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I am currently studying the textbook Handbook of Modern Sensors - Physics, Designs, and Applications, fifth edition, by Jacob Fraden.

In this textbook, the author states the following:

All sensors may be of two kinds: passive and active. A passive sensor does not need any additional energy source. It generates an electric signal in response to an external stimulus. That is, the input stimulus energy is converted by the sensor into the output signal. The examples are a thermocouple, a photodiode, and a piezoelectric sensor. Many sensors are direct sensors as we defined them earlier.

In researching this further, I came across this Quora question, in which user Giulio Moro claims the following:

An active sensor would be like your garage door sensors. One side has a light and the other a sensor. A beam fires across the area when the doors closing to ensure it's clear. If the light beam is broken, the door stops, and then opens wide.

A passive sensor would be like a metal detector. It creates a field, and when anything crosses it, it makes a sound based on the type of obstruction. A low pitch for say a plastic bottle or piece of wood, and a high pitch for a metal object.

There are two aspects of this definition of passive sensor that slightly confused with:

  1. Firstly, what is meant by an "additional" energy source? And analogously, if there is an "additional" energy source, then what would the base/default energy source?

  2. I think that this second confusion likely stems from the first, and clarification of the definitions of an "additional" and base/default energy source will probably clear this up, but I want to ask it anyway just in case. From a physics standpoint, I'm not totally convinced of this definition of passive sensor. In particular, I'm struggling to come to terms with this idea that a sensor can function without an energy source (although, again, the text does say additional energy source). The example of the metal detector also seems unconvincing, since my understanding is that metal detectors are certainly connected to some energy source (say, at airports or such)?

I would greatly appreciate it if people would please take the time to clarify this.

EDIT: After reviewing the answers (thank you all), one thing is obvious: There is a terribly large amount of contradiction with regards to what sensors qualify as active or passive.

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    \$\begingroup\$ Think of a thermistor that changes resistance with temperature, this is a passive device. It operates the same if it's in a circuit or not. \$\endgroup\$
    – Ron Beyer
    Jan 5 '20 at 14:38
  • \$\begingroup\$ @RonBeyer And it is able to do this due to the inherent chemistry/physics of the device, rather than any extra ("additional") energy source? \$\endgroup\$ Jan 5 '20 at 15:01
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    \$\begingroup\$ Yes, exactly right. \$\endgroup\$
    – Ron Beyer
    Jan 5 '20 at 15:07
  • \$\begingroup\$ @RonBeyer Ahh, ok, I understand now. Thanks for the clarifying comment. \$\endgroup\$ Jan 5 '20 at 15:08
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    \$\begingroup\$ @RonBeyer Haha, funnily enough, later in the text, the author specifically describes a thermistor as an active sensor: "For example, a thermistors is a temperature-sensitive resistor. It does not generate any electric signal, but by passing's electric current (excitation signal) through it its resistance can be measured by detecting variations in current and/or voltage across the thermistors. These variations (presented in ohms) directly relate to temperature through a known transfer function. Another example of an active sensor is a resistive strain gauge, ..." \$\endgroup\$ Jan 5 '20 at 15:23
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Adding my two cents. Maybe there are no formal definitions, but the one given in your textbook seems pretty formal to me, so let's stick with that.

And it is quite straightforward - no additional energy should be applied to the sensor in order for it to be considered "passive".

The keyword here is "additional". The caveat is that the physics laws require some energy to be applied to the sensor in order for it to detect something. In fact, energy IS the thing that is being sensed.

The thermocouple converts thermal energy into voltage, photodiode (in photovoltaic mode) converts light energy, and piezoelectric sensor converts mechanical energy. You can add a simple coil in the magnetic field to this list, which converts motion into voltage. And so on.

An "active" sensor also senses energy (once again, there is no way around this). However unlike "passive" sensor it does not generate voltage on its own. Instead it changes its property (usually resistance) in response. And at this point you need additional energy to read this change and amplify it to some useful level.

The same photodiode in photoconductive mode becomes active sensor, because you need to apply voltage to read its resistance. Hall effect sensor requires current across it to be able to detect magnetic field. MEMS sensor requires electrical excitation to detect motion.

