Digital boost pressure indicator (Articles)

Last edited on 16-06-2026, 13:42, edited 13 times in total.

DIY Digital Boost GaugeA boost gauge is useful for almost any engine with a turbocharger, but how do you create a visually appealing gauge that integrates well with the existing instrument panel?
Here's a solution to this problem, illustrated with an example of an Audi A3 manufactured in July 1998 with an AHF engine, where an electronic display, using the same colors as the original instruments (red), was retrofitted into a free switch slot on the center console.

Principle of operation
To minimize the number of components required, the output signal of the already installed electronic pressure sensor was utilized. This sensor, combined with a temperature sensor, is located in the intake manifold of the example vehicle.
The 250kPa pressure sensor installed in the AHF is powered by the engine control unit (ECU) with a 5V DC voltage. It provides a linear output signal of 0.40V at 20kPa and 4.65V at 250kPa pressure in the intake manifold. Therefore, at atmospheric pressure, the sensor output is approximately 1.86V. Since the sensor can only provide a small amount of current, it is important not to use significantly lower resistor values than those mentioned below when designing the voltage divider.

The pin configuration of the sensor (4-pin connector) is as follows:

Therefore, the intake air temperature is measured between pin 1 and 2, and the output signal of the boost pressure sensor is measured between pin 1 and 4.
There are different versions of pressure sensors available, with varying measurement ranges.
"On my AHF (110hp TDI) engine, the turbocharger is factory-installed for a maximum pressure of 250 kPa (approximately 2.5 bar absolute)." The ASZ engine uses a version with a 300kPa (3.0 bar) pressure rating.

This voltage is now measured using a digital LED panel meter. A skillful calibration of the panel meter ensures an accurate display of the boost pressure.

For a 250kPa boost pressure sensor: At an output voltage of 1.86V (normal pressure, approximately 1013 mbar) from the sensor, a pressure of 0 bar must be displayed, and at 4.65V, a pressure of 1.50 bar.
The necessary offset adjustment of the display can be achieved by using a 2kΩ potentiometer connected between the +5V and ground pins of the pressure sensor: by adjusting the wiper, you can set any voltage between 0 and 5V. The potentiometer is set to 1.86V.
To obtain an initial voltage of 150mV at a boost pressure of 1.50 bar, which is then displayed as 1.50 bar on the LED panel meter, a voltage divider is used.
4.65V - 1.86V = 2.79V. This needs to be reduced to 150mV, so use a resistor of 180kΩ in series with a resistor of 10kΩ.

For a 300kPa boost pressure sensor: Under normal pressure, an output voltage of 1.62V is generated at 0 bar boost pressure; adjust the potentiometer to 1.62V. To achieve an initial voltage of 200mV at a boost pressure of 2.00 bar, the voltage divider must be designed as follows:
4.65V - 1.62V = 3.03V -> reduced to 200mV using a voltage divider with resistors of 150kΩ and 10470Ω (10kΩ + 470Ω).

For 7-segment displays, it's best to use super-bright versions. This ensures that the display remains legible even during the day.

Since the space available in the center console is very limited, I connected the 7-segment displays to the panel meter circuit board using shielded (radio!) cable. Diese ist unter der unteren Cockpitverkleidung befestigt und wird über eine Sicherung aus der Komfortelektrik versorgt.

Zum Schluß noch eine Warnung: Weil direkt das Signal des Ladedrucksensors angezapft wird, kann bei falschem Schaltungsaufbau die Ladedruckregelung nicht mehr funktionieren. Dies kann bei Ignoranz des Fahrers schnell zu Schäden an Turbolader und/oder Motor führen.
Die Schaltung ist supereinfach, richtig zusammen bauen und anschließen muß man sie trotzdem!

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