Pub. Date:
Elsevier Science
Mims Circuit Scrapbook V.I. / Edition 1

Mims Circuit Scrapbook V.I. / Edition 1

by Forrest Mims, Harry Helms


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Mims Circuit Scrapbook V.I. / Edition 1

Here it is—a collection of Forrest Mims's classic work from the original Popular Electronics magazine! Using commonly available components and remarkable ingenuity, Forrest shows you how to build and experiment with circuits like these:

analog computers color organs digital phase-locked loops frequency-to-voltage and voltage-to-frequency converters interval timers
LED oscilloscopes light wave communicators magnetic field sensors optoelectronics pseudorandom number generators tone sequencers and much, much, more!

Product Details

ISBN-13: 9781878707482
Publisher: Elsevier Science
Publication date: 09/01/2000
Pages: 154
Sales rank: 353,911
Product dimensions: 0.33(w) x 8.25(h) x 11.00(d)

About the Author

Forrest Mims has been an electronics hobbyist since building a one tube radio kit at the age of 11. Following graduation from Texas A&M University in 1966 and service as a photo intelligence office in Vietnam, he worked for three years with high-powered lasers, solid-state instrumentation, and trained monkeys with the Air Force Weapons Laboratory in New Mexico. Since becoming a full-time writer in 1970, he's written several hundred magazine articles and scholarly papers. His articles and columns have appeared in virtually every significant electronics magazine, including Popular Electronics, Radio-Electronics, and Modern Electronics. His articles on other scientific topics have appeared in a wide range of other publications, including National Geographic World, Science Digest, Highlights for Children, and Scientific American. His editorial exploits have included an assignment from the National Enquirer to evaluate the feasibility of eavesdropping on Howard Hughes by laser (it was possible, but Forrest declined to take part) and getting dropped by Scientific American as their "The Amateur Scientist" columnist because he admitted to the magazine's editors that he was a born-again Christian. His book sales total in the millions, and he is likely the most widely-read electronics writer in the world.

Read an Excerpt

Analog Circuits

In operation, resistors R2 and R3 form a voltage divider that places half the supply voltage (4.5 volts) at the 741's inverting input. This voltage is called the reference voltage. R1 functions as an adjustable voltage divider that delivers a variable voltage to the noninverting input of the 741. This voltage is called the input.

When the amplitude of the input voltage is below that of the reference, the output of the 741 comparator is low (near ground). Therefore, the LED is switched on. When the input voltage rises above the reference, the output of the 741 suddenly switches on, rising to near the positive supply voltage. The LED is then extinguished.

If the input voltage is made very close to the switching threshold, the 741 may oscillate in an unstable fashion by rapidly and unpredictably switching on and off. But practically speaking, the comparator output is either full off (ground) or full on (near the positive supply voltage).

Note that the inputs of the comparator are designated Mverting (pin 2) and noninverting (pin 3). You can reverse the operation of the circuit in Figure 2-3 simply by reversing the two inputs. Be sure to keep this in mind when you experiment with the following circuits.

Adjustable Light-Dark Detector

The basic circuit in Fig. 2-3 may seem simple, but it can be readily adapted for many applications. Fig. 2-4, for example, shows how to use the basic circuit as an adjustable light-dark detector. This circuit can be used to signal the arrival of dawn (or dusk) and to provide a warning when a refrigerator door has been left open. It can also be used as a simple break-beam object detector. Though the circuit usesa piezoelectric buzzer or alerter, an output relay can be included to control an external motor, lamp or other device.

The circuit's light detector (PC1) is a low cost, but highly sensitive, cadmium sulfide photoresistor. The circuit activates the alerter when the photoresistor is illuminated by even a very low light level. After a simple modification is made, the circuit will trigger the alerter when the photodetector is dark. In either case, the circuit consumes only about 0.5 mA in its standby mode and about 4.5 mA when the alerter is sounding.

Comparing the two circuits, note that the photoresistor in Fig. 2-4 has replaced R2 in Fig. 2-3. Therefore, the photoresistor and R1 in Fig. 2-4 form a light-dependent voltage divider. Potentiometer R2, which forms a second adjustable voltage divider, permits the reference voltage at the noninverting input of the 741 to be alerted.

