REED SWITCHES VS HALL EFFECT &ELECTROMECHANICALIntroduction
Purpose› Explore the different technologies to find the right one for agiven sensor application
Objectives› The sensor technologies we will focus on are – Reed, Hall andElectromechanical Sensors, all of which, provide a potential
switching function› We will define the key functions of these technologies› Cover the different parameters and compare them to thedifferent technologies

Introduction
With the availability of new technologies more sensing
requirements in:› Telecommunications› Instrumentation› Medical› Automotive› Household› Marine› Consumer products› General Purpose

Key Terms›

We need to define the following technologies
Reed Sensors› Hall Sensors› Electromechanical Sensors››

We will use the word load often. A load is thatwhich needs to be powered when the above
technologies are switched on.

Key Terms – The Reed Sensor
Reed Switch Sensor›

The reed switch has two leads hermetically sealed in a glass cylinder›

Sensitivity to closure is measured in milli-Tesla (mT) or ampere turns (AT)›The reed sensor typically uses an external magnet to close the contacts›››

When the magnet is brought into the sphere of influence of the reedsensor the contacts will close or Pull-in (PI)
When the magnet is withdrawn from the proximity of the reed sensor thecontacts will open or Drop-out (DO)Hysteresis is the ratio of the pull-in divided by the drop-out (PI/DO)

Key Terms – The Reed Sensor
Reed Switch Sensor› The reed sensor does not need any other circuitry tooperate› The reed sensor has only two leads and are notpolarity sensitive› They can be used in all environmental conditions,over wide temperature ranges and dirty
environments

Key Terms – The Reed Sensor
Reed Switch Sensor›The reed sensor comes in hundreds of different sizes and shapes to meet given applications

Custom Packaging

Bare Glass Surface Mount

Float Type Level Sensors

Threaded Screw & Cylindrical Panel Mount
Rectangular Screw Mount

Molded Surface Mount

Key Terms – The Hall Sensor
Hall Sensor››››

The hall sensor is asemiconductor device
An electric current must
always be flowing in the hallsensor
The hall sensor produces avoltage when a magnet or
magnetic field is broughtclose enough to the device.
The voltage is measured inmilli-volts

Key Terms – The Hall Sensor
Hall Sensor› Hall sensors are not affectedin any way with shock andvibration› They can be used in mostenvironmental conditions› Can be used in dirty
environments

Key Terms – The Hall Sensor
Hall Sensor› Because of the semiconductor
integration process, the hall chip canbe very small› Hall sensors come in several packages
with a minimum of 3 electrical leads› Hall sensors need added electroniccircuitry to function

Key Terms – The Electromechanical Sensor
Electromechanical SensorThe electromechanical Sensor
requires a mechanical movementthat presses against a mechanicallever on the sensor
The mechanical movement willclose a set of contacts usuallyinternal to the sensor
They can have two or more leads
They can have spst and/or spdt
(single pole with single throw anddouble throw contacts)Key Terms – The Electromechanical Sensor
Electromechanical Sensor
››

›

›

Electromechanical Sensor
The electromechanical sensors
require no external circuitry forproper operation
They come in many different
package styles
These sensor are not suitable inall environments

Parameter Analysis››

In the following slides we will look at the various
critical specifications that help define ourapplications
We will look at these parameters and how they
compare for each of the technologies

Parameter Analysis
Sensor Technology
Reed Sensor
Hall Sensor
Electromechanical Sensor

Additional Circuitry
Required?
NOYESNO

Parameter Analysis
Sensor Technology

Reed Sensor
Hall Sensor
Electromechanical Sensor

Current Draw
Required in OFFState?
NOYESNO

Parameter Analysis
Sensor Technology

Reed Sensor

Input
Polarity
Sensitive?
NOHall Sensor

YES

ElectromechanicalSensorNO

Comment

None required
Input polarities must beobserved for proper
operation
None required

