Page 304 - Electromechanical Devices and Components Illustrated Sourcebook

266 Electromechanical Devices & Components Illustrated Sourcebook

Light Sensors

There are a variety of applications that call for the detection
of light. Most notably is in the area of opto-sensor sets. The
photosensitive vacuum tube, as shown in Figure 15-71, is Drill Through Hole in Top of Can
most commonly used in movie projectors that use an optical
sound strip. A bias voltage is applied across the cathode and
anode. When the cathode is exposed to light it emits electrons
in proportion to the intensity of the light.
Package
Cut & Discard Base Lead Wire
Light
Lead Wires

Figure 15-73 Photo Sensor Made from an Ordinary
Transistor
Cathode
Anode
Glass Envelope
and the light operates as the base signal. Figure 15-73 shows
an ordinary transistor that has been modified to operate as a
photo sensor.

Vibration Sensors

Detecting vibration can provide important information as to
the internal condition of many kinds of industrial equipment.
A vibration sensor generally consists of a floating coil sus-
Battery
Microammeter pended within a magnetic field, as shown in Figure 15-74.
Figure 15-71 Photosensitive Vacuum Tube Schematic The base of the sensor is fixed to the equipment to be moni-
tored. As the equipment vibrates, so does the base and per-
manent magnet assembly. The coil is suspended between the
poles of the magnet, at the end of a leaf spring, which is fixed
Figure 15-72 shows a photosensitive diode. These units
to the base. The inertial mass of the coil has a tendency to
operate in the same fashion as a diode, except that they can be
resist any movement and, therefore a voltage is generated by
turned on and off by exposure to light. These devices are used
the differential movement of the magnet.
in most opto sets and are very inexpensive.
Most transistors are photosensitive and can be modified to
operate as photo sensors by simply drilling a hole in the top
Coil Mount
of the can that packages the device. The base wire is removed Leaf Spring
Magnet Poles Coil
Spring Mount
Permanent Terminal
Magnet

Schematic Symbol Terminal Base
Figure 15-74 Moving Coil Vibration Sensor
Lens
Package
The output of the vibration sensor can be rectified and the
Lead Wires
signal is fed to a voltmeter. The meter will display the relative
amplitude of the sensor. A rising voltage indicates a deterio-
rating condition within the equipment being monitored.
Figure 15-75 shows a schematic of a vibration sensor circuit.
Figure 15-72 Photosensitive The filter capacitor is intended to smooth out the pulses and
Diode provide a more steady voltage reading.