Monthly Archives: June 2021

Homemade Cardiac Monitor: 1961

If Junior is looking for a spectacular science project in the field of biomedical engineering, this one is a sure winner.

Sixty years ago, the June 1961 issue of Popular Electronics showed how to construct this simple cardiac monitor. The circuit was simple, amounting to a 4 transistor audio amplifier. The diagram called for 2N279 transistors, which are apparently unobtainium today, but this substitution guide lists the 2N2431 as equivalent, and it is available at a reasonable price.

The input comes from two electrodes, one of which is placed on each arm. In this position, they will pick up the currents from the heart, which are alternating currents of up to 100 kHz. The audio portions will be audible in the headphones, and also displayed on the meter. The magazine notes that you are not hearing the actual sound of the heart, merely the amplified voltage sent to the heart muscle. Other muscles can be monitored by placing the electrodes on either side of the muscle in question.

The electrodes are simply pieces of metal placed on the body. Prior to placing them, the skin needs to be scrubbed to improve the conductivity. This is done by scrubbing with Lava soap
or Ajax cleanser. (However, since Ajax is now billed as “non-scratching”, I’m not sure it would still work.) The article notes that the device is completely safe, even for children. First of all, it runs off only three volts. And the connection to the electrodes goes through a capacitor, so even that voltage has no pathway to the body.

More advanced versions are available today, but the advanced student will almost certainly bring home the blue ribbon in the science fair by building the medical device at home.

Emergency Portable Radio Batteries: 1951

This young woman shown above in Popular Mechanics 70 years ago this month, June 1951, is using her portable radio to pull in some entertainment at the beach. But she also realizes that in a civil defense emergency, the set might have a vital role to play if the bombs started falling.

The problem, of course, is that you need batteries for the radio, and many sets had oddball shapes and sizes, some of which might not be available in the runup to an emergency. Therefore, the magazine gives some pointers on substituting what’s available. Even though the batteries might not fit in the case, with a little thought, it was a relatively easy matter to power the radio for emergency information.

Annular Eclipse, June 10, 2021

Eclipse of May 31, 2003, Bavaria. Wikimedia Commons image by Sch, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

On Thursday, June 10, 2021, much of North America will experience a sunrise similar to the one shown above. As we previously reported, on that day, there will be an annular eclipse of the sun. Normally, this isn’t a really big deal, since enough of the sun remains visible so that it doesn’t get noticeably dark outside. But this one is an exception, since it takes place at sunrise. Depending on your location, as the sun rises Thursday morning, the sun will be either a crescent or a ring.

The map below (courtesy of Michael Zeiler, GreatAmericanEclipse.com) shows approximately what the sun will look like at sunrise. In some cities, such as Minneapolis and Chicago, there will be a noticeable chunk missing from the sun as it rises that morning. Other cities, such as New York, Philadelphia, Washington, and Toronto, will have only a tiny crescent sun come over the horizon.

This map shows simulated views of the partial eclipse at sunrise. To get this view, you’ll need approved solar viewers. In general, you will get the most dramatic views of the rising eclipse along the middle curve of the three curves in this map.

As the sun is rising, it’s as safe to view as any other sunrise. But as it goes above the horizon, if you want to keep watching, you’ll need eclipse glasses or an indirect viewing method. If you still have glasses from the 2017 eclipse, you’re all set. You can also use indirect methods, as we discussed in 2017.

My original plan was to view the eclipse near Longlac, Ontario at Macleod Provincial Park. Unfortunately, the border remains closed, and it looks likely that the park will remain closed to camping due to COVID. Therefore, the official OneTubeRadio.com eclipse headquarters will be located at Cascade River State Park, near Lutsen, Minnesota. While there probably won’t be a good view of the sunrise from the campground, we will use the day before the eclipse to scout out a spot along the shore of Lake Superior to view the sunrise. That campground is fully booked, but the last I checked, there were a handful of sites available at Judge C.R. Magney State Park and Gooseberry Falls State Park. There are also some hotel rooms available in the area.

