sign in sign up
<<
Home , Crystal radio, Electrolytic detector

7A 3v7

Most us believe that the age of solid- MARTIN CLIFFORD DID YOU KNOW: TI IAT SOLID-STATE ELEC- of tronics can trace its roots back to 1835? state electronics began with the invention In this, the first installment That radio signals can be demodulated of the ; Bardeen and Brattain of using sulfuric acid or nitric acid? That oil - Bell Laboratories produced that first crys- of our new, occasional series filled variable were once used tal in 1948. However, lost in the about the early days of radio, in radio receivers? That a single crystal mists of history is the fact that true solid- can be used as a ? state receivers have been with us since we look at the original That there are some radio receivers that about 1918. "solid-state" receivers. never need to be turned off, have no on/off In more recent times, the term solid- , and do not require battery or AC state has been so firmly associated with power? Or that lenzite, zincite, bornite, germanium, and subsequently with sil- tellurium, and chalcopyrite are all semi- icon, that no one should be blamed for conductors? thinking that those are the only materials

The Early Days o

1

i

I

y

m y,o- .. J'

-,

- suitable for use in . Yet least some improvement over those with there are numerous materials that are just no tuning at all. as suitable. Among them are carhorun- Another problem was that the output of dum ( carbide): (a crystal the consisted of both an sulphide of lead); molybdenum: lenzite: and an RF carrier: both were ¿incite (an oxide of zinc): tellurium: bar- passed directly to the . Subse- nite (a sulphide of iron and cooper): chal- quently, a small was placed copyrite (also known as copper ): a across the headphones to bypass the RF and cerusite. Except for carborundunt, a HEADPHONES carrier. manufactured material also used as an Also, it was found that selectivity could abrasive. all are materials that are found in U he further improved by adding a second nature. slider to the tuning coil. The radio shown CRYSTAL DET. in Fig. 3-a incorporates both those im- Early solid state SINGLE -SLIDE provements: the schematic for that circuit The fact that certain materials have rec- TUNING COIL is shown in Fig. 3-b. tifying properties (that is. they allow cur- rent to flow in one direction only) has been FIG. 2-TO IMPROVE SELECTIVITY a slide Improved designs known since 1835. thanks to the research tuned coil can be used (see a). The schematic of As time went on, various methods were a crystal set that used such a coil is shown in b. of one Munk Al. Rosenshold. At the time used to improve tuning. In one arrange- a laboratory curiosity, his discovery was ment, shown in Fig. 4-a, the single -slide largely forgotten until it was unearthed was used as the primary winding of again by F. Braun in 1874. an RF . The secondary wind- However. a practical use for that discov- 2 SLIDE ing, which was tapped, was wound on a ery was not found until many years later. It TUNING COIL form that could travel into the primary by - -0 came at at time when interest in radio was means of a pair of rods. That resulted in heating up and many early experimenters triple tuning. The primary was tuned by had radio setups in their attic or basement the single slider: coupling was adjusted by workshops. III III II III Illllllllllllllillllllllllll!Illllllllilllu>oo moving the secondary' in or out of the are key components in sim- primary, and finally, the appropriate sec- ple radio detectors. Along with vacuum ondary' tap could he switch selected by the tubes, early experimenters turned to sol- operator. The schematic for the circuit is id-state rectifiers made from one of the shown in Fig. 4-b. substances that exhibit natural rectifying Since most receivers are properties. Since most of those sub- "powered" by the radio signal itself, they stances are crystalline in nature, such rec- require no supply or battery. (They tifiers were called costal detectors; radi- os that used such detectors were called radio sets, cn.cta/ or simply crystal sets. a The simplest radio Early vacuum -tube rectifiers, such as the UX-201A and UV -199 , cost about $15. Since $15 a week was consid- ered good pay for a workman back then. CRYSTAL DET. ImIiiliiiiiUiIIVIIIIIIIIIIIIIIpII911111Vi and since crystal detectors cost only a small part of that amount, many experi- TUNING menters turned to crystal sets. Such radios COIL were among the simplest possible, con- HEAD- qp sisting of just a crystal detector and a PHONES- headset. See Fig. I. b The crystal set of Fig. 1 has its virtues. but it also has plenty of faults. Selectivity FIG. 3-FOR EVEN GREATER selectivity, re- is non-existent; after all, that radio has no ceivers with double -slide -tuned coils were de- veloped (a). The schematic of such a is tuning circuits. So whatever the radio re - set shown in b. Note the inclusion of a capacitor across the headphones. Its purpose was to by- pass the RF carrier.

