Nuts and Volts - March 2015

#3 You can use either type and expect the same results. Both diodes are germanium and both have a forward voltage drop (often called “ turn-on” voltage — where a diode begins conducting) of about 0.3V. A diode with even lower forward voltage is the 1N5817 — a Schottky diode — which has a forward voltage of about 0.16 volts. The forward voltage determines how weak a signal can be heard.

The author at the following URL presents a comprehensive table of 1N34 and 1N60 subtypes and a few Schottky diodes used as detectors in crystal radios. If you view it, look in the column labeled Measured Vr: http://wiki.waggy.org/dokuwiki/crys-tal_radio/detector. However, long before the sensitivity of the diode becomes the limiting factor, four other factors will limit the performance of the radio you propose. Those are:

Selectivity — only one tuned circuit is used and it is not impedance matched at input or output.

Antenna length — definitely use more than 10 feet — a goal would be

50 feet and as high as possible.

Ground losses — connect a wire to earth or to a large expanse of metal.

Frequency of operation (also related to selectivity) — with this type of circuit, as you increase frequency, the bandwidth increases. This means it lets through more and more stations at the same time.

If you haven’t had the opportunity to read them, you’ll probably find the insights in the Wiki entry on crystal radios time saving. Especially note the sections on tuned circuits, impedance matching, and the problem of selectiv-ity.It can be found at http://en . wikipedia.org/wiki/Crystal_radio. The following URL shows how to connect your tuning circuit and diode directly to your LM386 without the LM741 you have in the middle of the circuit: http://makerf.com/posts/ an_lm386_powered_crystal_radio_in _an_altoids_smalls_tin.

You specified a number of turns for your coil but I didn’t see any diameter for it. Starting coil designs would be 56 turns for a five inch diameter oatmeal box or 75 turns on a 2-1/8 inch diameter coil — each of which could be used with your 365 pf variable capacitor; 22 to 24 AWG bare enameled wire would work for the 5" diameter; 28 to 30 AWG would work for the 2-1/8" diameter.

To help optimize selectivity, you want what's called a "square coil." This means the coil length is about equal to the diameter. Not critical, but helpful. Small diameter wire increases resistance which degrades selectivity. It’s likely to be less frustrating to first get your design working at the lower AM broadcast band frequencies before pushing up into the shortwave frequencies.

Last, it looks like you might put taps on your coil. The following URL has photos that might give you helpful ideas: www.midnightscience .com/downloadfiles/XSOB1-manual- 050108.pdf. Once on the website, click on the “Oatmeal box crystal set.” Please accept my apologies if I've included too much information. Best wishes for your success.