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About RF Cafe
Copyright: 1996 - 2024 BSEE - KB3UON RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while typing up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived... All trademarks, copyrights, patents, and other rights of ownership to images
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Carl & Jerry: The Sparking Light |
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In this adventure, Carl and Jerry use a pair of General Electric pnpn junction photoelectric switches to exact revenge on an engineering student 'friend' at Parvoo U. It involves embarrassing the guy in the presence of his YL (Hamese for young lady) date. The switches, per Jerry's tutelage, work like a silicon-controlled rectifier (SCR), except light is used to trigger the conduction path rather than an electrical gate signal. The devices are "solid-state kissing cousins of vacuum tube thyratrons." That's not necessarily the way I would have put it, but OK. You are also treated to a discussion of how and why to tame a chattering relay with a diode rather than a big capacitor. Author and creator John T. Frye ceased writing the stories before the two boys graduated from college, so we'll never know what became of them. Carl & Jerry: The Sparking Light A Carl and Jerry Adventure By John T. Frye W9EGV "Hey, Jer," Carl called as he came swinging through the door of the Parvoo University residence hall room he shared with his hometown pal, Jerry Bishop, "guess what I just heard down ... " He stopped in mid-sentence at the sight of the intriguing array of equipment spread out on the desk in front of Jerry. This included a VTVM, a bell transformer, some pilot-light bulbs, a multi-cell flashlight with the lens removed and two wires leading from an adapter screwed into the bulb socket, plus several tiny objects that looked like elongated clear glass beads with gold-colored wires protruding from opposite ends. "What are you up to behind my back ?" Carl demanded accusingly. "Not a thing, but while you were shooting the breeze up and down the halls I've been experimenting with these developmental General Electric subminiature silicon pnpn light-activated switches," Jerry retorted. "Two of them are Type ZJ235A; the other two, Type ZJ235B. I conned a lab Prof into the loan of them." "What are they? Come to think of it, where are they?"
"Right here," Jerry replied, poking the little glass beads, each of which was about three tenths of an inch long and one eighth inch in diameter, with a forefinger. "You know how a silicon controlled rectifier works. In spite of voltage applied across it, it passes no appreciable current in either direction until a signal voltage is applied to the gate lead; then it conducts heavily in the forward direction like an ordinary silicon rectifier, even after the signal voltage is removed from the gate. When the applied voltage is removed, the rectifier lapses again into its non-conducting state. These switches work the same way except that light, instead of a gate signal voltage, triggers them into conduction. Both devices are solid-state kissing cousins of vacuum tube thyratrons. "Let me show you," Jerry offered. "See: I have a pilot lamp and a ZJ235A connected in series across the secondary of this bell transformer whose primary is plugged into the a.c. line. Watch what happens when I shine this penlight on the little rectifier." When the cone of light struck the semiconductor, the lamp bulb glowed at about half its normal brilliance. When the penlight was shut off, the light bulb went out. "Current flows through the bulb only during the half of the a.c. cycle being rectified," Jerry explained. "Remember, this 'switch' passes current only in one direction even when 'closed' by the presence of light. Now I'll parallel the ZJ235A with another unit that's reversed so it will pass the other half of the cycle during the presence of light." He did so and demonstrated that when the light beam shone on either switch, the lamp glowed dully as before; but when the beam covered both silicon units simultaneously, the lamp glowed brightly. The VTVM, with the meter pointer adjusted to rest at center scale with no applied voltage, was then connected across the lamp. Rectified d.c. voltage across the bulb made the pointer swing right or left according to which switch was illuminated; but when both switches were receiving the light, the a.c. voltage present across the bulb left the meter pointer quivering in the center. One of the light-activated switches was removed, and a relay was substituted for the bulb. Now light shining on the switch would cause the relay contacts to close; however, the relay hummed and chattered until Jerry connected an ordinary silicon diode across the relay coil. This quieted the relay completely. "That diode is connected so that its polarity presents a very high reverse resistance to the d.c. pulses delivered by the semiconductor switch," Jerry continued; "but it has a very low forward resistance to the e.m.f. produced by the collapsing field of the armature coil between pulses. The result is that current flows through the relay coil at all