Home Chapter 17 Electronic switches, Hall effect sensors, break beam, capacitance, piezo
Electronic switches, Hall effect sensors, break beam, capacitance, piezo

Electromagnetic relays can use a smaller voltage, often from a micro controller to switch a larger voltage.  They isolate the switching portion of the circuit from the voltage and current that is being activated, though they tend to create electrical noise in the process. Therefore it is advisable that you use a diode with the circuit to stop the backward EMF or electromagnetic Force that can happen when you deactivate the relay.  Such a back EMF can fry your micro controller. 

You can imagine the electrons bouncing around inside a coiled wire and the electromagnetic noise it would cause. Electromagnetic relays require more current than other relays to switch larger voltages.

Solid-state relays on the other hand, as detailed in chapter 9 work with TTL logic to switch voltages on a off and in the process do not consume very much power.

The relays you see directly below is an electromagnetic relay switch. With the closing of one switch it can activate many lines at one time.  This relay uses an electromagnetic coil to  create a magnetic field and pulls a switch down. Most microcontroller do not have the power to close an electromagnetic relay directly though you can use a small transistor or TIP Darlington transistor to activate larger relays. 


Electromagnetic Relay. As a coil creates magnetic field it pulls a switch down


Some very small relays such as reed relays can be activated by a microcontroller directly if the pin can source or provide enough current to allow the relay to close. One that will work with some microcontrollers is Radio Shack 5VDC/.5A SPST Reed Relay product number 275-0232.  Remember SPST means single ple (one switch) closes and ST means Single line closes. This relay can be driven off of some controllers directly. 


Reed Relay


Hall Effect sensors are magnetic sensors, which are effected by increasing or decreasing Gauss magnetic flux. There are Hall effect sensors, which just switch, on or off as a digital switch they sense the magnet field or they do not. There are also linear Hall effect sensors called radiometric hall effect sensors, which are extremely sensitive and accurate to varying quantities of magnetic flux. Hall effect sensors are valuable because they do not require a contact to make the switching and therefore they do not show mechanical wear over time. They can be used for contactless sensing, sensing of turning objects like gears, speed sensing, and direction sensing.

A good source of hall effect sensors is Allegro micro at http://www.allegromicro.com and Xensor at http://www.xensor.nl/index.htm.

Keep in mind that while you can find data sheets and valuable information at these web sites that you will have to go through a distributor to purchase these manufacturers parts.


Hall Effect Sensor

Break Beam sensors are created by having a reference beam sometimes a laser or laser diode. The focused light source on a phototransistor or photo resistor. When the reference beam goes away as people break the beam by passing in front, the switch is activated.

A good source is for a matched set is Parallax infrared receiver and sender. It is wise to get a matched set of infrared sensor and receiver. Radio Shack also has a matches set 276-142 is there product number.


Matches infrared receiver and diode from Parallax

Capacitive Touch switches are switches that are solid state with no moving parts and are often capacitive and are used for sensing touch or other conductive materials. A good source for these is HVW Tech at: http://www.hvwtech.com. This is an analog sensor so in order to use this with the BS2 you would need to use an analog to digital converter.


Board Dimensions: 3.0cm x 3.8cm


Piezo pickups are great ways to sense minute vibrations propagating through materials. Amplified guitars often use piezoelectric pickups however when you purchase these from music stores they are very pricey and set to pick up the frequencies they are designed to work optimally within.

When purchasing make sure you have a good sense of the frequency you are trying to amplify or sense and then purchase at that frequency. A good source for piezo products is Digikey and American Piezo at http://www.americanpiezo.com and MSI USA Inc at: http://www.msiusa.com and Sparkle Electronics at: http://www.sparklerceramics.com.

For an excellent application note and some entertaining examples of how to use them Google search the name Richard Lerman and piezo pickups and you will see how he amplified a spider web to make a wind harp. The sound artist Richard Lerman has used these sensors extensively.



Piezo Disks


Future developments

 Nanotechnology will play a major role in the future development of sensors and their interfaces to computers. One can imagine ultra small nanotech surfaces that are mini linear actuators, which can create pressure on the body and receive electrical signals from muscle activation. The pressure waves that are created with these nano devices could allow us to have more immersive experiences as the multisensors provide input to computers and create feedback to the computer.

Takao Someya and his research team at the University of Tokyo have created pressure-sensors that have a high degree of sensor density. They have created a sensor array with 16 sensors per square centimeter that will permit robot to have an artificial skin and use this to have better tactile feedback.


Sensor Matrix by Takao Someya. 2004. University of Tokyo.


The "skin" is constructed of rubber and organic transistors, which allow sensing and conduction. These sensor arrays could be used in the future to monitor the heart and breathing rate of hospital patients or they could be imbedded into carpets to distinguish family members by the rhythm of their walk thereby adjusting the lighting in the room. This artificial skin may allow the more domestic robots that are sensitive to their human habited environments.