Troubleshooting "Radio Project" in ScienceWiz Kit- 矿石收音机调试

Perhaps inspired by my recent post, many friends start building science projects of ScienceWiz / Inventions Kit with their kids,  However, they could not get the last project of "Building a Radio" work. 

Frankly, my son and I did not make it at our first attempt, either. Apparently, we are not alone. For example, this amazon reviewer complained it even though he "followed the instructions to a tee". This boy on YouTube was not able to get any sound even though he double checked everything. 

Almost gave up until I read this successful case with the
same Inventions Kit as ours. Then I did some online research and troubleshooting, eventually made it work! My son was so excited to hear the music out of the homemade radio.

Now, I like to share some troubleshooting tips. Hopefully, these trigger your a "Aha moment" and help you fix your Radio...

多半是受我帖子的影响，很多朋友都在和孩子做ScienceWiz/Inventions Kit 的科学制作。但是，他们告诉我该书最后一个矿石收音机项目做不出来。其实，我和儿子第一次也没成功。直到看了这个帖子，我坚信应该可以做成，但需要掌握一些调试技巧。在网上做点儿研究，再继续调试，我们最终成功了！这篇帖子就和大家分享一些调试窍门......

Double check your circuitry 

Check the wiring carefully against the schematic diagram, making sure that all the connections make good contact. The cardboard should sit on top of an insulated material (wooden table or plastics), so no short circuit between the paper fasteners.

Earphone

Don't use your iPhone or smartphone headset, only used the earphone in the kit. A low impedance earphone that you typically use in your MP3 player or smartphone will not work.

Sand magnet wire completely

Unlike the motor project,  you have to COMPLETELY remove the protective enamel off the two ends of the coiled wire. I used a piece of 220-Grit sandpaper (not the low quality pieces from the kit) and re-sanded them.

Grounding

The ground is critical and should be True ground, something like a metal water pipe in your house all the way to the earthen ground pipe. 
The tip I got is shown on the right picture. I found a cable outlet (BNC connector) on the wall, so I just connected my red ground wire to the outer surface of the cable outlet, which is a better ground.

Antenna

A long enough antenna (>90ft). If space is a problem, you can try winding the antenna around the room in a big coil, but having the antenna outdoors, and as high up as you can get it, will be best. 
The tip I got is is to use your body as an antenna - simply hold the slidable outer foil on the towel tube (one electrode of the variable capacitor), but don't touch the inner foil (if you do so, you short circuit the variable capacitor).

For our case, after re-sanding, grounding to the cable outlet, and touching the outer foil, we immediately heard the radio sound. What an exciting moment!

Background

This radio is actually called a Crystal Radio, the first widely used type of radio receiver, and the main type used during the wireless telegraphy era. This inexpensive, reliable, non-battery-powered radio was a major driving force in the introduction of radio to the public, contributing to the beginning of radio broadcasting around 1920. wiki