Construction of a digital LDA and a digital oil thermometer (Articles)

DIY LDA, Oil Thermometer and Voltmeter

Introduction
IMPORTANT! This guide is provided without warranty. I specifically disclaim any responsibility if damage results - the construction or following of this guide is done entirely at your own risk!

Furthermore, this is purely a DIY project for me. I am not pursuing any commercial purposes with it, and I have no intention of selling or otherwise profiting from any parts of the circuit. Of course, I cannot prohibit anyone from doing so, but I would like to request that someone does not attempt to market the instructions.

Of course, the details were not solely based on my own ideas, but also stem from suggestions from numerous interesting discussions in the forums of Dieselschrauber.org and the Seat Ibiza Forums.

Idea
In the Ibiza 6K GP01 with TDI engine (ASV in my case), there is neither a boost pressure gauge nor one for oil temperature. However, both are very important.

One could try to adopt or adapt the advertising displays from the sports model, however, due to their deep placement below the center console, they do not meet my expectations, nor do they comply with the principle of traffic safety.

The goal is therefore to place a small display unit so that nothing essential is obscured, and that the whole thing also looks somewhat like something.

Functions
In addition to charging pressure and oil temperature, the system voltage is also an interesting parameter to monitor, as it can help detect problems with the alternator or regulator. Furthermore, controlling the glow plug system is interesting in a TDI, as it is a misconception to believe that the glow plugs are only active when the indicator light in the instrument panel is lit. In addition, there are the two stages of the glow plug heating, if available. And... finally, I also find the feedback from the fog lights in the 6K to be poorly solved, especially since I have already had to pay a fine for forgetting to turn them off. That makes the requirements document clear:

Download Pressure
• 
  
• Oil Temperature
  
• Incandescent bulb monitoring
  
• Headlight indicator
  
• Voltmeter
  
• Glow lamp heater (I have omitted this as my NL re-import doesn't have it)
  

Digital or Analog?
For voltage and temperature, digital displays make sense, while for charging pressure, an analog display is preferable, so that you can better recognize overshoots during the VTG's startup. However, conventional LDAs are not available in the desired size, so that an electronic charging pressure measurement is unavoidable, whose output signal can then be connected to a small voltmeter. I, however, have not found any that are available in an analog version, small and precise enough.

Therefore, LDA is also the solution of my choice for digital instruments.

Architecture
First, a note about the images. I don't have much time at the moment, so neither the photos are artistically valuable, nor is this description. The schematics and sketches were also created with minimal effort, which means that the drawings are freehand sketches on the organizer.

I would like to apologize and hope that you can still understand.

The entire structure can be broken down into the following components:

Dashboard-mounted display unit
• 
  
• Electronics block for under the dashboard
  
• Protective housing for the charging pressure sensor with distributor for the engine compartment
  
• Pressure tapping
  
• Sensor for Oil Temperature
  

This is the spatial layout. In the following, I will describe the components based on their functions, focusing only on their location. Therefore, the schematics mainly represent the principle circuit, rather than the detailed circuit of individual hardware elements.

However, I assume that the person interested in replicating this is capable of doing so independently.

If relevant, I will provide (without warranty) part numbers for some components, but I assume that standard parts such as cables, capacitors, resistors, etc., can also be obtained without these.

Function Elements
">Power Supply

The entire setup requires a stable power supply, and since the pressure sensor needs 5 volts, and the display I chose works with both 5V and 9V, I've decided to use 5V.

You can achieve this by using the 12 volts (or 14 volts, if the motor is running) from the car's battery in a very simple way with a 7805 regulator (175030):

In the following, 5V always refers to the output of this circuit.

It is installed in the electronics box.

Voltage Measurement">
Voltage measurement is straightforward; you simply need to apply the voltage to be measured (10...15 volts) to the measuring range of the voltmeter (-200 mV ... 200 mV in my case). The best way to do this is with a measuring voltage divider, such as the 415650, which provides a 1:100 conversion.

