Chapter 21: Semiconductors Input Devices

What happens to the resistance of a PTC thermistor when it is heated?

The resistance increases as the temperature increases. This is because in a PTC (Positive Temperature Coefficient) thermistor, heating causes the device to become more resistive.

What happens to the resistance of an NTC thermistor when it is heated?

The resistance decreases as the temperature increases. This is due to the increased thermal energy generating more electron-hole pairs, allowing more current to flow.

What is the most commonly used thermistor type?

The NTC (Negative Temperature Coefficient) thermistor is the most commonly used type, particularly in applications like alarms, temperature sensors, and fan controls.

What happens to the resistance of a photoconductive cell as light on it increases?

The resistance decreases. As light increases, the semiconductor material allows more charge carriers to flow, reducing resistance.

What happens to the current flowing through a photoconductive cell as light on it increases?

The current increases. With more light, there is less resistance, which allows more electrical current to pass through the photocell.

What happens to the resistance of a photodiode when light is shining on it?

The resistance decreases. When light hits the photodiode, it reduces resistance by exciting electrons, making it easier for current to flow.

What happens to the current flowing through a photodiode when light is shining on it?

The current increases. As resistance decreases due to light exposure, more electrical current is allowed to pass through the diode.

If a pressure sensor that is rated to output 0 VDC to 10 VDC with a specified pressure operating range of 0 psi to 500 psi outputs 2 VDC, is it working properly according to manufacturer specifications?

Yes, it is working properly.

The pressure sensor is rated for a 0 to 500 psi range and outputs 0 to 10 VDC proportionally.

0 V = 0 psi - 10 V = 500 psi

So: 2 V = (2 V / 10 V) × 500 psi = 100 psi If the actual pressure is around 100 psi, the sensor is performing correctly.

If a pressure sensor that is rated to output 4 mA to 20 mA DC with a specified pressure operating range of 0 psi to 500 psi outputs 2.5 mA, is the pressure sensor working properly according to manufacturer specifications?

No, it is not working properly.

The lowest it should output is 4 mA, which corresponds to 0 psi. A reading of 2.5 mA is below the minimum range, which may indicate:

A faulty sensor

A wiring issue

Or incorrect calibration This reading is outside the expected operating range, so it does not meet specifications.

In a thermal-type flow detection sensor, does the sensor's thermistor produce a higher or lower electrical signal when there is no flow?

The thermistor produces a higher electrical signal when there is no flow.

With no movement, the heated sensor head retains heat, and the thermistor detects this elevated temperature as a stronger signal.

Is a thermal-type flow detection sensor a fast-acting or slow-acting change detection type?

It is a slow-acting change detection type.

Flow sensors usually have a time delay (less than 30 seconds) to allow for system startup and to prevent false alarms due to momentary pauses.

Is the output of a Hall effect sensor of the digital (ON/OFF) type or analog (varying) type as a magnet moves closer to the sensor?

It can be both, depending on the application.

If the sensor is designed for simple detection, it will output digital ON/OFF signals.

If designed for position or distance sensing, it will output a varying (analog) voltage as the magnet moves closer.

Can a magnet actuate a Hall effect sensor by moving in a sideways or straight manner?

Yes,

A straight (head-on) movement activates the sensor by aligning the magnet directly over it.

A sideways (slide-by) movement works by dragging the magnet across the sensor’s face at a constant distance.

What type of proximity switch detects metallic objects?

An inductive proximity switch detects metallic objects.

It works by creating a magnetic field and detecting the eddy currents formed by conductive metals.

What type of proximity switch detects any object that has a high dielectric constant?

A capacitive proximity switch detects any object with a high dielectric constant. These objects can be metallic or non-metallic, including materials like water, plastic, wood, and glass.

What is the operating mode of an ultrasonic sensor called when the emitter sound waves travel in only one direction to the receiver?

This is called Direct Mode. In direct mode, the emitter and receiver are placed opposite each other, and sound waves travel directly from one to the other.

What is the operating mode of an ultrasonic sensor called when the emitter sound waves travel in one direction to the detected object and bounces back in the opposite direction to the receiver?

This is called Diffused Mode. In diffused mode, both emitter and receiver are in the same housing. Sound bounces off the object and returns as an echo for detection.