Nicholas Kinsman is interested in inventing solar-powered devices to reduce our dependence on other energy sources. He's also a winner of a Science Buddies Clever Scientist award for his 2007 California State Science Fair project (Kinsman, 2007). Nicholas set out to build a simple, inexpensive device to desalinate sea water, using readily available materials and easy construction methods.
Typical sea water contains dissolved salts at concentrations between 32 and and 37.5 parts per thousand. That means that if you started with one kilogram of sea water and then you allowed all of the water to evaporate, you'd be left with between 32 and 37.5 grams of salts (also called "total dissolved solids").
With all of that salt, sea water is not suitable for drinking nor for watering most plants. The fluid circulating in your body (blood plasma), contains much less salt than sea water (on the order of 9 grams of total dissolved solids). If you were to drink sea water, your body would actually lose water, because the high salt concentration of the sea water causes an osmotic pressure gradient which drives water out of your cells. Desalination is the process of removing the dissolved salts from water, making it pure enough for drinking or irrigation
Our lives would be completely different without semiconductors. All integrated circuit (IC) chips are made out of semiconductors. ICs are in our cars, in airplanes, and in our kitchen appliances. But what is a semiconductor? A semiconductor is a material whose function falls somewhere between an insulator (like plastic) and a conductor (like copper). Semiconductors have the ability to conduct electricity under certain conditions, which leads to some interesting and useful devices, such as transistors and lasers.
One semiconductor you've probably heard of is a light-emitting diode (LED), which is a semiconductor device that converts electricity into light. Current in semiconductors is carried by electrons (negatively charged) and holes (positively charged). An LED consists of a layer of electron-rich material next to a layer of hole-rich material. When a current is applied to the LED in a certain direction, the electrons move toward the holes and the holes move toward the electrons. When an electron and a hole meet, they create light. The wavelength, or color of the light, depends on the materials that are used for the layers and how electron-rich layer 1 is and how hole-rich layer 2 is.
The law of refraction, which is known as Snell's Law, applies to our everyday life. For example, when you answer the door and see your friend's face through the window, you see light that is refracted through the glass. Snell's Law compactly describes what happens to the trajectory of a beam of light as it passes from one medium, for example, air to another, for example, glass. As we apply Snell's Law and the definition of index of refraction in this project, we are able to measure the speed of light in Jello. The beauty of this project also lies in how we can verify one of the most basic laws of optics experimentally by using readily available and inexpensive components.
Note that Snell's Law not only applies to our case of the laser beam passing through air and Jello but also to other examples of how the incident object changes direction as it passes from a faster medium to a slower medium, and vice versa. For example, a marching band walks together in time with the music and take the same length steps. What if the band moves across a grassy football field at an angle, and as each band member crosses the 50 yard line, he suddenly finds the field very muddy and slippery? As a result, he/she steps in time but takes steps that are 20% shorter because of the mud. What happens? Answer: Those who have crossed the 50 yard line are traveling at 80% the speed of those who have not, and the line of band members bends at the 50 yard line, just like light in this experiment. With a little thought, one can even compute the angle at which the line bends (actually the reverse of what we are trying to do in this experiment).
CDs and DVDs are everywhere these days. In fact, you probably receive one free in the mail every month or two as an advertisement for an Internet service provider. CDs and DVDs store huge amounts of binary data (patterns of 0's and 1's) which your player can "read" with a laser, lenses, light detector, and some sophisticated electronics.
CDs and DVDs are both multi-layered disks, made mostly of plastic. The layer that contains the data (DVDs can have more than one data layer) consists of a series of tiny pits, arranged in a spiral, tracking from the center of the disk to the edge. The data layer is coated with a thin layer of aluminum or silver, making it highly reflective.
Several weeks ago I became aware, via the Internet, of an experimental AM 'crystal set' style of receiver, that uses what is known as a ZVB (Zero Voltage Bias) device. This is in the form of an IC, which contains either a 2 pack or 4 pack of specialised FETs (Field Effect Transistors) that require no bias voltage on their gates to conduct current from source to drain. They simply rely on signal voltages to turn them partially or fully on. This new IC, dubbed the ALD110900A, is made by Advanced Linear Devices in the
Many years ago, crystal sets and such were basically abandoned by manufacturers and constructors, as a very poor cousin to modern superhet receivers, and even the novelty effect of 'toy' radios for the kids had all but lost its appeal. The discovery of the germanium diode during WW2 and the subsequent discovery of the point contact transistor by Bell Labs in 1947, rekindled the joys of simple receivers, as those components came onto the market via disposals shops and hobby stores. During the 1970's and 80's, the Ferranti ZN414 and its later incarnation, the MK 484 (AM radio chips) enjoyed tremendous success in the hobby market. Now, they have all but had their day, as manufacturers and retailers once again, move away from the hobby end of the market. If it were not for a band of very dedicated enthusiasts, the hobby part of radio may well have died a long time ago. I'm so glad they didn't give up, as we now have a whole new generation of up and coming technicians, engineers and operators, who can once again "cut their teeth" on 'crystal set' style radios, albeit with the context of a 21st Century update.