AERIAL ceives is what you hear-the strongest LEAD-IN signal dominates, and all the rest provide background noise. CRYSTAL DETECTOR The earliest effort at improving selec- tivity was to add a tuning coil like the one shown in Fig. 2-a. EARPHONES (HEADSET) That coil consisted of enamel insulated wound on a round form: experimenters often used cylin- b drical oatmeal boxes as the coil form. The FIG. 4-IN THE CRYSTAL SET SHOWN in a the amount of could he selected slide -tuned coil is used as the primary of an RF transformer; the secondary had multiple taps using a metal slider. The tuning method FIG. 1-THE SIMPLEST CRYSTAL RECEIVER and was wound on a form that could be moved consists of an , a detector, headphones, was crude. but the radio circuits that in- in to or out of the primary. The schematic of the and a ground. cluded that coil (see Fig. 2-h) did offer at circuit is shown in b.

62 was weak. Those two factors eventually caused the carborundum detector to fall out of favor. but not before it gave rise to N some interesting circuit innovations. CARBORUNOUM One of those was the concept of the DETECTOR . The early amplifier shown in Fig. 6 was a mechanical one known as the Brown amplifying . The leads identi- lied as A and n were connected to the output of the detector. The signal current through relay coil Ml caused relay arma- FIG. S-BATTERY POWER was needed in crys- ture v to vibrate. thus varying the - tal sets that used carborundum detectors. ic field around winding R2 of the second relay. That caused the current supplied by the battery to be varied at an audio rate. That current flowed through the head- phones. p oducing sound. In some in- stances a series arrangement was used: the current from the battery drove still another mechanical amplifier. the need for clumsy tuning coils was finally eliminated through the use of a FIG. 8-A VARIO -COUPLER, or variometer, con- . then known as a con- sisted of two coils mounted in such a way that one :ould be rotated within the other. denser. For a while, however. variable ca- pacitors were used in conjunction with ADJUSTING single- and double -slider variable coils. KNOB PIVOT Two examples of capacitor -coil -tuned cir- CAT cuits are shown in Fig. 7. WHISKER FIG. 6-BECAUSE OF THEIR LOW OUTPUT, In some of the more advanced sets. a some sort of amplification was often used with TERMINAL TERMINAL carborundum sets. Here, a popular mechanical vario coupler, sometimes called a varí- amplifier of the time is shown. ometer (see Fig. 8), was used. That con- sisted of a pair of coils mounted in such a way that one could he rotated within the other. a Ordinarily, the variable capacitors used were air types; that is. the dielectric he- CRYSTAL tween the stator and rotor plates was air. CONTAINER CAT WISKER Air. though. has a dielectric constant of I. To increase the without adding more plates. one design had the variable capacitor positioned in a leak- 'i~INSULATING BEAD _ proof. transparent case, filled with oil. Oil WAX SEAL has a dielectric constant of five. so the LEADS capacitance of such a capacitor is live b times that of one with an air dielectric FIG. 9-TWO CATWHISKER DETECTORS. A a variables to be the \in a; a sealed, (assuming all other variable contact type is shown same). One such capacitor had 17 plates ftxec unit is shown in b. and a total maximum capacitance of 0.(K04 µF. while another had 43 plates we've not looked at the detector itself and and a maximum capacitance of 0.0011.1.F. how it was made. It is time now to correct Anyone who has had any contact with that oversight. radio knows that the antenna plays an im- Crystal detectors were available in two portant role in determining the quality of basic forms: contact and combination. Of the received signal. Antennas were even those, the contact detector was cheaper more important in the days of the crystal and more popular. set. That's because that design made no A single crystalline substance was used provision for amplification. Thus. the in the contact detector. A small bit of ' b strength of the signal heard was w holly spring wire. called a catw/tisker, was FIG. 7-THE SELECTIVITY OF CRYSTAL SETS dependent on the strength of the signal placed so that it made contact with a point improved by the addition of a varia- was further delivered by the antenna. Some of the on he crystal. In early detectors, the cat - ble capacitor. Here two designs, one (a) using a whisker was to he variable be- single -slide -tuned coil, the other (b) a double - schemes devised were quite elaborate: designed slide -tuned coil, are shown. others were quite simple. For apartment cause finding the most sensitive spot on dwellers, a bedspring was a popular alter- the crystal was a trial -and -error pro- also require no on/off switch.) llowever, native to an outdoor design: the ground cedure. See Fig. 9-a. that is not always the case. connection was made to a cold -water or The catwhisker was usually made from One detector that was popular for a radiator pipe. stiff phosphor bronze, hut in more expen- while was made from carhorundum. As sive detectors silver or I8 -karat gold was you can see in Fig. 5. a receiver design The crystal detector used. The wire was coiled. with a small using that detector required a voltage So far, we've spoken about how crystal straight extension ending in a blunt source. Further, the output of the detector detectors were used in early radios. But point-an arrangement much like that

