A Voice Activated Light Switch

Build fun circuits! Impress your friends! (Or at least the ones who aren’t in-the-know 😉 )

With some inspiration from The Fifth Element and Iron Man, here’s a voice-activated light switch that provides the illusion of a more advanced artificial intelligence, with the simplicity of “the clapper”.

Schematics and board design files are available on the github project page.

Fun With Analog Multipliers

Need a frequency doubler? Want to plot a cubic function on your ‘scope? How about a square root extractor, or a voltage controlled amplifier? Analog multipliers make all this (and more) a snap!

Description Reference
Analog multiplier background and basics MT-079: Analog Multipliers, Analog Devices
A variety of multiplier applications and circuits ADI Multiplier Applications Guide, Analog Devices
How the Gilbert cell works Basics of the Gilbert Cell, W2AEW
Datasheet for the AD633 used in the video AD633 Datasheet, Analog Devices

Crystal Motional Parameters

Ever tried searching through your datasheets for the motional parameters of that quartz crystal you just bought? Good luck! Vendors simply don’t specify these parameters to general end users, and for most applications that’s OK. But for high Q oscillator and filter design, measuring and matching crystals can be important.

This video discusses crystal motional parameters, how to measure them with a crystal impedance meter, and finally examines the measured values of 150+ real world crystals.

Below are some interesting correlations/statistics gathered from the measured data; raw measurement data is available here.

Average C0 capacitance for each crystal holder type

Average C0 capacitance for each crystal holder type

Average motional resistance vs frequency

Average motional resistance vs frequency

Average motional inductance vs frequency

Average motional inductance vs frequency

Average unloaded Q vs frequency

Average unloaded Q vs frequency

Average unloaded Q for each crystal holder type

Average unloaded Q for each crystal holder type

Unloaded Q of glass-sealed crystals vs average unloaded Q

Unloaded Q of glass-sealed crystals vs average unloaded Q

Overall statistical analysis of unloaded crystal Qs

Overall statistical analysis of unloaded crystal Qs

References and additional reading:

Description Reference
Crystal motional parameters and relevant equations Understanding Quartz Crystals and Oscillators, Ramon M. Cerda, Chapter 1.27
Measuring quartz crystal parameters Measurement of the equivalent circuit of a quartz crystal, Omicron Lab
Crystal measurements with a VNA Crystal Bandpass Filters, W0QE
Crystal measurements with a Spectrum Analyzer Crystal Parameters — Experiments with a Tracking Generator + Spectrum Analyzer, QRP HomeBuilder
Comparison of crystal measurement techniques Crystal Motional Parameters: A Comparison of Measurement Approaches, Jack R. Smith, K8ZOA
Crystal test fixture design Assembly and Usage Notes for K8ZOA Crystal Test Fixture Revised for Version 1.2 PCB, Jack R. Smith, K8ZOA

The RC Filter Transfer Function

Most people are familiar with the simple first-order RC low pass filter:

RC Low Pass Filter

Also well known is the equation for calculating the -3dB (aka, half-power) cutoff frequency of the RC low pass filter:

\(
\begin{equation}
\mathbf{f}_{c} = \frac{1}{2 \pi RC}\\
\end{equation}
\)

It’s an easy equation to memorize, but if you’re interested in where this equation comes from, read on; if you’re familiar with resistor voltage dividers, this will be a piece of cake!

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HP5385A LCD Repair

The LCD in my beloved HP 5385A frequency counter (with the OCXO option, naturally) has been gradually deteriorating, and it was high time I replaced it:

The HP5385A's failing LCD

The HP5385A’s failing LCD

Of course it’s not a standard HD44780 LCD, but a custom branded HP part (#5061-5212, revision A).

Luckily, I was able to salvage a replacement from an HP 6622A power supply front panel off ebay. The front panel cost about 1/3 of what I paid for the HP 5385A in the first place, but it’s a great frequency counter, so it was well worth the investment.

Repairing an HP3466A Multimeter

My main bench multimeter is an HP3466A that I found at a hamfest a couple years ago. Mind you it’s older than I am, but it looked to be in good functional order, and $30 seemed like a fair price for a 4.5 digit bench meter, so I picked it up.

The HP3466A

The HP3466A

After a while though, I noticed that its DC voltage readings seemed to be low when probing circuits containing larger resistances; anything around 100K ohms caused a noticeable discrepancy in the expected voltage reading, and it worsened with larger resistances.

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