Temperature Measurement

">There are several ways to measure temperature, such as the oil drain screw, oil filter housing, or oil dipstick. There is a more detailed discussion about the advantages and disadvantages of each measurement point in the TDI forum, which I do not want to repeat here (use the search function!).

I have chosen the pipette method because it makes the measurement very simple, and the temperature can be measured quickly, as the oil basin does not cool, and there are no distortions caused by the water-oil heat exchanger.

As a sensor, I purchased an Equus level indicator, but I rejected it for two reasons:

Non-linear characteristic of the NTC element, which would require a significant amount of effort in the circuit to implement, without achieving high accuracy.
• 
  
• Mechanical instability of the structure - the NTC is essentially unprotected (only paint for insulation) and is attached to a rod. If something falls off, there could be small pieces of debris in the engine.
  

The idea, instead, came from the TDI forum, to use a pt100 sensor, as well as Dieter's excellent idea for the mechanical construction.

Both will be explained in detail in the following sections:

Mechanical ConstructionYou will need: A brass tube with a 4mm outer diameter from the Bauhaus, a PT100 sensor (e.g. 181250, PT1000 would also work), solder, solder flux, a little silicone, and two thin, silicone-insulated cables.

You cut the pipe to the desired length (use the oil measuring rod as a measuring tool, and either attach a piece from the old oil measuring rod, or, as I did, use the remaining pieces from the measuring rod).

You make sure that both cables fit together into the tube, and also that the sensor fits comfortably, and then you connect the cables to the sensor (be careful, it's very sensitive and very small). Now you need to ensure that the sensor is short-circuit-proof (both to each other and to the tube) and that it is brought to the front end of the tube, about 8mm in front of the end. This is not easy, and I cannot recommend a specific recipe. I did it like this: I pushed everything in, aligned it, measured to make sure there was no contact, and then simply pressed in some silicone. The only problem is that it doesn't have to work, and that the silicone can burn when soldering. Alternatively, it might also be possible to use thermal paste and epoxy resin instead, and avoid soldering. I made it with silicone, and then I soldered the pipe.

At the top, you choose a suitable connection option (I, as mentioned, had to replace the probe), and then you assemble everything.

Afterwards or in between, test with a multimeter in the resistance measurement range and a pot of boiling water, as well as other known temperature sources. The characteristic for the pt100 can be found, for example, here.

If all goes well, you have a highly accurate measuring rod for a low price, and nothing can go wrong with it if you work properly.

Finally, mark the oil level indicators on the pipe, and you're done.

The following images show how it looks for me.

Output CircuitThe PT100 provides a resistance that is linearly proportional to the temperature, according to the above curve.

However, the voltmeter measures voltages, so we need a constant current source.

One idea for this is presented here, but you only need the top part if you use a very powerful mixer like me. Important is the note further down regarding a load resistance.

Therefore, you need an LM317 (175978 or similar) and a circuit of the following type:

The 60 Ohm resistor (or one of similar size) provides the aforementioned load. R1 is a trim potentiometer with 1 kiloohm (I used a precision trimmer - 424641) and is used to adjust the current flowing through the pt100.

R2 together with the 250 kΩ resistor forms a voltage divider for setting the zero point - I used a 50 kΩ trimmer potentiometer here (424692).

Voting proceeds as follows:

Replace the pt100, which is already connected externally to the circuit, with a resistor between 100 and 150 Ohms, which you will need to obtain (in my case, it was a 127 Ohm resistor).
• 
  
• Instead of the voltmeter on the display, a multimeter is used, as when adjusting between M+ and M-, voltages of more than 200 mV occur.
  
• You should set R2 so that there is 0 volts between M- and the mass (i.e., to the limit).
  
• One should set R1 approximately to the midpoint (0 Ohms could endanger the LM317).
  