Parameter Analysis
Sensor Technology

Reed Sensor

OutputPolarity
Sensitive?
NOHall Sensor

YES

ElectromechanicalSensorNO

Comment

No output sensitivity
Output polarities must
be observed for proper
operation
None required

Parameter Analysis
Sensor Technology Sensing Distance
Reed Sensor>1” or 2.54cm

Hall Sensor>1” or 2.54cm

ElectromechanicalSensorSensing can only works bymechanically touchingand depressing thesensor lever

Comment
No physicalcontact
required
No physicalcontact
requiredParameter Analysis
SensorTechnology
Reed Sensor

Minimum Sensing
Distance0.04” or 1.0mm

Hall Sensor

0.04” or 1.0mm

Electromechanical Has no sensing
Sensordistance

Comment
No physical contact
required
No physical contact
required
Physical contact isrequired

Parameter Analysis
Sensor Technology
Reed Sensor
Hall Sensor
ElectromechanicalSensorHysteresis
40% - 95%
No hysteresis
Hysteresis is fixed and can becontrolled

Parameter Analysis
Sensor Technology
Reed Sensor
Hall SensorElectromechanicalSensor

ON ResistanceComment
Typically 50 –100 mΩ
Typically > 1000Ω
Typically 50 –Can widely vary
100 mΩ
depending on
voltage/ current
flowing in load

Parameter Analysis
Sensor Technology
Reed Sensor
Hall Sensor
ElectromechanicalSensor

Switching Loads Directly?
Can switch loads directly. Requires noother circuitry
Cannot switch loads directly. Requires
other circuitry
Can switch loads directly. Requires noother circuitry

Parameter Analysis
Sensor Technology
Reed Sensor
Hall Sensor

ElectromechanicalSensorVoltage Switching Range
0 – 1000 Volts
Puts out milli-volts.
Requires other circuitry for higher
voltages.
> 5 Volts – 240 Volts

Parameter Analysis
Sensor Technology
Reed Sensor
Hall Sensor
ElectromechanicalSensorCurrent Switching Range
Up to 1 Amp directly and carry up to 3
Amps
Requires other circuitry
Up to 1 Amp directly and carry up to 3
Amps

Parameter Analysis
Sensor Technology
Reed Sensor
Hall Sensor
ElectromechanicalSensorOperate Time< 100 µsec
< 10 µsec< 100 msec

Release Time< 25 µsec< 10 µsec< 100 msec

Parameter Analysis
Sensor Technology
Reed Sensor
Hall Sensor
ElectromechanicalSensor

Output Capacitance
Typically 0.2 pico-farads
Typically 100 nano-farads
Typically 50 pico-farads

Parameter Analysis
Sensor Technology
Reed Sensor
Hall Sensor
ElectromechanicalSensorOutput Isolation
Typically > 1 x 10 12 Ω
Typically > 1 x 10 7 Ω
Typically > 1 x 10 8 Ω

Parameter Analysis
Sensor Technology
Reed Sensor
Hall Sensor
ElectromechanicalSensorLife Expectancy
Up to 1 Billion operations
Unlimited
Up to 1 Million operations

Parameter Analysis
Sensor Technology
Reed Sensor
Hall Sensor
ElectromechanicalSensor

EDI / RFI Susceptibility
None
VeryNoneParameter Analysis
Sensor Technology
Reed Sensor

Hall Sensor
ElectromechanicalSensorShock
Can be susceptible to shock. Newer
MEM micro-reed sensors are notsusceptible to shock.
No susceptibility to shock
Relatively low susceptibility to shockParameter Analysis
Sensor Technology
Reed Sensor
Hall Sensor
ElectromechanicalSensorOperatingTemperature-55°C to 200°C-0°C to 70°C
-20°C to 100°C

Storage
Temperature-55°C to 100°C-55°C to 100°C-55°C to 100°C

Summary››››Each technology has its own best operating
characteristics
Each technology needs to be selected based on therequirements
Choosing the wrong technology for a given
application can result in a lot of time and money lostalong with the loss of several potential customers
Care must be taken when pairing a technology to agiven application

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