Negisters, Unijunction Transistors, Precision Resistors, Hall Effect Devices
Nothing has impacted our lives more than the electronic devices we now use in our everyday lives. More and more, sophisticated devices are becoming commonplace and research is constantly adding to the list. Some of the new devices are the result of new technology while others are just new techniques applied to conventional devices. We survey a few of the more interesting examples here.
One of the most unusual devices in electronics is the negister. A common NPN transistor can be turned into a negistor by wiring it in reverse. The emitter is connected to the power source through a resistor and the collector is connected to ground. When wired this way the transistor exhibits a phenomenon known as negative resistance, also known as avalanching.
In the diagram we see that a capacitor across the transistor charges until the reverse breakdown voltage is reached. At this point the collector-to-emitter resistance actually reaches a resistance of less than zero and then the capacitor is discharged through transistor. Once the capacitor is discharged, the transistor returns to its resistive state and the capacitor starts to charge again. While not all transistors will operate in this mode, a surprisingly large number do. Even transistors that test bad may operate as negisters.
Amazingly, the circuit is extremely stable and produces a linear ramp wave. It can feed a speaker directly, or an amplifier stage can be added to use the oscillator as a function generator. Its wide range and stability make it suitable for use in music synthesizers. The components used in the circuit are just a starting point, as different transistors will react differently, and you may have needs that require other component values, so experiment. It is advisable to use a regulated power source of at least 15 volts.
White Noise Generator
The avalanche mode can also be useful in the production of white noise. The illustration below shows a transistor Q1 connected differently than the oscillator. Instead of the emitter lead, the base of the transistor is used, and since there are no frequency-selecting components, a cascade of random frequencies is generated. Transistor Q2 amplifies the signal using resistor R2 as a current limiter, while R3 and capacitor C1 smooth response and increase the overall gain. Potentiometer R1 will customize the drive current to obtain the best results from a particular transistor. It should be adjusted to get the maximum volume. White noise is all frequencies at the same level, and it is used in electronic music production as well as a tone source for audio testing.
One of the most unusual solid state devices is the unijunction transistor. Conventional bipolar and FET transistors use a base or gate lead to control the resistance between the two remaining leads. Unijunction transistors do not operate like other transistors. Unlike other transistors, the unijunction transistor actually passes the input signal through to ground. This makes it suitable only as a wave shape generator. It produces a negative pulse at base 1 and a reverse exponential ramp at the emitter.
In the unijunction circuit capacitor C1 charges through resistor R1. When a trigger voltage is reached (approximately 1.3 volts), the capacitor is discharged through base 2.
Capacitor C2 charges through resistor R2 and is discharged at the same time as capacitor C1, producing a negative pulse. The schematic symbol of a unijunction transistor looks like an FET except for the input lead which turns down, indicating the capacitor discharge to ground function.
Because the unijunction transistor is not a linear device, an FET amplifier stage is required to produce a linear ramp and an illustration shows how this is accomplished. Because of the complex harmonic content of a linear ramp, it is more useful as a synthesizer tone or test tone.
There are times when a specific value resistor falls outside the normally available values. You can use a trimmer potentiometer, however, the exact value may be hard to tune and vibrations can change the setting. Precision resistors are available commercially, but again they may not be the exact value you need. Because of new components and more demanding circuit designs, the need for precision resistors is growing.
One way to make your own precision resistors is to file them down. The illustration shows how to prepare carbon and film type resistors. Choose a resistor that is closest to, but greater in value, than you need. In the case of a carbon resistor, use a square or triangle file and work it across the resistor to cut a groove the resistor. Connect the resistor leads to an ohmmeter with alligator clip-leads to monitor the change in resistance. Film type resistors should be filed using a flat file.
One way to simplify the process is to solder the resistor tightly in place on the circuit board and gently file the resistor. When you have achieved the proper value, clean away the excess particles and then coat the resistor with airplane glue to keep out moisture.
Another way to make your own resistors is to use pencil lead or graphite. Even the resistance of a line on a piece of paper can be measured on an ohmmeter. Wire wrapped around a pencil lead can function as a resistor. Shrink-wrap will operate as an insulator or again, use airplane glue. Even center-taped resistors can be made from pencil lead. Different number pencils, such as a number 2, will act differently than a number 4, so again experiment.
In 1879, American physicist Edwin Hall experimented with magnetic forces and their effect on wire and metal surfaces. His work led the way for research that resulted in almost all of the electronic devices we depend on today.
If a conductor carrying an electrical current comes in contact with a magnetic field, a voltage, known as the Hall voltage, is generated across the conductor. The magnetic field distorts the natural flow of electrons through the conductor.
In 1985, German physicist Klaus-Olaf von Klitzing won the Nobel Prize in physics for his expansion of Hall Effect principles to semiconductor technology and discovered the quantum Hall Effect.
In the diagram we see how magnetic force displaces charged particles from one electrode to another, carrying with it a flow of current from point A to point B. Early experiments in the development of the Hall Effect sensor were not very successful. Semiconductors comprise electrons and holes that are naturally attracted to each other. Triggering a current flow through semiconductors with magnetic force was far too marginal and unstable using the doping techniques at the time. With the development of artificial semiconductor structures, a matrix was designed to hold electron and holes in thin layers of material separated by a barrier layer.
With careful attention paid to new doping ratios, along with new materials, the right chemical combination was found and a practical Hall Effect sensor prototype was built. Further improvements, such as signal amplification, differentiate signal levels to assure a digital output.
Although a Hall Effect sensor acts like a relay, it has two specific advantages. The first is no bounce, like the mechanical relay. The second is that the Hall Effect sensor can operate at very high frequencies (100k or more, and new research is continuing to expand this number).
Hall Effect Sensor
The package for the Hall Effect sensor is very much like a transistor. It is a three-lead in line device in a transistor size black plastic case. Some manufacturers also have a surface mount model. The only difference is a transistor has a curved back while Hall Effect sensors are flat. Inexpensive devices that I picked up in a surplus store had a circle on the front, indicating the side that is to come in close proximity to a magnetic source.
In the basic circuit we see an LED with current limiting resistor. When a magnetic source is brought close to the HES, the LED turns on. The backside of the sensor will respond to the opposite pole of the magnet.
Hall effect sensors can be used to trigger alarms. The alarm circuit illustration displays a simple alarm. A magnet should be placed so it passes near the sensor when opening a drawer or door of a locked enclosure. Once triggered the alarm will stay on until switch S1 is turned off. To reset, just turn the switch back on.
Motor Speed Indicator
One of the most useful functions that the sensor can perform is a motor speed indicator.
In the diagram we see four magnets added to the rotor of a motor and a sensor fixed in close proximity. As the magnets pass by the sensor, it clicks on momentarily; four clicks equal one turn in this case. A 4027 J-K flip-flop divides the clicks by four to provide an accurate spin count that is then fed to a frequency counter.
There are a wide variety of frequency counters on the market. Large expensive test bench units, and small circuit boards that can be incorporated in your designs—and all available from surplus electronic companies.
Although Hall Effect sensors are not exactly standard at your local supplier, you might try Allegro MicroSystems, in Worcester, Mass or All Electronics Corp, PO Box 567, Van Nuys, CA 91408. Some manufacturers of motors are including sensors in their new designs, anticipating the demand for high tech motors for new products, so you should also check with them. There is a sense of satisfaction that comes with making your own components or enlisting an unusual application of a conventional device. It may also stimulate your imagination and awaken the mad scientist within you. So get to your lab and experiment, experiment, experiment.