• Now, let's consider the pt100-table and do some calculations: 0 degrees correspond to 100 Ohms, and 267 degrees correspond to 200 Ohms. If we want to linearize the conversion so that one millivolt corresponds to one Ohm, then at 0 degrees, we should have 267 mV on the display (this allows us to capture the relevant range, but at very low temperatures, it won't quite work, but that's not a problem, hopefully it won't get colder than -50 C). Okay, let's calculate what temperature our resistor (127 Ohms corresponds to 70 degrees) corresponds to, and add 267 (which is 337 in the example).
  
• Now we set R1 so that there is a voltage of 337 mV between M+ and M- (or mass, since M- is at 0). Each change of 1 degree changes the voltage by 1 mV.
  
• Finally, the zero point must be adjusted - R2 must be set so that the exact mV corresponding to the temperature, i.e. 70 in the example, lies between M+ and M-.
  
  
• Now, remove the resistor and replace it with the PT100 – if everything was correct, the room temperature should now be displayed on the multimeter.
  
• As desired, the fine-tuning can now be carried out with ice water and boiling water.
  

WARNING! The presented circuit has a known problem! Without the pt100, there are approximately 3.5 volts between M+ and M-, which is far too much for a 200 mV voltmeter! Therefore, this situation must be avoided (either disconnect the pt100 when the circuit is not in use), or use a voltmeter that is unaffected, or install a protection circuit. I am using a series resistor that is low enough to not distort the measurement result, but large enough to limit the current according to the voltmeter's instructions.

So, let's just take a look at what's best for the voltmeter in this case.

Charging Pressure">
CircuitThere are two ways to get the charging pressure signal:

Original sensor tapping.
• Rainer has also created a guide in the TDI forum for this.
  
• Using a Custom Pressure Sensor
  

I have chosen the second solution because it also allows for the detection of errors in the original sensor system, and also minimizes the risk of interference with the motor control.

As a sensor, I chose the MPX4250AP from Motorola. You can also use others, as long as they have a linear characteristic. It is also possible to use sensors that measure differential pressure (e.g., relative to the environment) (called ...Dx in Motorola), which saves you from having to set the zero point for a relative display. These observations apply to the MPX4250AP, which can be purchased for approximately €28 at Segor.

The datasheet for the sensor can be found here, where the different types and connections are also explained.

Therefore, the sensor provides a DC voltage that is linearly proportional to the pressure, with a certain offset. The circuit must therefore allow for both adjustment of the slope and the offset again:

The 250 kΩ resistor is there to make the trimmer's range of adjustment more useful, and the trimmers themselves are 50 kΩ trimmer potentiometers (part number 424692).

The measurement can be done in several ways - both mathematically, and through experimentation. Mathematically, it works like this: You carefully examine the curve in the datasheet and realize that the output voltage is linearly proportional to the pressure, with a specific offset that, in turn, depends on the minimum pressure (0.2 bar = 20 kPa absolute) that the sensor can measure. The standard atmospheric pressure is 1.013 bar (101.3 kPa), so we are 0.813 bar (81.3 kPa) above this minimum. At the minimum, the sensor typically provides 204 mV, and 20 mV more per kPa. This results in 1.83 volts at standard pressure. The increase, i.e., the 20 mV per kPa, is given in the datasheet, but the offset (i.e., the 204 mV) can range between 133 and 274 mV. This allows us to calculate the offset of our sensor by measuring the voltage at a known air pressure – for example, mine provides 1.84 volts. This means an offset of 214 mV. We want linearity in such a way that 1 mV corresponds to an increase of 1 kPa (0.01 bar), so we need to set the circuit so that it divides the increase exactly by 20. This means that at normal pressure, we need to have 92 mV (one twentieth of the measured value at normal pressure). Therefore, we set R1 so that this value is displayed exactly. Subsequently, the R2 is used to calibrate the zero point, so that at ambient pressure, the display shows exactly 0 - this results in a relative display that honors every change of 0.01 bar with a 1 mV increase - therefore, 1 bar is equivalent to 100 mV, which corresponds to 1000 on a 3.5-digit panel meter.

Alternatively - or additionally - one can also determine the value through experimentation, by comparing the results using a pressure source and a very precise measuring device.

