Louder Sidetone
Sometimes you need a louder sidetone than a piezo sounder can deliver. One way is to feed the sidetone into an audio amplifier. But that can be a pain, square wave audio has thumpy artifacts that are unpleasing. Another way is to use the Piezo Loudenator kit
previously mentioned here:
https://wb9kzy.blogspot.com/2022/05/the-4007-or-what-about-that-piezo.html
One way that I mentioned back in the days of the PK-2 keyer was to use a dynamic speaker.
See: http://wb9kzy.com/apntpk2a.pdf
Page 4 has a simple transistor circuit to buffer the keyer chip output. And you can see the difference in audio output between a small piezo and a dynamic speaker on the next two pages. One reason why a small piezo isn't loud is that the fundamental frequency isn't being output. This gives an idea why piezo elements are used in filters.
For the heck of it I tried a slightly different circuit:
 |
| slightly different circuit for driving a dynamic speaker |
And got similar results. MicFFT was replaced with Spectran to see the output spectrums.
First, the small piezo (almost no fundamental frequency output):
.JPG) |
| the 600 Hz fundamental is down at -95 dB |
Second, with a large piezo (the CPE-6800 I sell now
, the fundamental is there this time:
.JPG) |
| the 600 Hz fundamental is now at -70 dB |
Third, the dynamic speaker (from an old cassette player), visibly louder:
.JPG) |
| The 600 Hz fundamental is now at -40 dB |
Conclusions: small piezos aren't so good with 600 Hz sidetones. Large piezos are OK. Dynamic speakers can be quite loud and sound fine. But of course the speaker uses a lot more current (100 mA in this case) than the piezo.
But if the sidetone can't be heard and there's no large piezos around it's easy to find an old transistor radio with a speaker in it.
BTW, the easy way to tell the difference between a piezo and a speaker is with an ohmmeter. The piezo will show an open circuit, the speaker will have a DC path.
The large piezos are sometimes called Telephone Ringers. In a pinch they can also be used for a crystal radio earphone.
Can't believe that it's been 24 years since that PK-2 document was written.
Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm