TT555 Soldering Kit now available on Tindie!

Probably the most-viewed post on this blog so far has been my project to construct a discrete-transistor replica of the 555 timer IC. I’m pleased to report that, due to popular demand, the TT555 is now available in kit form!

This is your chance to prove that you’re a real SMD soldering expert: to assemble it, you need to place 43 components onto a 10×10 mm2 PCB, pick and place 01005 size resistors (0.4×0.2 mm2), and solder DFN-1006-3 packages (1.0×0.6 mm2). A microscope, sharp tweezers and a fine-tipped soldering iron are essential tools to complete this project.

Please click here to find the TT555 on Tindie!

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First Alert Smoke and CO Detector

I bought this smoke and carbon monoxide detector several years ago, and it’s been doing its job just fine ever since: mostly being silent, screaming when I burn my toast, and beeping every two years or so when its batteries run out. Recently however it began beeping for no reason, and I couldn’t get it to stop other than by removing the batteries. So I bought a new one and decided to tear down the old one.

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TI LPV801 and LPV821

Back in 2016, TI introduced a line of what they called “nanopower op amps”. Where older op amps like the 741 use around 2 mA, and more modern ones might reduce that to perhaps 100 uA or so, TI’s ultra-low power devices consume just a few hundred nA. This enables the design of things like smoke alarms and temperature monitors that can work for a decade on a single battery charge.

This is the LPV801, a single channel op amp that uses just 450 nA. It’s not very fast: with just 8 kHz of unity-gain bandwidth it’s useless for audio, but ideal for slow-moving things like temperature sensors. A dual version (LPV802) is also available, as are single and dual versions with reduced offset voltage (the ‘811 and ‘812 respectively).

Inside we find this neat little design. Five bond pads are bonded to the five pins on the package; two additional ones on the top row are used for testing. In the top-right corner is an L-shaped alignment marker, which is used during laser trimming.

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Philips LED Bulb

LED light bulbs are the most commonly sold type nowadays, especially since incandescent bulbs have been gradually banned from sale starting around 2009. Other low-power types like halogen and compact fluorescent bulbs were also commonly sold until a few years ago, but advances in LED technology, along with a precipitous drop in price, have made LED bulbs the most common type by far.

This 5.5 W, 470 lumen bulb had been lighting my home for a couple of years until it burned out a month ago. It was one of the cheapest types sold under the Philips brand; I don’t recall exactly how much I paid for it, but it must have been around eight euros or so. Philips, founded in 1891, is one of the oldest manufacturers of light bulbs, although all lighting products were spun off into a separate company called Signify in 2016.

If we cut off the translucent plastic dome, we find a small PCB carrying the LED chips. It’s screwed onto a thick aluminium body that acts as a heat sink. There are five LEDs mounted on the board, but as we can see there’s place for three more. Clearly, the same basic design is also used for a higher wattage version that contains eight LEDs.

All LEDs are connected in series; the three unused ones are bypassed by two zero-ohm resistors (JP1 and JP2). In the middle is a two-pin connector that supplies power from a regulator PCB in the base of the bulb.

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π120u30 Digital Isolator

Digital isolators are a modern replacement for optocouplers: components that can bring a signal from one place to another without connecting those two places electrically. They’re essential parts in equipment that connects to a dangerous voltage on one end (mains power usually) and comes into close contact with something sensitive on the other (humans, usually). Since they’re safety-critical components, manufacturers show off all kinds of safety certificates and qualifications to convince their customers that their isolators won’t electrocute anyone by mistake.

Today we’ll look at one of the cheapest digital isolators out there: the π120u30, made by 2Pai semiconductor, which costs less than 20 cents in large quantities.

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TI DRV8876

Texas Instruments has a nice selection of motor driver ICs. Where in the old days you’d often have to make your own H-bridge out of discrete transistors, figure out how to drive their gates, and then generate the right signals to spin up, reverse, brake or coast your motor, nowadays you can get all these functions integrated into a single chip. Today we’ll look at the DRV8876, which is a rather small chip that can nevertheless dump up to 3.5 A into a motor’s windings.

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The 555 timer – part 2

Today we’ll look at a couple more versions of the 555 timer. Like the 741, this chip has been produced by many different manufacturers in the nearly five decades since its introduction by Signetics in 1972.

First up is RCA’s CA555. Packaged in an 8-pin DIP (which is what the “E” in “CA555CE” stands for), this is a “C” spec which can work at up to 16 V, unlike the CA555E that is spec’ed up to 18 V. I’m not sure what the actual difference between these two would be; I guess the chips were sorted after production, with parts that marginally failed some spec at 18 V being demoted to “C” versions.

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ST LIS3DH Accelerometer

The LIS3DH is an accelerometer, designed and manufactured by STMicroelectronics. Like most accelerometers today it is a MEMS device (Micro-Electro-Mechanical Systems), which means the sensing function is made from silicon and integrated into an IC process. Special etching techniques are used to create tiny moving parts that can bend in certain directions, along with sensors that can detect that movement.

MEMS accelerometers are used in many electronic devices: your phone that detects whether you’re holding it horizontally or vertically, your games console that can sense which way you’re moving the controller, or your car that can detect the speed and direction in which you’ve just crashed so it can properly deploy the airbag.

The LIS3DH is housed in a little LGA package that measures just 3 x 3 mm2. Oddly there’s no ST logo or marking on the package. The “ON5nn” production code could actually trick you into thinking this is an ONSemi part.

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The 741 op amp – part 4

After a bit of browsing on eBay I found several “new” versions of my favourite classic op amp, the 741. We’ve already seen about twelve different layouts so far, but today we’ll see that there are still more out there.

Starting with the oldest, we go back more than four decades to 1974. Texas Instruments at that point sold the SN72741, adapting the original Fairchild part number to their own nomenclature: SN stands for “Semiconductor Network”, meaning “integrated circuit”, while the numbers indicate the product series and temperature range (0 to 70 °C in this case; there was also an SN52741 with a wider temperature range).

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The TT555: a discrete 555 timer plug-in replacement

After dissecting a whole batch of 555 timer ICs I thought it would be an interesting project to make a working copy of the 555’s internal circuit out of discrete components, in the same physical space as the original IC. I’ve done this before with the 741 opamp, and the steps are similar. First, I drew the schematic in KiCAD:

I used the original circuit as designed by Hans Camenzind, as a tribute to his design but also because it uses the smallest number of transistors among all different designs I found while dissecting the various 555 ICs. Discrete transistors are larger than discrete resistors, so the original design saves space compared to newer versions that include several more transistors. Note that it’s the opposite situation when you’re designing an actual IC: integrated resistors are usually the largest parts in your layout.

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