Following on from a previous post discussing the Analog Devices AD587 precision 10.000V voltage reference, I built a portable device to utilise the chip.
Some requirements of the project were:
10.000V ±5mV output
Battery operated device
Visual, low battery indication
Small, aluminium housing
Clear front panel
Low cost (under $50), readily available components
A low battery indication was a desired feature to prevent the device being used in an important test and the battery level drops low and compromises the AD587’s performance. A simple green LED will suffice. Output performance of 10.000V (± 5mV) couldn’t be compromised so there is no protection to prevent high current draw from the chip, I’ll just have to be sensible.
Following on from a recent teardown of a low cost appliance energy meter, I’ve done a bit more hacking of the device. As you may recall, I identified that one of the pins on the meter’s chip (EOUT) output a train of pulses proportional to the energy consumed. Some tests with a multimeter seemed to confirm this because the average voltage of EOUT changed with the appliance wattage.
I was hesitant to connect my logic analyser to the meter to measure the EOUT pin because of potential differences in voltage levels. To resolve, this I quickly whipped up a small board with a 4N25 opto-isolator to provide some voltage isolation between the internals of the energy meter and my logic analyser.
Again, I will repeat the obligatory warnings prevail. Do not:
Attempt or copy any of this if you do not fully understand or appreciate the hazards of mains power
Open the meter whilst connected to mains power
Perform any measurements whilst the meter’s case is open
Connect another mains powered device to measure the chip. The energy meter’s power supply is not isolated from active, neutral or earth.
The circuitry that allows for isolation of voltage levels is based around a 4N25 opto-isolator. An opto-isolator allows for a signal to be transferred using light, this provides an airgap which provides electrical isolation.
New Scientist recently detailed the construction materials for the simplest flex sensor I’ve ever heard of – a graphite pencil and a piece of paper. It is obvious when you think about it, but as New Scientist details in in their article Pencil lines as sensors, 21st March 2015 (No 3013) edition,
“MacGyver would be proud. Drawing a rectangle on a piece of paper with an ordinary pencil can create a sensor.”
A friend of mine recently asked me if I knew that there are two package types for humble TO220 devices. I didn’t understand what he meant. He then asked if I had heard of single gauge and dual gauge TO220 packages. Nope still no idea. Finally, he asked if I had seen TO220 voltage regulators such as a 78L05 device with a thin heatsink?
Yes, I had seen these before, in fact I have a couple. I hadn’t really paid a lot of attention to the first time I had seen one of these components with a thin heatsink. I thought that it was perhaps a counterfeit device. That is not the case (excuse the pun). A thinner heatsink is a type of TO220 package.
Below are two TO220 devices, a 7812 12V regulator manufactured by On Semiconductor and a TIP31 NPN transistor from ST Microelectronics. A quick glance and their shape is instantly recognisable.
Honestly, each board that was made was a fail. Electrically, they all worked correctly. The failure was during the manufacturing stage – reflow soldering very small SMD components.
Just as an aside, Adafruit Neopixel devices use WS2812B LEDs which have some very strict timing requirements – this can be problematic for some devices that use software interrupts. Whereas, the APA102C LED is a chip that does not suffer from strict timing.
Each small PCB, contains 9x APA102C SMD LEDs and 8x 0.1uF 0603 ceramic capacitors for power supply decoupling. Each APA102C LED is the large white square with white circle. Between the rows of the APA102C LEDs are the SMD pads for the 0603 size capacitors.
Old seven segment displays definitely have a cool factor. These TIL302 displays from Texas Instruments are no exception.
Five of these displays were picked up cheap on eBay recently. I plan to use them in an upcoming project that requires a display that is a little more special than a standard seven segment display. Continue reading →