What is presented in this article, is my version of the QST arrangement, and I'm using the humble 2N5484 JFET, purchased from JAYCAR Electronics for around $2.00. Yes, a JFET, not a MOSFET! Why? Well, my basic knowledge of FET devices at the time of acquiring the QST design was a little rusty, to say the least, not having experimented with them for some time, so after a bit of scratching around on the net, and down at the local library, I reclued myself as to their peculiarities. Devices that are static sensitive have protection diodes on the inputs, and the ALD device certainly has those. Basic JFETs also have a protective diode between the gate and source connections, presumably for the same reason, but neither the QST design, nor my adaptation of it, use any internal diodes as a rectifier. You can experiment with that as a start - simply replace your germanium or Schottky diode with the gate and source of a JFET device, and you'll get reception all right - but it does seem a bit mushy or scratchy, somewhat like a poor Schottky diode that distorts on low signal levels (BAT46's come to mind…) You can still obtain reasonable results by tapping the gate further up the tuning coil, if you want to.
The basic premise behind the QST article, is that you drive the gate of the FET with an RF voltage derived from the top of the tank circuit via C1, and the gate switches on and off very rapidly, at the resonant frequency that you are tuned to. The source connection is tapped into the tuning coil low down, as a means of impedance matching with the headphones and acts as the anode, while the drain connects with your headphones as the cathode, to complete the detector part of the circuit. According to the experts on the net, much success has been enjoyed by one and all, and there's a lot of chatter about this most recent innovation in the realm of simple AM receivers (292 posts so far, to the 'Rap'n'tap' chatroom of the American Crystal Set Society alone!- www.midnightscience.com). One of the most curious aspects of this little beauty is that the antenna/ground system I employ basically entails a 'short' antenna, and a water pipe ground. Ideally, short antenna wires work best near the top of the tuning coil, but in this case, the best position seems to be right at the bottom tap! Normally, this would send all my weak locals into a spin, and shift the whole band up towards the top end of the tuning cap's range. The JFET device seems to act like a FET, (not a diode) as connecting it to the tank circuit via the gate lead does not appear to cause any loading, and flatten out the 'Q' of the LC tank, in any discernable way.
Another aspect of using a JFET in this manner, is the actual sound quality that you get in the headphones. The audio is very clear ans is in no way muffled, or distorted. Diode detectors often produce poor results, and while this can sometimes be attributed to bad antenna/ground systems & poor layout and construction of the receiver, at the end of the day, a FET device has it over a simple diode on a number of fronts. Diodes tend to introduce various distortions, and they also exhibit very high output impedance. With this design using the 2N5484 device, it is possible to get rid of most distortions and at the same time, use just about any kind of audio transducer that you may have on hand. I have successfully used a crystal earphone, my pair of 'Scientific' 2KR headphones, and a couple of low impedance telecom style inserts, and all work reasonably well, without the need for matching transformers, or extra passive components, beyond the usual 0.001uF cap, or 47KR ballast resistor. Diode detectors can also cause loading on the tank circuit if they are tapped too high up the coil windings. The JFET in this circuit is tapped way down, at around 10 turns, with the antenna lead sitting just under that on 5 turns from the grounded end of the coil. Sensitivity is OK, and selectivity is quite good, for a simple AM receiver
Using this circuit, you can obtain the following voltages (approx.) at a current limited to one ampere: 3.3V, 5V, 6V, 9V, 12V and 15V. The AC main is stepped down by transformer X1 to deliver the secondary output of 18V AC at a maximum current of 1A dependant upon the load. The transformer output is rectified by the bridge rectifier comprising diodes D1 through D4, filtered by capacitor C1 and fed to regulator IC LM317, which is a 3-terminal positive regulator capable of providing 1.2V to 37 volts at 1.5A current to the load. Resistor R13 and selected combinations of resistors R1 through R12 are used to produce approximately 3.3V, 5V, 6V, 9V, 12V and 15V at the output. The desired resistors are selected by switching into conduction one of the six pnp transistors T1 through T6 by grounding the corresponding transistor base using rotary switch S1. For example, to get regulated 3.3V, simply rotate the knob of rotary switch to 3.3V position. Consequently, transistor T1 is forward biased to switch resistors R1 and R2 (in series) across Adj pin of LM317 and ground to produce 3.3V. Other voltages can be produced in the same way by using rotary switch S1. Capacitor C2 bypasses any ripple
in the output. Diode D5 is used as the protection diode. Use a heat-sink for dissipation of heat from IC LM317. The fuse-rated lamp provides protection against short circuit. This 1A rated power supply can be used for testing of various circuit ideas as well as construction projects published in EFY.
Fig 1 Hearing aid circuit
This low-cost, general-purpose electronic hearing aid works off 3V DC (2x1.5V battery).
In this circuit, transistor T1 and associated components form the audio signal preamplifier for the acoustic signals picked up by the condenser microphone and converted into corresponding electrical signals. Resistor R5 and capacitor C3 decouple the power supply of the preamplifier stage. Resistor R1 biases the internal circuit of the low-voltage condenser microphone for proper working. The audio output from the preamplifier stage is fed to the input of the medium-power amplifier circuit via capacitor C2 and volume control VR1. The medium-power amplifier section is wired around popular audio amplifier IC TDA2822M (not TDA2822). This IC, specially designed for portable lowpower applications, is readily available in 8-pin mini DIP package. Here the IC is wired in bridge configuration to drive the 32-ohm general-purpose monophonic earphone. Red LED (LED1) indicates the power status. Resistor R8 limits the operating current of LED1. The audio output of this circuit is 10 to 15 mW and the quiescent current drain is below 1 mA. The circuit can be easily assembled on a veroboard. For easy assembling and maintenance, use an 8-pin DIP IC socket for TDA2822M.Proposed enclosure (with earphone socket) for the assembled unit is shown in Fig. 2.
Fig 2 Enclosure