63 used in the first point -contact many years later. There were two connec- tions to the crystal detector: one to the cat hisker and another to the cup holding the crystal. I.ater on, fixed -position catwhiskers in sealed containers became available. See Fig. 9-b. The combination crystal detector con- sisted of two different crystals in close contact. Various combinations of miner- als were tried. including hornite and ei- ther ¿incite or copper pyrites. Another combination detector used tellurium and ( 3-

Illlllllllll ' : 0 'P o-, ,

part amateur station 1 -WP, Warwick, AN EARLY RECEIVER. With this set-up, which was of Maine to Florida. Note the variable -contact RI, circa 1913. the operator was able to log stations from -tuned coil. catwhisker crystal detector, 23 -plate tuning capacitor, and double -slide

yuently had to he repositioned or re- placed. Secondly. the detector used a ADJUSTMENT y. is dangerous. KNOB exposed acid, hick

FINE PLATINUM WIRE HEAD PRIMARY VSECONDARY PHONES

CRYSTAL

b 00055 .001

NITRIC OR DILUTE SULPHURIC ACID SOLUTION a

TUBE LONGI FIG. 11-IN AN the Ye" (4" rectifying material was nitric acid or diluted sul- S phuric acid. COIL PRIMARY SECONDARY 2. WINDING c chalcopyritc (copper pyrites). The chief i advantage of the combination detector 90 was that it was less susceptible to vibra- 16 45 20 11.1~=1_ PRIMARY - tion than the catwhisker type. 4 32 TURNS 2 Some of the many different types of OF NO. 28 WIRE SECONDARY-90 TURNS crystal detectors are shown in Fig. 10. TAPPED AS OF NO. 28 WIRE d SHOWN TAPPED AS SHOWN We've so far looked at all of those save b invented by one-the Perikon detector. FIG. 12-YOU CAN BUILD this crystal set (a) PRESSURE "á 1906. and shown in Fig. 10-b, Winding details for ADJUSTMENT a Pickard in using modern components. -shaped the coil are shown in b. SCREW E That detector used a small. cone piece of zincite. 'Ihe ¿incite could he posi- wmcwn tioned so that it contacted one of several Building your own segments of chalcopvrite located on a cir- We hope this article has aroused your SLUG cular plate. By selecting which segment of chalcopvrite was used. the detector interest in the electronics or days gone by. some of the flavor could he "tuned." If you want to recapture CARBORUNDUM you'd As v,e slated earlier. most detectors of the early experimenters, perhaps own crystal re- were made using a crystalline material. like to try building your Fig. 12-u However. "most" is not "all." One detec- ceiver. If so, the circuit shown in parts. tor that was at least brief!\ popular was the can be built using modern COMPRESSION electrolytic detector. shomn in Fig. II. In that circuit we've replaced the slide - SPRING (Finding a The design of that detector Vas similar to tuned coil with a tapped one. would he near- that of the catwhisker. but nitric acid, or a slide -tuned coil these days the are dilute solution of sulphuric acid, was used ly impossible.) Full details of coil e Just about any diode can in place of the crystal. Despite that detec- given in Fig I2 -b. Varia- results. FIG. 10-FIVE POPULAR DETECTORS: tor's high sensitivity. it had disadvan- he used for the detector. For best catwhisker (a), Perikon (b), com- that's at ble -contact tages. One was that the wire tended to curl plan on using an outdoor antenna bination crystal (c), fixed -contact catwhisker the wire Ire - least 75 feet long. R -E (d). and carborundum (e). away from the acid-so

64