Bil Herd: Computing with Analog

When I was young the first “computer” I ever owned was an analog computer built from a kit. It had a sloped plastic case which had three knobs with large numerical scales around them and a small center-null meter. To operate it I would dial in two numbers as indicated by the scales and then adjust the “answer” by rotating the third dial until the little meter centered. Underneath there was a small handful of components wired on a terminal strip including two or three transistors.

Science Fair Analog Computer

Science Fair Analog Computer

In thinking back about that relic from the early 1970’s there was a moment when I assumed they may have been using the transistors as logarithmic amplifiers meaning that it was able to multiply electronically. After a few minutes of thought I came to the conclusion that it was probably much simpler and was most likely a Wheatstone Bridge. That doesn’t mean it couldn’t multiply, it was probably the printed scales that were logarithmic, much like a slide rule.

Analog slide rule on digital calculator

Old meets new: Analog and digital computation

Did someone just ask what a slide rule was? Let me explain further for anyone under 50. If you watch the video footage or movies about the Apollo Space Program you won’t see any anyone carrying a hand calculator, they didn’t exist yet. Yet the navigation guys in the first row of Mission Control known aptly as “the trench”, could quickly calculate a position or vector to within a couple of decimal places, and they did it using sliding piece of bamboo or aluminum with numbers printed on them.

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Analog Drum Machine

Drum Machine Schematic

This analog drum machine project synthesizes a kick and snare drum that are clocked to a beat. It pulls together a few analog circuits to do the timing and synthesis.

The beat timing is a product of a hysteretic oscillator used to create a ‘shark wave,’ which is a friendly term for the output of a relaxation oscillator. This waveform can be compared to a set point using a comparator to create a slow square wave that clocks the drum beat.

The kick drum is synthesized using another hysteretic oscillator, but at a higher frequency, creating a triangle-like waveform at 265 Hz that provides a bass sound. The snare, however, uses white noise provided by a BJT’s P-N junction, which is reverse biased and then amplified. You can spot this transistor because its collector is not connected.

The resulting snare and kick drum wave forms are gated by two transistors into the output. Controlling these gates allows the user to create a drum beat. After the break, check out a video walk-through and a demo of the build.

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Pulse Oximeter from LM324, LED, and Photodiode

pulse-oximeter

This pulse oximeter is so simple and cheap to build it’s almost criminal. The most obvious way to monitor the output of the sensor is to use an oscilloscope. The poor-man’s stand-in for that is a sound card, which is what [Scott Harden] demonstrates in his write-up.

It uses a concept we’ve seen a few times before. The light from an LED shines through your finger and is measured on the other side by a phototransistor. It’s that light grey plastic thing you see on a patient’s finger when they’re in the hospital. [Scott] went with a common wooden clothes pin as a way to mount and align the sensor with your finger. It is monitored by the simplest of circuits which uses just one chip: an LM324 op-amp. There are three basic stages which he explains well in the video after the jump. The incoming signal is decoupled before being fed to the first amplifier stage. From there it is fed to an adjustable low-pass filter to help eliminate 60Hz noise from AC power in the room. The last stage amplifies the signal again while using another low-pass filter in parallel.

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Understanding op-amps from simple to hard

[Tim] wanted to help out a ECE student struggling with some Op-Amp problems. He put together a video which does a good job of explaining what an Op-Amp does, then tackles each of the questions one at a time.

His analogy is illustrated in this image. There’s an operator using a crane to lift a crate. He is watching a ‘radio man’ in a window of the building to know how high it should be lifted. These roles are translated to the function of an Op-Amp in a way that makes understanding the common parts quite easy. The crane is the Op-Amp and the floor to which it is trying to lift the crate is the input pin. The current height of the crate is the output signal. The radio man is the feedback resistor which is trying to get the desired height and current height to equal each other. Watch the video after the break and all becomes clear.

After this analogy is explained [Tim] tackles the actual homework problems. He’s going through everything pretty quickly, and doesn’t actually give the answers. What he does is show how this — like most circuit solving problems — depends on how you group the components in order to simplify the questions. Grab a pen and paper and put on your electron theory hats to see if you can solve the questions for yourselves.

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Analog soil moisture alarm

The lion’s share of soil moisture monitors we see are meant as add-ons for a microcontroller. So we’re glad that [Miceuz] tipped us off about this soil moisture alarm he built with analog parts. It’s really not hard to take the concept and build it in the analog world. That’s because you’re just measuring a resistance value. But for those of us who never really got started with analog parts this is a great project to learn from.

A high-efficiency op-amp is doing the brunt of the work. When the soil is moist the resistance is rather low compared to a reference voltage provided by a separate resistive divider. But when the plant gets thirsty and the soil dries out the resistance increases, triggering the op-amp to illuminate an LED and create some noise on the buzzer (we’re a bit confused on how that buzzer works).

Unfortunately this isn’t a viable long-term solution as the battery calculations show it lasting only about four months. That’s where a microcontroller-based circuit really shines, as it can put it self in low-power sleep and wake infrequently to take readings.

Fill in the bass on your PSP

[Michael Chen] felt the sound his PSP was putting out needed more dimension. Some would have grabbed themselves a nice set of headphones, but he grabbed his soldering iron instead and found some space where he could add a bigger speaker.

Mobile devices tend to cram as much into the small form factor as possible so we’re surprised he managed make room. But apparently if you cut away a bit from the inside of the case there is space beneath the memory card. [Michael] cautions that you need to choose a speaker rated for 8 ohms or greater  in order to use it as a drop-in replacement for one of the two original speakers. But he also touches on a method to use both stock speakers as well as the new one. He suggests grabbing an LM386 op-amp and a capacitor and hooking them up. Yep, there’s room for that too if you mount it dead-bug-style. We wonder how the battery life will be affected by this hack?

A Salinometer built for the Science Olympiad

This is a Digital Salinometer which [Daniel Kramnik] built as a Science Olympiad entry. He’s a Junior in High School and when looking for a project to enter into the Water Quality event he was interested in achieving greater accuracy than a mechanical hydrometer provides.

We think the circuit design is very impressive for anyone who hasn’t complete formal training as an engineer, and outstanding for someone as young as [Daniel]. Measurements depend on two main parts, a temperature control and a salinity sensor. These are both necessary because fluctuation in sample temperature will affect the salinity reading.

A Peltier element is used to heat the water sample if it doesn’t fall within a set range of temperatures. From there, an Op-Amp circuit conditions a signal running through the sample, passing an output to the ADC converter chip which drives the three-digit readout. [Daniel] calculates an accuracy within 0.0014%. He must be on the mark because he’s won his regional competition and will soon compete at the state level.

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