times. During the pulse, current flows from the power supply through the coil. Between pulses, self-induced current of the coil flows through the diode. The continuous current gives the relay no opportunity to chatter." "Wouldn't a big capacitor connected across the coil accomplish the same thing by feeding stored current through the coil between pulses?" Carl wanted to know. "Yes, but that arrangement has two drawbacks. First, the presence of the capacitor would slow down the pull-in and drop-out time of the relay. Second, the light-activated switch would be working into a capacitive load instead of the resistive or inductive loads for which it is rated. The d.c. voltage stored in the capacitor would appear in series with the a.c. voltage applied and would substantially reduce the r.m.s. voltage that can safely be applied to the switch without exceeding peak voltage ratings. But let's see how the educated speck of silicon acts on d.c." Jerry connected one of the ZJ235A's in series with a lamp bulb across the leads coming from the batteries in the big flashlight. When the flashlight switch was closed, nothing happened; but when the penlight beam struck the semiconductor switch, the bulb glowed brightly. Its light continued undiminished after the penlight was shut off. But when the switch on the flashlight was opened, the bulb went out and refused to light again even when this switch was closed until light from the penlight once more "closed" the pnpn switch. "On d.c. that thing acts like a latching relay," Carl observed. "Once it starts conducting, you have to remove the power to make it stop. How much light is required to trip it?" "Between 80 and 500 foot-candles, with 125 foot-candles being a typical value. And in some applications the ZJ235D, which is rated at 400 peak volts, will handle 160 watts. Unlike ordinary photocells, it needs no amplifiers to control considerable power. For example, it can operate heavy-duty relays directly. At the same time, its tiny size permits it to be mounted behind a small hole in a meter face so that the shadow of the pointer cutting off light shining onto the unit through that hole could operate it. Since the input is light, the input and output circuits are entirely separate from each other ... "What were you going to say before we got started on all this?" Jerry finally asked. "Oh, I was going to tell you that Jodi, the nice YL kid from Florida we met when we were tunnel-stomping a couple of months ago, has a date tonight with that big ox, Bruce, down the hall. How he talked her into it I'll never know, unless he used some of that hypnotism of his. Anyway, he was telling a gang in his room how he plans to park with her at The Wall tonight under the pretext of showing her an imaginary satellite about which he is supposed to have some inside info. It makes my blood boil to think of him using a cheap trick like that on our - I mean Jodi. Anyway, we still owe him one for making you look silly with that post-hypnotic-suggestion bit." "Yes-s-s-s-s, that we do," Jerry said thoughtfully as he rolled one of the little light-activated switches between a thumb and forefinger; "and this may be the time to pay off. Doesn't he have classes all afternoon?" "Yes, but what have you got in mind?" "Come down to the parking lot for a look at his car and I'll show you. Just let me collect a few things first." The Wall was a Parvoo tradition. It was a secluded area at the edge of the campus alongside a retaining wall where couples were permitted to park unmolested by the university police. School officials apparently felt it was better to have the students park where they would be safe than invite robbery and attack by parking on back roads. The parking lot was just across the street from the H-3 Residence Hall. Bruce's car was not locked, and Jerry quickly set to work. First he disconnected the battery. Then he removed the wire going from the fuse block to the door-operated switches for the dome light of the car. A wire was run from the hot side of the fuse block through one of the light-activated switches and directly to the dome light bulb. The threads of the screw-on glass cover of the dome light were coated with Duco cement and the cover was screwed into place. The light-activated switch was mounted in a small cardboard tube so that the light gathered by a small lens in the end of the tube focused on the light-sensitive silicon area. The tube was mounted underneath the car at the rear with the lens pointing backward. A little paper cap was sipped over the lens, and the battery was reconnected. Now, opening the doors did not cause the dome light to come on, but removing the cap from the end of the cardboard tube did. Naturally, one the switch was triggered "closed" by the daylight, there was no way to turn the dome light off except to disconnect the battery. Pulling the bypassed dome light fuse or working the bypassed dome light switch had no effect whatever. The battery was disconnected again while the lens cap was replaced. One end of a short length of string was cemented to the lens cap and the other end was cemented to the concrete beneath the car. Finally, the battery cable was replaced.
