Sensor overview

A sensor is a device that detects or measures a physical quantity. In this part we are only interested in the sensors that have an electric output. This part is a short overview of the different types of sensors. If you want to read more about all kind of different sensors, it’s a good idea to look at this link:  
http://www.infusionsystems.com/products

  Resistive sensors
The resistive sensor changes value (electrical) when the physical quantity (e.a. temperature, force, light etc) changes. These sensors can (generally) be used in combination with an extra resistor, like described above.

Switches
The most common sensor is a switch. A switch is actually a resistor with two values: infinite and zero. When a switch has to be connected to the analog input of the MicroLab, the same series resistor as mentioned before is needed.

 

 

 

 

A good value of the series resistor R1 will be between 10K-50K. In rest situation (switch is not pushed), the output will be +5V (MIDI value 127). When the switch is pushed, the resistance will be zero (shortcut to gnd) and the output voltage will be 0V.

Potentiometers
A potentiometer is well known ‘sensor’, which you can find in almost every electronic device. A potentiometer is a resistor that changes value when the position is changed; i.e. the volume button of your radio, the fader of an audio-mixer etc. The potentiometer is the only resistive sensor, which doesn’t necessarily need a series resistor. The potentiometer itself can be seen as two different resistors.

 


It has a resistance element that is attached by three contacts, or terminals. The two ends of the resistor are directly connected to two terminals. The third one (the middle) is usually a moveable terminal that slides across the resistance element, effectively dividing it into two resistors; for this reason it is sometimes called a voltage divider.


In position 1 the potentiometer is in the middle. This means: R1=R2. In position 2 the value of R1 is almost zero, and the value of R2 is maximal. Position 3 shows the opposite: R1 is maximal and R2 is almost 0.  

Temperature sensors
There is a wide range of temperature sensors. In general these type of sensors change resistor value, when the temperature changes. The simplest forms of temperature sensors are NTC or PTC. This stands for Negative Temperature Coefficient or Positive Temperature Coefficient. These components have two connections (like a resistor). In case of a NTC, the value of the resistor will decrease if the temperature rises. The PTC will do the opposite. If you want to know more about temperature sensors, look on the net. For example: http://www.temperature.com

 

 

Force Sensing resistors  
Nicknamed “the electronic gas pedal” an FSR is a tactile sensor, which responds with variable resistance. The harder you press on this thick film device, the more its resistance drops. The FSR was first developed for use in music keyboards. Musicians wanted their electronic pianos to play louder when the keys were hit harder, and softer when they were hit softly. More information:

l  

Light sensors
Light sensors are resistors that change resistance when the intensity of the (visible) light changes. The most common form is the LDR. This stands for Light Dependent Resistor. They are available in all different kind of forms and ranges.

 

http://www.doctronics.co.uk/ldr_sensors.htm

 

 

 

Active components
Not all sensors create a resistor change. There are also active sensors or components that generate voltage change, pulse width modulation (PWM), current change or frequency change. The practical problem with using these active sensors will be the fact that you almost always need some extra electronic circuit to create the right dimension or voltage range. In this part a few examples of sensors that are often used in combination with the MicroLab.

 

Hall effect sensors
A Hall effect sensor is a sensor that measures the change of the strength of the magnetical field. When a little (or big) magnet changes position relative to the hall sensor, the voltage output of the (active) sensor will change. This sensor can for example be used measuring the moving parts of a device. The movable parts, applied with a magnet will generate a changing magnetical field and thus provide the MicroLab (or other device) with a voltage change.

The only problem is that this voltage change is very little.  If you want to connect this directly to the MicroLab, the MIDI range you will get will be too little. This means you first have to design or make a little amplifier.

 

The circuit to use will look like the figure above. The output of the Hall sensor is connected to the Opamp (Operational Amplifier). The output will be amplified xx times. The amount of amplification (xx) depends of the values of the resistors that are in use. Because it will be a bit too much to explain the complete circuit here (for this moment), I will point out a few links.

http://www.cmsa.wmin.ac.uk/~alan/components/opamps/
http://www.dself.demon.co.uk/webbop/opamp.htm#basic

If you just look for some basic opamp design examples, you will find a lot of information on the internet.

 

 

 

Acceleration
With an accelerometer you can measure dynamic acceleration (e.g. the moving of your hand, vibration) and static acceleration (gravity). The accelerometer used in combination with the MicroLab is called the ADXL202. This is a 2-axis acceleration sensor on a single chip.  

The accelerometer has a digital output that can be used when it is directly connected with a processor. This same digital output can also be used in an analog way. Both X and Y outputs connected to a capacitor (check the datasheet for the right values) produce a little DC-voltage, which changes when the chip is moved. Also this relative little voltage change must be amplified first, before it can be connected to the MicroLab. The same circuit is used as mentioned in 2.4.1. The amount of amplification will be different!

 

 

Pressure sensors
Pressure sensors produce an electrical signal proportional to the pressure. There are five main pressure sensor types: absolute, gauge, vacuum, differential and sealed.  Each of these sensors measures different pressure variables.  The first of these sensor types are absolute sensors.  These devices test for absolute pressure, which is a measurement of pressure, relative to a perfect vacuum.  Gauge sensors are the most common type of pressure sensors.  They can be calibrated to measure pressure with respect to atmospheric pressure at a given location.  Vacuum sensors are used in situations where the pressure level that is being tested for, is lower than the localized atmospheric pressure.  Differential pressure is measured by reading the difference between the input of two or more pressure levels.  Finally, sealed gauged sensors measure pressure relative to one atmosphere at sea level (14.7 PSI) regardless of local atmospheric pressure’.

Most of the pressure sensors we used in combination of the microlab were made to measure the air pressure difference generated by the breath of a player.  If you want to use a pressure sensor, start looking at the following links:

http://content.honeywell.com/sensing/prodinfo/pressure_all/
http://www.ti.com/snc/docs/sensors.htm

 


Ultrasonic transducers
With the ultrasonic sender and receiver it’s possible to measure distance. These sensors can be directly connected to the microlab; no extra electronics is needed. An ultrasonic sender / receiver have a resonation frequency of around 40kHz.

In case of the microlab, the sender generates bursts (short pulses) of 40KHz. The receiver picks up these bursts and ‘tells’ the processor to stop sending the bursts. The time delay (the distance) is being converted to MIDI.

Check out:
http://www.msiusa.com/piezo_documentation.htm