How do digital sensors work?

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I always wondered how electronic sensors work. I mean, there is some kind of analogue sensor that detects a value that is not quantifiable (temperature, pressure, speed etc.). How exactly does conversion from analogue detector get transformed to some kind of numeric value that gets shown to user?

In: Engineering

It depends on what you’re sensing!

Temperature is usually sensed with a device called a thermocouple. Thermocouples use the thermoelectric effect – certain types of devices will create voltage when there’s a temperature difference between the two sides of the device. The amount of voltage they make is directly in proportion to how big the temperature difference is. So you can make a thermocouple and then calibrate it with known temperatures (like how you know an ice bath is 0° and you know boiling water is 100° at sea level,) and then you can hook your thermocouple up to a multimeter. The amount of voltage it makes tells you how hot of a temperature you’ve exposed it to.

Pressure is measured a few different ways. You can have a flexible membrane hooked up to a strain gauge – more pressure means it pushes harder on the membrane, which means more strain is measured by the strain gauge – and again that makes a signal that your multimeter or other detection software can read.

The basic gist to measure X digitally is find a device that will produce a varied current or voltage with respect to X, and connect it to something that can really precisely measure and record voltage. And then calibrate the device.

Sensing real world phenomenon is inherently analog. Once that analog value is read, it then has to be converted into a digital signal via an analog-to-digital converter.

There are many ways ADC can be implemented.

However, a good way to understand the principle would be to imagine I’ve got a stick hidden behind my back and you want to know how long it is. You can ask me yes/no questions to get the answer.

For example, you could ask me “is it between 6 and 7 inches?” followed by “is it between 7 and 8 inches”… and so forth until I answer yes. Indeed, you could get a bunch of friends and you could all ask me questions about the entire range simultaneously if you wanted to speed up the process.

Or you could ask questions like “is it bigger than 6 inches?” and, depending on the answer, “is it bigger than 9 inches?” or “is it bigger than 3 inches?”.

If you asked enough questions, you’re eventually going to get the answer to any arbitrary level of precision you desire.

All those ‘yes/no’ answers are bits in a digital number.