Below is the pressure gauge of the heavy equipment. The pressure gauge needle is moved by a hydraulic pressure line.

We are looking to capture the hydraulic pressure into an IoT device that takes an analog voltage as input.

How do I convert the hydraulic pressure into a corresponding analog voltage which can be given as input to the IoT device?

Front side

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  • 9
    \$\begingroup\$ use a pressure transducer \$\endgroup\$
    – jsotola
    Apr 12, 2022 at 6:39
  • \$\begingroup\$ glue a magnet to the needle and attach an analog hall effect sensor tot he bottom to read "distance", or use a camera and math. \$\endgroup\$
    – dandavis
    Apr 12, 2022 at 6:40
  • 2
    \$\begingroup\$ why analog? ... digital output would require a simpler interface \$\endgroup\$
    – jsotola
    Apr 12, 2022 at 6:40
  • 5
    \$\begingroup\$ @this.__curious_geek why do you assume digital to mean binary? You should research the different gauges and sensors available. \$\endgroup\$
    – Solar Mike
    Apr 12, 2022 at 7:13
  • 1
    \$\begingroup\$ If you get a digital pressure transducer that might make interfacing easier as you don’t need a ADC. CAN seems to be a popular interface for the digital ones. \$\endgroup\$
    – Kartman
    Apr 12, 2022 at 9:04

3 Answers 3


You use a pressure transducer which gives some sort of electrical output. On the hydraulic side, common interface is national pipe thread (NPT). You need to tap in to the hydraulic line either at the distribution manifold or inserting a 'T' connection. There are many companies that offer pressure transducers. Keller and Omega are two such companies. Omega has good user information on use of pressure transducers.

Common output formats are:

  • Analog current output, specifically 4 to 20 mA current. 4-20 mA current loop output is often used in industrial environments since it only requires two wires and accuracy isn't affected by cable length. You need additional circuitry to convert 4-20 mA to the desired voltage range for your analog to digital (A/D) converter input on the microprocessor. There are solutions you can buy off the shelf for interfacing to 4-20 mA current loop. Not knowing your installation, this is a good solution since it is robust.
  • Analog voltage output. You can feed this directly to an A/D converter with some protection circuitry. I would steer clear of this solution due to voltage drop on the ground line that can affect the accuracy of the measurement unless you know what you're doing.
  • Digital output, specifically I2C, which is a standard communications scheme for microcontrollers. This is fine for shorter runs in a nice environment, but probably not suitable for industrial environments where you need long cable runs. This is the easiest interface for a microcontroller that has an I2C interface.
    [edit] As mentioned by @Kartman, pressure transducers are also offered with CAN bus.

Pressure transducers can be damaged by spikes in pressure. Protect the pressure transducer with a snubber designed for hydraulic useage.


How to convert hydraulic pressure gauge into analog output?

The Bourdon tube pressure gauge may be replaced by a digital pressure gauge with a pressure transducer, an LED / LCD seven segment display and analog (0-10 V / 4-20 mA) or digital (BCD) output.

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If you want it done in a couple of days without tearing the existing panel apart: Let your IoT device have a camera. Jevois would be one example brand. Then use image recognition to determine the angle of the arrow.

Such tasks are well suited for neural networks: one network will detect the instances and sizes of the dials in the image, so the camera orientation and framing won’t be critical. Another network will then work on each dial individually and extract the angle.

Today this is all done using existing open source building blocks, so it should be a quick task for anyone who has played with Jevois or similar systems before.

IoT devices can get input in more ways than just as analog voltages :)

  • \$\begingroup\$ This could work, however, lighting must be considered. If the sensor needs to work when lighting is extremely poor, then the camera solution won't work. This looks like it might be in a factory of some sort. If the lights ever go out in that factory, the camera may not have enough light to determine the needle position. If the lighting is very consistent then this might work really well. Obstruction should also be considered. If people will be walking around in between the camera and the gauges (for example) then the camera solution will not work properly during the time it is obstructed. \$\endgroup\$
    – user57037
    Apr 13, 2022 at 1:51
  • \$\begingroup\$ Glare on the glass could prevent the camera from getting a good reading also, if the lighting is variable (for example if direct sunlight strikes the gauge faces from windows or skylights). \$\endgroup\$
    – user57037
    Apr 13, 2022 at 1:56

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