Your second source sounds confusing, and in fact it is. That is because it mixes complex sensor systems into the picture, and tries to categorize them by simple sensor criteria.

Now, the problem is that in addition to simple sensors described above we also call "sensors" some complex devices of which the sensor itself is just a tiny component.

For example, garage door sensor built on photodiode in photovoltaic mode will be an active "sensor", even though diode itself is passive sensor. Why? because you have to apply energy to the circuit that detects voltage on a diode, amplifies it and switches external relay. Not to mention energy consumed by the laser, which can be considered a part of the whole "garage door sensor" device.

Same with metal detector. The coil that generates voltage in response to magnetic field is passive sensor by the definition above. However in order to induce magnetic field in a metal you need to generate alternating magnetic field of your own, and for that you need additional energy. So, if you take the entire metal detector device and call it a "sensor" then it becomes active by the same definition.

I hope the above makes things clearer. Having said that [insert mischievous grin here], a bonus question for you:

Is laser sensor card an active or passive device?

On one hand, it detects IR radiation without any energy applied to it. On the other, it has to be recharged (i.e. energy must be pumped in) by visible light before use.

UPDATE:

After reading all the answers and comments here I believe the root of the confusion and the fuel of all the discussions is in the definition of a "sensor" itself.

Let's get this straight:

  • some devices can function by themselves, directly converting measured physical phenomenon (energy) into useful output signal (e.g. voltage, visible light, mechanical movement). By the textbook definition these all are passive sensors.

  • some devices cannot produce meaningful output signal without additional energy input, whether directly used for excitation or indirectly for required amplification circuits. In this case we should call the device a "sensing element" (not a sensor!), which makes the containing device an active sensor.

  • finally, even passive sensors can be equipped with the additional circuitry to simplify their usage (various current/voltage transducer modules, for example), which makes them active again.

So, if we count only sensing elements as "sensors", then classification is straightforward. If we allow complex devices to be called "sensors" even though the actual sensing element is just a tiny part of them, then majority of the modern sensors will be active devices.

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  • \$\begingroup\$ is a diode thermometer sensor passive or active to you? \$\endgroup\$ Jan 5 '20 at 19:24
  • \$\begingroup\$ Nice answer. And thanks for introducing me to laser sensor cards; I never even knew that such a thing even existed. The website you linked says that "card must be charged with daylight or artificial light", so, if I'm understanding what you've said, I would say that this is an active sensor, since it first needed external energy in order to allow its operation. \$\endgroup\$ Jan 5 '20 at 19:41
  • \$\begingroup\$ that argument applies to resistors as well \$\endgroup\$ Jan 5 '20 at 19:46
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 if by "diode thermometer sensor" you mean using diode temperature coefficient under constant current condition, then it is an active sensor by the quoted textbook definition. Simply because diode cannot function as temperature sensor without all the additional circuitry required to get useful output, and that circuitry does need energy. \$\endgroup\$
    – Maple
    Jan 5 '20 at 19:46
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    \$\begingroup\$ @ThePointer That was a tricky question and the answer is open to interpretation (as by the way this entire topic). Myself, I'd go with your reasoning too. Since the card cannot function without additional energy when the stored one is used up. \$\endgroup\$
    – Maple
    Jan 5 '20 at 20:02
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There are no strict, formal definitions of "passive sensor" and "active sensor". These terms can have different meanings based on the context in which they are used. Both of the quoted paragraphs are basically correct but they have different contexts.

Regarding the "additional energy source", this would typically be a battery or similar power supply. The word "additional" here means a source in addition to any energy provided by the sensor itself. In many cases the sensor itself provides no energy so the "additional" energy source is "in addition to the sensor" not "in addition to a another energy source".

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Writers can divide the sensors to classes as they want. I would make my division as follows:

Active sensors make some actions which affect the environment. They for example send radio waves or apply a force. The response is analyzed and the information is extracted.

Passive sensors only take what the environment offers and they change their state along the changes in the environment. For example a light dependent resistor changes its resistance along the variations in light. Or a dynamic microphone gives a voltage which depends on how loud voices there exist. The interesting data can be sometimes extracted easily or it can need complex decision making. But the most complex decision making system doesn't make a sensor active as long as the sensor doesn't send anything which is used to test the environment.