Pressure Control

Here, there are several options. One option is to connect the MPX4250AP directly to the pressure tubes, but I would not do this because of the sensitive connections, especially since repairs would then be very difficult. Alternatively, the sensor can be installed in a box that connects to the charging pressure tubes via a hose - this is the version I chose.

Here is the pressure sensor in its plastic box, from which the cable to the dipstick also exits, and the orange cable leads into the interior:

The green hose leads to the pressure pipes, which I drilled nearby the original sensor and glued a pressure fitting in with 2-component adhesive:

One should naturally drill into the pipe while it is in its expanded state, and also ensure that no drilling debris remains in the pipe.

Otherwise, these can cause significant damage to the engine.

Alternatively, one can also use Ulf's idea, which is described here alongside the need for LDA.


LED Monitoring for additional features">There isn't much to say about this.
A LED is switched on by a limiting resistor (depending on the brightness, 500...5000 Ohms) connected to the voltage to be monitored.

Display Unit">
IMPORTANT! You can use any panel meter (i.e., an installed voltmeter), however, you should make sure that it is compatible if the power supply and the measuring object are not galvanically isolated!

I learned this the hard way, for example, the inexpensive advertisement 126594 (7.95 Euros) does not apply, it simply doesn't work, unless you want to power it with a battery, which doesn't make sense in a car.

My final choice has fallen on the DPM951 (121142) model for 17.95 euros, which has all these features and also supports many modes (see instructions). I don't want to say much about the wiring here, but I just want to refer to page 13 of the instructions - for us, the version on the left in the upper dotted box is relevant, i.e. the one where LK4 and LK6 are to be bridged. Connection details, etc., are explained in the instructions.

With the measurement circuits presented so far, they must be connected in such a way that M+ goes to terminal 1, M- goes to terminal 2, and everything else is connected according to the instructions (it is also important to choose the decimal points).

The DPM951 has connectors for electrical connection, it is therefore recommended to use flat cables with corresponding connectors. I simply took care of it at that point, and I separated the old floppy cable lengthwise and cut off the connectors.

The lighting of the DPM951 is green, and it can be easily converted to other colors by disassembling it (carefully removing the circuit board) and separating the reflector from the circuit board, then cutting the LEDs from the circuit board, slightly modifying the original holders attached to the reflector, breaking off the module holders, and finally allowing super-bright LEDs to shine in from the side. In this way, I have converted my panel meters to red.

I used two panel meters for the mechanical construction, of which the left one used two 2xUM relays (one for M+ and M-, and one for the decimal points) to switch between temperature and voltage, and the right one for the charging pressure. Alternatively, you can also put everything on a single panel meter, or use three... as you prefer.

WARNING! I have stabilized everything with hot glue... I strongly advise extreme caution, because hot glue on the circuit boards can cause unwanted symbols (Fahrenheit in my case - see pictures) to be activated, which can only be removed with great effort if everything has been glued together as I have done.

Here are two pictures of the advertisement - there are still missing covers on the base of the instrument box - the unsightly adhesives will also be concealed, just like the cables. The trimmers on the bottom right are for adjusting the pressure gauge, and the switch on the left is for switching between oil temperature/voltage. The trimmers for the temperature are located in the electronics box. In the middle between the displays are the LEDs, of which the top three are parallel for pre-heating (I only realized too late that my car does not have a headlight heating system, and two of them were intended for that purpose), and the bottom one is for the fog lights.

Addition from 11.01.2004: New photos

The cover frame is installed.

Display of the battery voltage, charging pressure is indicated by the low external pressure being in the negative range (since it is an absolute pressure sensor, in this way, with appropriate calibration, you can also estimate the external pressure at the initial value).

Oil temperature display after a night of inactivity.


Concluding RemarksOf course, there are many errors in this text, which was typed in a hurry, and I would be grateful for any corrections (email below).

And, to reiterate: Replication and use are at YOUR OWN risk!

Thank you
A special thank you goes to everyone who participated in this thread on the TDI forum, and of course to RainerK for the TDI forum itself.

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(C) 2003 Jan Richling