Similarly, if you’re in Chicago or Detroit, it’s worth the drive to Sault Ste. Marie to view the crescent sunrise. And if you live in New York, Philadelphia, Toronto, or Washington, all you need to do is set the alarm clock early, and find a good place to watch the spectacular sunrise.

Ration Challenge 2021

For a week starting June 13, I will be taking part in the “Ration Challenge,” a fundraiser for Church World Service (CWS). Funds are designated to feeding Syrian Refugees in Jordan (or wherever the need is greatest). CWS does not engage in proselytizing, but they are sponsored by numerous Protestant and Catholic denominations. They are a highly rated charity, as you can see at sites such as Charity Navigator.

For that week, I will be eating essentially the same rations as those refugees, as explained in the video below.

Please consider donating at the following link:

https://www.rationchallengeusa.org/richard-clem

Both small and large donations are greatly appreciated. I’ll be posting updates during the challenge, here and on Facebook.

Two Beginner’s Sets: 1951

This young man is pulling in the proverbial strong local station thanks to one of two sets described in Popular Mechanics 70 years ago this month, June 1951. Both sets are billed as low-cost sets for young radio experimenters, and the magazine noted that such designs re not as plentiful as they should be.

The set above is a crystal set using a galena detector. The tuning coil is wound on a rectangular piece of wood under the chassis, and there is an opening on top for a slider.

The father-son team at left have upgraded the set. First of all, instead of a galena detector, this set uses a 1N34 diode. And for added volume, it employs a 3Q5-GT tube as audio amplifier. For the B+, the set uses 9 volts, but the article points out that for added volume, as much as 90 volts can be used.

If you are thinking of recreating one of these designs, you might find ideas for sourcing parts at my crystal set parts page.

1921 Homemade Catamaran

A hundred years ago this month, the June 1921 issue of Popular Mechanics showed these self-explanatory plans for a homemade raft. The magazine specifies that the hulls should be logs of about 12 feet. However, in many states, such a craft would require a license. In most cases, this can be avoided by making the craft slightly smaller. For example, Minnesota requires no license for a non-motorized craft of 10 feet or less.

For any size boat, including this one, most states have a certain level of minimum equipment. For example, Minnesota requires life jackets and a white lantern or flashlight visible for at least two miles.

While most states do not require adults to wear the flotation device, in the case of this homemade expedient raft, it would be highly recommended.

1941 Headphone Crystal Set

Eighty years ago this month, as shown in the June 1941 issue of Popular Mechanics, Junior was so pleased with his new crystal set that Dad decided to make a second one for himself. The set was so small that it was built right in to an old set of “Baldwin” style headphones, which had an extra large Bakelite shell and cap to house the set.

Into the bottom of the shell was placed a spiderweb coil, and the the shaft of a small tuning condenser was mounted in a hole drilled in the bottom. A wire with a spring clip was used for the optional but recommended ground connection, with another for any convenient antenna.

Keeping Volume Controls In Stock: 1941

Eighty years ago, the hapless radio serviceman at left was making his customer wait for a relatively simply job to be completed. The one at the right, however, was able to proceed immediately with the job, thanks to having in stock a kit consisting of 18 volume controls, along with various switches and shafts. He didn’t have to wait for the exact replacement. He just located the resistor of the correct value, and then added the shaft himself, tapping it in place with a hammer. If a switch was part of the control, it was an easy matter to add that as well.

The set of 18 controls sold to the dealer for $14.97, with a total list price of $29.95. The kit came from the International Resistance Company of 401 N. Broad Street, Philadelphia, and this ad appeared in the June 1941 issue of Radio Retailing.

1946 Portables

Seventy-five years ago, the summer of 1946 was the first summer after the war, and Americans were ready to enjoy some leisure. The June 1946 issue of Radio Retailing reminded its readers that this meant there would be a big demand for portables, and manufacturers were in the process of delivering the first new sets. The season would last through fall, and dealers were advised to stock up.

There were currently 24 models on the market. Some of these included a record player, with a maximum price of $149.50. Of the 16 straight portables, the average price was $35.43, with the lowest being $17.50.

The magazine noted that only half of the new receivers were from old-name manufacturers. The remainder came from postwar startups.