Active sensors surely need some energy source to be able to make physical work. That energy can be collected from the environment, it can be taken from a source which is assembled in the sensor box or there can be supply lines from an external source such as a computer or AC mains.

Passive sensors need energy to transfer their state to the information user which can well be a device. That user can supply everything which is needed to read the state (for example some voltage to cause a current which depend on the resistance which depend on lightness, temperature humidity or other interesting quantity). Or the sensor can have the tools which use energy and convert the state to more robust or more informative form of data. It can for ex. have a numerical display or to WiFi connected computer. That energy can as well be collected from the environment, stored in the sensor box or supplied with wires.

A dynamic microphone collects from the environment the energy which carries the state to the user. The sensed sound directly makes the needed electricity. But that starts to be false as soon as one decides that a preamp is needed for more robust signal and within the same breath he says that the preamp is a part of the sensor.

Intruder alarm sensors for example can as well be passive as active and both need energy to do their job. PIR sensor does not send anything, but their photodiodes need quite complex amplification, filtering and decision making circuits. In addition a clear intrusion alarm signal must be formed. There are also radar based intrusion alarms which I consider active.

Even a door switch can be active or passive. It's active for ex. if it contains a spring which moves a contact if a door opens. It has got its energy beforehand loaded to the spring when the door was closed. A passive door switch hasn't that spring, the contact must be moved by the door to both directions.

As said everyone can write his definitions as he wants. Nobody cares as long as the writer doesn't fool the readers or collide against law.

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The textbook also says:-

The active sensors require external power for their operation, which is called an excitation signal. That signal is modified (modulated) by the sensor to produce the output signal... For example, a thermistor is a temperature-sensitive resistor. It does not generate any electric signal, but by passing electric current (excitation signal) through it its resistance can be measured

That means power for the sensor's output does not come directly from the phenomenon being sensed. So a 'crystal' radio set would be passive, but one that amplified the rf before detection would not. A passive infrared sensor element is passive, but a PIR is not. A photodiode used in photovoltaic mode is passive, but in photoconductive mode is active, etc.

This definition is useful for industrial applications where it's important to know whether a sensor needs to be powered or not. But it's not the only definition.

NASA defines passive sensors as instruments "designed to receive and to measure natural emissions produced by constituents of the Earth's surface and its atmosphere.", while active sensors measure "signals transmitted by the sensor that were reflected, refracted or scattered by the Earth's surface or its atmosphere."

The subtle difference here is that the excitation signal is not 'modulated' by the sensor, but by the (remote) object being sensed.

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  • \$\begingroup\$ Thanks again, Bruce. You meant "crystal"? And what is a "PIR"? \$\endgroup\$ Jan 5 '20 at 19:27
  • \$\begingroup\$ Yet one cannot measure resistance without applying an external current to measure the voltage to say measure the temperature change of copper. With author's definition, the wire in a transformer is an "active" sensor to measure hotspot temp rise. Yet we always consider this passive like thermistors and resistors \$\endgroup\$ Jan 5 '20 at 19:44
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    \$\begingroup\$ Arrgh! There were so many words that Firefox doesn't think are spelled right (but are) that I missed this one. PIR is the abbreviated acronym for Passive InfraRed sensor, a device used in alarm systems. Confusing, right? like calling a computer monitor or TV set a 'CRT'. No, the cathode ray tube is the thing in it! \$\endgroup\$ Jan 5 '20 at 19:45
  • \$\begingroup\$ NASA's def'n suits them where one must send a certain frequency range to get an active response of conditions. Like the Mother -Daughter split payloads we had in the 70's using a spectrum analyzer with TX on mothership, Rx on daughter-ship then exploding water in between to create a medium of humidity in upper atmosphere then measure the s21 parameters < 500 miles above earth \$\endgroup\$ Jan 5 '20 at 19:51
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 "Yet we always consider this passive like thermistors and resistors". We consider them passive components, not "sensors". And that's key difference, IMHO. As sensors they cannot be used alone, and that is why we usually call "sensor" the whole active device that is based on passive component. \$\endgroup\$
    – Maple
    Jan 5 '20 at 19:55

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