my recent reads..

LEAP#198 Hall Effect Switch

Hall-effect sensors generally support one of three modes:

  • switch - turn on in the presence of a magnetic field of a specified polarity
  • latch - turn on in the presence of a magnetic field, and stay on until exposed to the reverse polarity
  • linear sensor - output is proportional to the magnetic field strength
The components I have for testing are marked "44E/938" and are nominally compatible with the A3144 or OH3144. These are switches that turn on when facing a south magnetic polarity

The chip is rated for continuous output current of 25mA, so that is sufficient to drive an LED (as demonstrated here). For other switching applications, the output can be used to switch a transistor or pull a microcontroller output low.

Testing with some neodymium magnets, I get a strong full-on when the south pole of the magnets are within 23mm directly to the front of the chip. The output remains on until I pull back to over 40mm.

A common use for Hall-effect sensors is to detect and measure rotation. A good demonstration of this is to sequence a PoV display as demonstrated in Great Scott's latest video - HACKED!: Old Fan becomes a POV Display

As always, all notes, schematics and code are in the Little Arduino Projects repo on GitHub.

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LEAP#197 Wien Bridge Audio Tone Generator

A Wien bridge oscillator is essentially an RC Band Pass Filter with a high Q factor at the resonant frequency, and generates a nice sine wave. I wanted a simple audio-frequency test signal generator, and a Wien Bridge turned out to be perfect for the job.

As always, all notes, schematics and code are in the Little Arduino Projects repo on GitHub.

A good old LM324 does triple duty in the circuit:
  • a non-inverting amplifier with a gain of ~3 provides the feedback required to sustain the oscillator
  • one unit buffers a half-supply voltage to provide a "virtual ground" for the non-inverting amplifier
  • a third unit buffers the output signal to avoid load interference with the oscillator
The LM324 doesn't have great upper and lower limits, and clipping is severe in the basic circuit. This is solved with diode stabilisation.

The result is a pretty decent sine wave at ~1.574kHz, very close to the theoretical resonant frequency of 1.592kHz.

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LEAP#196 Driving a 7-segment display with CD4026 Counter

Here's yet another way to drive a 7-segment single-digit display unit - using a CD4026.

The CD4026 is a 5-stage Johnson decade counter with decoded 7-segment display outputs and display enable. With RESET and CLOCK INHIBIT low, and DISPLAY ENABLE IN high, the 7-segment display outputs progress through the 0-9 sequence on the rising edge of the CLOCK pulse.

It's an interesting alternative to a shift register for driving a 7-segment LED (as in the ShiftDrive project). While a latched shift register provides random addressing and clean transitions to any digit, it requires the 7-segment display outputs to be decoded externally (like in code). On the other hand, the CD4026 takes care of the decoding, and external circuits just need to send a counter pulse.

As always, all notes, schematics and code are in the Little Arduino Projects repo on GitHub.

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LEAP#195 Coil Resonance and Inductor Testing

I've been trying to do some RF experiments, but instead being driven mad by hand-wound coils that never seem to behave as the standard coil inductance calculations would lead us to believe.

As always, it's w2aew to the rescue with a neat deconstruction and demo of a circuit for coil or inductor measurement.

I built the circuit out, and was easily measuring down to 10µH on a breadboard, and sub-1µH once I put the circuit on a hacked up copper board with islands. Using the LC circuit resonant frequency formula, it's possible to work backwards from known frequency and capacitance to determine the inductance (wolframalpha is great for this).

So now I hope to get back into some RF without the nagging doubt of not really knowing what my coils are doing!

As always, all notes, schematics and code are in the Little Arduino Projects repo on GitHub.

Here's a beautiful trace of a (nominally) 10µH choke with a 150pF capacitor. I measure the frequency at 4.26MHz, therefore an actual inductance of 9.3µH .. pretty close!

Even with a super-bodgy 4.5 turn coil whipped up on the spot and put in parallel with a 30pF capacitor, I'm still getting a decent oscillation at 34.09MHz for a calculated inductance of 0.73µH

And here's the board delivering the results:

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