Universal Active Filters: Part 2

An easy way to conceptualize active filters is thinking about audio speakers. A speaker crossover has a low-pass, high-pass and band-pass effect breaking a signal into three components based upon frequency. In the previous part of this series I took that idea and applied it to a Universal Active Filter built with a single chip opamp based chip known as the UAF-42. By the way, it’s pretty much an older expensive chip, just one I picked out for demonstration.

Using a dual-ganged potentiometer, I was able to adjust the point at which frequencies are allowed to pass or be rejected. We could display this behavior by sweeping the circuit with my sweep frequency function generator which rapidly changes the frequency from low to high while we watch what can get through the filter.

In this installment I’ll test the theory that filtering out the harmonics which make up a square wave results in a predictable degradation of the waveform until at last it is a sine wave. This sine wave occurs at the fundamental frequency of the original square wave. Here’s the video but stick with me after the break to walk through each concept covered.

Continue reading “Universal Active Filters: Part 2”

Universal Active Filters: Part 1

Today I am experimenting with a single chip Universal Active Filter, in this case I made a small PCB for the UAF-42 from Texas Instruments. I chose this part in particular as it facilitates setting the filter frequency by changing just a pair of resistors and the somewhat critical values that are contained on the chip have been laser trimmed for accuracy. This type of active filter includes Operational Amplifiers to supply gain and it supports various configurations including simultaneous operating modes such as Band Pass, Low Pass and High Pass make it “Universal”.

Filter Basics

Speaker Crossover Example
Speaker Crossover Example

Looking at the block diagram you can see where I have inserted a dual-ganged potentiometer to change both resistors simultaneously which should allow a straight forward adjustment for our purposes here.

Looking into the components of a simple RC filter which can easily implement a simple Low Pass or High Pass filter, we see that the math is fairly straight forward and swapping the components with each other is all that is needed to change the type of filter. Continue reading “Universal Active Filters: Part 1”

Capacitive Garage Door Opener Hides Behind Your Dash

[Pyrow] wanted to upgrade his garage door opener remote. It worked just fine, but changing those tiny batteries out can be an inconvenience. Plus, the remote control was taking up valuable storage space and would always rattle around while driving. [Pyrow] decided to make use of an Omron E2K-F10MC2 capacitive touch sensor to fix these issues.

[Pyrow’s] circuit still makes use of the original remote control. He just added some of his own components to get it to do what he wanted. The circuit is powered by the car’s battery, so it never needs a battery replacement. The circuit is protected with a fuse and the power is regulated to prevent electrical spikes from burning up the original remote control. The actual circuit is pretty simple and uses mostly discrete components. It’s all soldered onto proto board to keep it together. He only had to solder to three places on the original remote control in order to provide power and simulate a button press.

Next, [Pyrow] took his dash apart. He used double-sided tape to attach the touch sensor to the back of the dash.  After securing the electronics in place with tape, he now has a working hidden garage door opener. Full schematics are available in the writeup linked above. Also, be sure to watch the demonstration video below.

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Ask Hackaday: How Would You Build a Clock Clock?

Hotel art often elicits less than a glance from most patrons. But we have to admit if we came across a piece like this we’d be compelled to record a video for later reference. That’s actually where the video came from, this was spotted in a hotel called Ham Yard.

The concept seemed familiar to us and a bit of Google-fu brings up our previous coverage of the concept back in 2010. The display is made up of circular analog clocks and we’d wager this is a version of “a million times” by Human Since 1982, the same artist who brought us the earlier concept.

Since we’re covering this once again we thought it would be fun to ask: how would you go about building your own? There are several challenges that come to mind. First, notice both hands of the analog clocks appear to be exactly the same (there is no short hour hand). Driving the coils of a cheap clock directly (a la Lord Vetinari clock hacking) seems an obvious approach. But look closely and you’ll see the hands sometimes move in opposite directions. There must be a simple way to implement the control, or are we chasing a pipe dream of a low cost version for our workshop clock?

[Thanks Munit]


Printing Text with a Chart Recorder

A chart recorder printing 'Hello World'

Chart recorders are vintage devices that were used to plot analog values on paper. They’re similar to old seismometers which plot seismic waves from earthquakes. The device has a heated pen which moves across a piece of thermally sensitive paper. This paper is fed through the machine at a specified rate, which gives two dimensions of plotting.

[Marv] ended up getting a couple of discontinued chart recorders and figured out the interface. Five parallel signals control the feed rate of the paper, and an analog voltage controls the pen location. The next logical step was to hook up an Arduino to control the plotter.

However, once the device could plot analog values, [Marv] quickly looked for a new challenge. He wanted to write characters and bitmaps using the device, but this would require non-continuous lines. By adding a solenoid to lift the pen, he built a chart recorder printer.

After the break, check out a video of the chart recorder doing something it was never intended to do. If you happen to have one of these chart recorders, [Marv] included all of the code in his writeup to help you build your own.

Continue reading “Printing Text with a Chart Recorder”

The Disintegrated Op Amp

741By now we’ve all seen the ‘Three Fives’ kit from Evil Mad Scientist, a very large clone of the 555 timer built from individual transistors and resistors. You can do a lot more in the analog world with discrete parts, and [Shane]’s SevenFortyFun is no exception: it’s a kit with a board, transistors, and resistors making a very large clone of the classic 741 op-amp, with all the parts laid bard instead of encapsulated in a brick of plastic.

[Shane] was inspired by the analog greats – [Bob Pease], [Jim Williams], and of course [Bob Widlar], and short of mowing his lawn with goats, the easiest way to get a feel for analog design was to build some analog circuits out of individual components.

[Shane] has a few more kits in mind: a linear dropout and switching regulators are on the top of the list, as is something like the Three Fives kit, likely to be used to blink giant LEDs.

A Motion Activated AC Switching Circuit using Mostly Discrete Components

AC motion switch

If you’ve ever dealt with a brightly lit Christmas tree, you might understand the frustration of having to crawl underneath the tree to turn the lights on and off. [brmarcum] feel’s your pain. He’s developed his own motion activated AC switching circuit to turn the lights on and off automatically. A motion sensor ensures that the lights are only on when there are people around to actually see the lights. The circuit also has an adjustable timer so [brmarcum] can change the length of time that the lights stay on.

The project is split into several different pieces. This makes the building and debugging of the circuit easier. The mains power is first run through a transformer to lower the voltage by a factor of 10. What remains is then filtered and regulated to 9VDC. [brmarcum] is using a Parallax PIR sensor which requires 4.5V. Therefore, the 9V signal is then lowered once more using a voltage divider circuit.

When the PIR sensor is triggered, it activates the timer circuit. The timer circuit is driven by a 555 timer. The circuit itself was originally borrowed from a classic Forrest Mims book, though it was slightly modified to accommodate the PIR sensor. The original push-button trigger was removed and replaced with the signal from the PIR sensor. The only problem is that the circuit was expecting a low signal as the trigger and the PIR sensor outputs a high signal. [brmarcum] resolved this problem with an NPN BJT to invert the signal. Once the timer is triggered, it flips on a relay that allows the mains electricity to flow through to the lights.

[brmarcum] soldered the entire circuit onto a piece of protoboard. The final product was then mounted securely inside of an insulated plastic case. This allows him to mount the circuit safely underneath the Christmas tree skirt. The PIR sensor is kept external to the enclosure and wired up into the tree itself. This allows the sensor to still detect motion in the room while the rest of the circuit is hidden away.

[via Reddit]