"When the sun sets," Jerry explained, "there won't be enough incident light to trigger the switch, even with the aid of the light-gathering lens. It will be almost dark when Bruce drives off for his date; so the automatic removal of the lens cap at that time will not trigger the switch." "Won't he think it funny that the dome light doesn't come on when he opens the doors ?" "He'll just think the bulb burned out and won't bother to replace it. After all, light in that car is not exactly what he wants tonight!" Carl and Jerry never waited more impatiently for the start of a date of their own than they waited to see Bruce waddle out to his car about eight o'clock. Both heaved a sigh of relief as he drove away from the parking stall with the dome light still dark. "So far so good," Carl remarked. "According to Bruce's announced plan, he intends to drive around for a couple of hours while he exposes Jodi to 'the full force of his winning personality' and sells her on the satellite story. That means he should be parking at The Wall about 10:00. What say we study for an hour or so and then amble over that way?" This they did, but judging from the frequent glances at their watches. it's doubtful either of them got much out of the studying. At 9:30 they took the powerful flashlight and strolled over to the field across the road from The Wall. It was a beautiful warm spring night, and the boys lay on their backs on the grass and studied the stars sparkling overhead. They became so engrossed in identifying the great rectangle of departing Orion, the sickle of Leo, and the parallel lines of Gemini, that they were astonished to see it was 10:30 when a car drove slowly down the road and joined several others parked at widely separated points along The Wall. "That's Bruce's car," Carl muttered as the tail-lamps flickered out. "It was thoughtful of him to park so that the rear of the car is aimed our way. How close do we have to be to trigger the switch with this flashlight?" "Well, the flashlight puts a lot of candlepower into a very small spot, and the lens in front of the ZJ235A increases the effectiveness of the light many times, but let's Indian-crawl a little closer to be sure. See if you can hit the lens with the first beam of light." When they were within fifty yards of the car, Carl took careful aim with the long barrel of the flashlight and pushed the switch. Instantly the interior of the car was bathed with light from the dome lamp. Jodi could be seen peering expectantly up through the windshield at the silhouette of the water tower on the hill in front of the car. She obviously had bought the satellite story. Bruce's fat hand reached up and worked the dome light switch, casually at first and then vigorously, with growing exasperation. He opened his door and punched the little push-button switch on the door jamb repeatedly. Then he heaved himself out of the car and went around to the door on Jodi's side and did the same thing, but the light kept right on burning. By this time his plight had attracted the amused attention of couples in the other cars. "That your sparking light, Bruce?" a voice called. "Smart girl, that one," a feminine voice observed. "She knows better than to be alone with you in the dark." "Drop dead, you jokers," Bruce snarled from where he lay on his back beneath the steering column reaching up for the fuses mounted on the rear of the firewall. But pulling the dome light fuse had no effect. Carl and Jerry could hardly restrain their laughter as they watched him wrenching vainly at the cemented dome light cover. "Hey, Bruce, your little see-the-satellite scheme isn't doing so good, is it?" a voice drawled from the darkness. That did it. Carl and Jerry could see Jodi talking fast and angrily. Then they watched Bruce switch on the headlights, back out into the road, and drive away with the interior of the car still brightly lighted. Wanting to see the finale of their efforts, Carl and Jerry took a short cut to X-Hall where Jodi lived and concealed themselves in some shadows near the door. Almost immediately Bruce's car came down the street, and it had scarcely stopped rolling when Jodi popped out of her door and slammed it hard behind her. "All I've got to say to you," she said indignantly in her rich Southern accent, "is that I've never been so embarrassed in my whole life. Don't ever ask me to go out with you again. And if I were you, I'd change schools. An EE who can't turn off a little old lamp bulb is going to make a pretty sorry engineer!" "Wow! That's telling him!" Jerry chuckled as Bruce slammed the car into gear and drove away with an angry screeching of tires. "Steamed as he is, he undoubtedly will disconnect the battery tonight and plan on looking the car over good tomorrow; so as soon as he leaves the car, we'll remove the ZJ235A, wash off the Duco with a little acetone, and restore the wiring to its original condition. Tomorrow, when he finds everything working normally, he'll think he's flipping his wig. And I'll bet Jodi will really appreciate our looking out for her when we tell her about it." Carl gave his pal a quizzical look. "Jer," he said slowly, "nobody makes better sense when he talks about electronics than you do; but this one time you'd better listen to me. Let's not say a thing to Jodi about this. If there's one thing a girl can't stand, it's having someone think she isn't capable of handling the curliest wolf that ever trotted down the path. If she learned we were protecting her without being asked, she'd be as mad at us as she is at Bruce." Jerry's round face puckered into a thoughtful frown in the moonlight and then smoothed out into a cheerful grin. "Could well be you're right," he acknowledged, "but suppose on the way back to the parking lot you tell me where you learn these interesting things about how girls think!"
Posted August 5, 2014 |
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