Fig.4: typical sensor and meter connections as found in a vehicle.

Fig.4 shows the typical sensor and meter connections as found in a vehicle. Generally, the sensor is grounded and the existing analog meter connects in series with this to a regulated supply. The other possible configuration is when the meter itself is grounded and the sensor connects to the regulated supply instead.

In either case, you can connect to the junction of the sensor and the meter (marked with an 'x') to obtain a signal for the Digital Instrument Display.

Alternatively, the sensor can be rewired as shown in Fig.5, using a fixed resistor (R1) to replace the meter. Note that R1 can be installed on the microcontroller board.

It is important to note that the Digital Instrument Display is designed to accept a signal voltage at its input which is within a certain range. So you will need to make some measurements to check whether the voltage range from the sensor is suitable. If the signal voltage is outside the limits, it can be tailored using several adjustments at the input to make it suit.

The voltage limits for the Digital Instrument Displays input are as follows:

(1) with R3 out of circuit, the unit can be used with voltages ranging from 0.5-4.5V.

(2) with R3 in circuit and VR1 adjusted so that the unit can read down to 0V, the Digital Instrument display can measure up to 2.7V when VR2 is fully clockwise (250kΩ) and up to 3.4V when VR2 is fully anticlockwise (0Ω).