This project started out as a quick idea, and the demand turned out to be a lot greater than I expected. I promised to publish it to the 200SX mailing list, but requests have come in from far and wide.
The gauge simply reads the output of a MAP Sensor, which gives us a nice 0 - 5V output to represent the boost or vacuum. It's complicated slightly by the fact that the 200SX (and all Nissans I know of) use an air flow meter, so we have to obtain a MAP Sensor. More about this later.
Because the MAP Sensor gives us a linear output, assuming a sensor capable or reading +/- 1 Bar (most factory units) the results will be:
|Output Voltage (V)||Manifold Pressure (Bar)|
Now this gets easy to do, because of a popular integrated circuit, the LM3914, which will drive a 10-LED display proportional to an input voltage. You should refer to the schematic below (click on the image for a big jpeg file)
Beside the LM3914, and the LEDS, there is an LF351 Operational Amplifier configured as a voltage follower. This buffers the input of the LM3914, and prevents it placing an load on the MAP circuit, should it be installed in a MAP-Sensed car. The remaining components are a 7808 voltage regulator (for the display) and a 7085 voltage regulator for the MAP Sensor (if installing from scratch).
Construction is pretty straightforward. A PCB can be
made up (you could make a 270-degree sweep display if you desire) or a piece of
VeroBoard will do just fine. Take care that all parts are aligned
correctly. Don't install the link (LNK1) at this stage.
Don't fit the electronics or the MAP Sensor to the car just yet.
The unit is very easy to adjust and is for the most part
self calibrating. First of all, without the MAP Sensor connected, connect
pin 2 of the 4-Pin connector to ground. Apply power and using a voltmeter,
adjust the variable resistor R4 until pin 6 of the LF351 (accessible
where LNK1 will be installed) is zero. This should be close to
mid-position if you don't have a meter handy.
Now, disconnect the power, insert LNK1, and connect the MAP Sensor to the 4-pin connector.
With the MAP Sensor exposed to normal atmosphere, re-apply power, and adjust the variable resistor R3 until LED 5 just comes on. You could attach a piece of hose and apply pressure and vacuum and see the display change. If you're blowing and sucking, only a superior pair of lungs is going to work here!
Please note this calibration assumes you are at, or close to sea level. If you live at high altitudes, you will need to use the 2.5V method, or adjust for the differing barometric pressure.
If you are installing in a MAP-Sensed car, you could use a variable power supply to provide a 2.5V input for the previous step.
Connect a switch to J2 and you can select between a bar or dot display. If you don't want to be able to select, shorting J2 will force a single dot display
The unit can be installed in the same manner as a conventional boost gauge.
Since the 200SX does not use a MAP Sensor, you will need
to obtain one. General Motors and Bosch both make units suitable.
You might find a Bosch unit in a wrecker, but the GM unit can be ordered as a
spare part for a supercharged VS or VT Commodore. (In the US, the
supercharged 3.8L V6 used in Buick and Pontiac cars).
Suitable units can also be obtained from Haltech and Motec as spare parts. Do not use a unit out of an AU or BA Falcon, it's output is a pulse, proportional to the MAP Reading.
|U1||LM3914 Display Driver|
|U2||7808 Voltage Regulator|
|U3||7805 Voltage Regulator (Optional)|
|U4||LF351 Operational Amplifier|
|D1 - D10||
Light Emitting Diodes
(You can use one of the 10-LED Arrays)
|R3||5K0 Variable Resistor|
|R4||10K Variable Resistor|
|C1||2.2uF 25VW Electrolytic|
|J1||Power connector to suit|