When the sensitive surface of the photoresistor is illuminated, its resistance is very low, typically a few hundred ohms. Therefore, the voltage appearing at pin 2 of the 741 can approach the supply voltage when the photoresistor is brightly illuminated. The 741 will switch on as soon as the voltage at pin 2 exceeds the reference voltage from R1 which is applied to pin 3. The alerter will then be actuated.

When the light level at the sensitive surface of the photoresistor is decreased, its resistance is increased. Indeed, the resistance may reach a million ohms or more when the lightevel is very low. When this occurs, the voltage at pin 2 approaches ground. In any case, when the light level falls to a point where the voltage at pin 2 falls below the reference voltage, the comparator will switch off. The trigger point, of course, can be conveniently altered simply by changing the setting of R2.

Incidentally, this operating mode can be reversed simply by exchanging the photoresistor and R1 in Fig. 2-4. The circuit then switches off when the photoresistor is illuminated and switches on when the photoresistor is dark.

The alerter in Fig. 2-4 can be easily replaced by a relay that can control external lamps, motors and other devices. The circuit in Fig. 2-5, which is described next, shows how.

Adjustable Temperature Detector

The photoresistor in the circuit in Fig. 2-4 can be replaced by a thermistor as shown in Fig. 2-5 to transform the circuit into an adjustable-threshold, temperature-sensing alarm. When properly calibrated, the circuit can function as a freeze detector.

In operation, the output from the comparator (pin 6) is connected via R3 to Q1 which functions as a switch that turns a low voltage relay on and off. When the comparator output is high, Q1 switches on and, in turn, allows current to flow through the relay coil. Q1 can be a 2N2222 or any general purpose silicon switching transistor. The relay is Radio Shack's 275-004.

Some electronics parts suppliers stock thermistors, and you can purchase them by mail order if they are not available locally. Check the ads in electronics magazines. Some of the many thermistor manufacturers include Keystone Carbon Company (Thermistor Division), Fenwal Electronics, Thermometrics, Inc., and Omega Engineering, Inc.

Many different kinds of thermistors are available. For best results, select a thermistor having a room temperature resistance of from 25 to 50 kilohms or so. I prefer to use glass bead thermistors since they are very small and can be safely calibrated in water. But they are more expensive than other types of thermistors.

If the thermistor you select can be calibrated in water, you can easily adjust the circuit to trigger at the freezing point of water simply by inserting the thermistor in crushed ice or snow. You can set other calibration points with the help of a thermometer. Just adjust the temperature of a small cup of water to the desired point, insert the thermistor and calibrate R2.

Sine- to Square-Wave Converter

The sine wave is among the most important waveforms in electronics. The comparator is well-suited for transforming the ubiquitous sine wave into square- and other kinds of waves. As you can see by referring to Fig. 2-6, this manipulation of waveforms can be achieved with the simplest possible comparator circuit. This circuit can also be used to clip that portion of a signal which rises above or below any preset level.

In operation, the sine wave (or signal) is applied to the noninverting input of the comparator. When the reference voltage applied to the inverting input is ground, the output of the comparator remains at ground until the positive (rising) voltage of the sine wave exceeds ground potential. The output then suddenly switches to its maximum positive value and remains there until the voltage of the wave falls to ground potential. The comparator then suddenly switches off. When the voltage falls below ground potential, the output voltage suddenly switches to its maximum negative value where it remains until the waveform voltage again reaches ground potential.

It should be obvious that this operating mode transforms a sine wave into a square wave. What is not obvious, however, is that the amplitude of the square wave at the output can be much higher than that of the sine wave at the input...

Table of Contents

Chapter 1: Analog Circuits
Chapter 2: Light and Light Communications
Chapter 3: LED Circuits
Chapter 4: Test and Measurement Circuits
Chapter 5: Power Sources
Chapter 6: Digital Circuits
Chapter 7: Experimenter, Hobby, and Game Circuits

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