fans when a preset temp is reached
  • Pulse over-temp warning lights or buzzers
  • Can switch with rising or falling temp
  • Automatically turn on lights when it gets dark
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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

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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.

  • Turns on when it gets hot

This is the approach to go for when things need to be switched once the temperature rises above a certain point.

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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).

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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.

Click for larger image

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.

  • Turns on when it gets cold

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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.

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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.

  • Turns on when temperature difference is high

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?

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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:

  • 2 x 100 kilo-ohm resistors

  • 2 x 100 kilo-ohm thermistors

Click for larger image

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

  • Turns on when it gets dark

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!)

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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.

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Here’s a bare thermistor

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Shorten the leads and then solder two insulted wires to the leads.

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Use insulation tape (when working with relatively low temps) or good quality heatshrink (high temp sensing) to insulate the connections.

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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.

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

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!

The Voltage Switch is available fully built and tested from the AutoSpeed Shop

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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