[Craig] recently built himself a version of the “hassler” circuit as a sort of homage to Bob Widlar. If you haven’t heard of Bob Widlar, he was a key person involved in making analog IC’s a reality. We’ve actually covered the topic in-depth in the past. The hassler circuit is a simple but ingenious office prank. The idea is that the circuit emits a very high frequency tone, but only when the noise level in the room reaches a certain threshold. If your coworkers become too noisy, they will suddenly notice a ringing in their ears. When they stop talking to identify the source, the noise goes away. The desired result is to get your coworkers to shut the hell up.
[Craig] couldn’t find any published schematics for the original circuit, but he managed to build his own version with discrete components and IC’s. Sound first enters the circuit via a small electret microphone. The signal is then amplified, half-wave rectified, and run through a low pass filter. The gain from the microphone is configurable via a trim pot. A capacitor converts the output into a flat DC voltage.
The signal then gets passed to a relaxation oscillator circuit. This circuit creates a signal whose output duty cycle is dependent on the input voltage. The higher the input voltage, the longer the duty cycle, and the lower the frequency. The resulting signal is sent to a small speaker for output. The speaker is also controlled by a Schmitt trigger. This prevents the speaker from being powered until the voltage reaches a certain threshold, thus saving energy. The whole circuit is soldered together dead bug style and mounted to a copper clad board.
When the room is quiet, the input voltage is low. The output frequency is high enough that it is out of the range of human hearing. As the room slowly gets louder, the voltage increases and the output frequency lowers. Eventually it reaches the outer limits of human hearing and people in the room take notice. The video below walks step by step through the circuit. Continue reading “Annoy Your Enemies with the Hassler Circuit”→
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.
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”.
UAF421 Universal Active Filter
UAF421 Universal Active Filter using a dual ganged potentiometer.
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″→
[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.
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?
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.
By 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.