Dear Bob: Regarding Jason Cook's question on eyelets.* I worked at Motorola in the '60s, when eyeleted pc boards were still being used. In the days before plated holes were common, eyelets did prevent foil from peeling when a board was repaired, and they were the only way to get vias on two-sided boards.
The eyelets were specially designed "double funnel" and had to be inserted properly so the funnel would split on the top side to allow solder to wick up.
But they did cause headaches because of solderability problems. We found that the plating had a shelf life. If the eyelets weren't used promptly, the surface would corrode or get contaminated. Or if they rattled around too long in the eyeleting machine's hopper, the plating would wear off. A bad eyelet was impossible to see and very difficult to repair. Not only that, a bad eyelet would cause an intermittent fault that might not show up until the board was temperature-cycled or shipped to a customer. As soon as plated holes became affordable, the eyelets were all designed out.
Ken Lundgren (via e-mail)
Pease: I always heard that the eyelets would adhere to some of the nearby foil, and then when thermal expansion occurred, the eyelet would lift this foil away from the main part of the foil, and the foil would crack in pieces. Maybe that's not the major fail mode, these days. I have used thin wire through holes and then soldered over, top and bottom. Any two of those, in parallel, are much better than an eyelet, in my opinion. (We have done this with homemade boards, when the delivery on plated-through holes was too slow.)
Dear Bob: Your column on "Resonance Stuff" (electronic design, Jan. 13, p. 22) was especially noticed by me because of my own experience in my childhood. In 1945, at the tender age of 12, I developed an interest in ham radio. I thought I would build myself a crystal radio using a galena crystal. Nobody seems to know what that is these days, but you surely do! (Galena is lead sulfide, a fair semiconductor, capable of making a rectifier. /rap)
Anyway, at that time I lived in Hackensack, N.J., just off Route 17, about a mile from where a 50-kW AM station on 770 kHz resided. The station was then called WJZ, the flagship station of ABC. During the summer, when it was hot, I used to leave the door open, with a screen door keeping out the mosquitoes that N.J. is noted for! The first night when I went to bed, I thought I heard some music and people talking. Using my ears as a directional antenna, I moved around until I found the source. The screen, in the screen door, was vibrating and causing, I guess, rectification of the RF coming out of WJZ! Wearing a pair of headphones, I poked around with the pin-jacks on the headphones, and I found a place on the mattress spring where I could connect the jacks with clip leads. I was able to fall asleep every night to music without a radio and batteries!
Diran Varzhabedian (via e-mail)
Pease: Thanks for the excellent story. I've heard of people whose tooth fillings can similarly rectify out radio signals, and they can hear the radio in their head. This could be used to fake psychic seances.
(If you wondered about the headline, Happy April Fools! /rap)
Comments invited! [email protected] —or:
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*electronic design, Feb. 17, p. 20
BOB PEASE obtained a BSEE from MIT in 1961 and is staff scientist at National Semiconductor Corp., Santa Clara, Calif.
