Wireless Temperature Sensor (nrf24L01 & DS18B20)

As I mentioned about IoT Gadgets in the last blog post, I've been developing an "home automation basestation" and gadgets which connects to that. The basestation (BS) itself is connected to wlan and my server via ESP8266. The BS has an DS18B20 temperature sensor and IR led, which controls my AC unit. It also has an nrf24l01 RF module for external wireless sensors and controllers. For now, I have only one wireless temperature sensor, which also has that same temperature sensor and RF module. More of that, the sensor module has ATtiny2313A microcontroller, 1.5 F 5.5 V super capacitor and a solar cell.

The sensor module would run almost forever with AA batteries but that would have been way too boring, so I used a super cap & solar cell combination to learn something new about the power management optimization, sleep modes and current measurement techniques. I also wanted to use an 8-bit AVR microcontroller at least for this revision, although for ex. Gecko EFM32 would have provided a lot smaller current consumption. Just for simplicity, I also left out the energy harvesting IC with solar cell MPPT tracking and buck-boost SMPS output.

Rev. 1 board. Click to enlarge

Just by looking at the operating voltage ranges of every component, the max. voltage of nrf24L01 is 3.3 V and min. voltage of DS18B20 is 3.0 V, so 3.0 V LDO regulator with a low quiescent current could power everything in this module. This sets the usable voltage range of the cap between 3 V and 5.5 V. The load acts as a constant current sink over the voltage range, if we average out the active and sleep state current consumptions. To calculate the time when the voltage of the super cap drops from 5.5 V to 3.0 V, we can use the formula: t = C * [(V1-V2)/I]. So for ex. 1.5 F and 20 uA would make 187 500 seconds, which is ~52 hours with the assumption that we don't put any charge in during that time.

The rev. 1 model was quite large because it had a jumper for every voltage rail, so I could measure the current consumption of any individual component. There was also 3 capacitors (1.5 F, 0.47 F and 330_uF) behind the jumpers so I was able to change the connected capacitance. Rev. 2 doesn't have these jumpers and has only one super capacitor, so it's a lot smaller.

Rev. 2 board. Click to enlarge

As I programmed the first working scratch (1 MHz clock) without any sleep modes, the current consumption was approximately 750 uA, which would drain the cap in 83 minutes. As the MCU stays in sleep state most of the time, we want to use the lowest power sleep state, which is "power-down". Power-down disables timers, except the watchdog, which can be used as a wake-up source even in that deepest sleep mode. Watchdog interrupt function is used to cancel the reset flag before the MCU resets. Just by adding the power-down sleep mode, the average current consumption goes down to 130 uA, which is still 5 times too much.

By lowering the MCU clock from 1 MHz to 250_kHz, enabling the "power reduction register" features in ATtiny, reducing the temperature resolution of DS18B20 from 12 bits (0.0625 °C) to 10 bits (0.25 °C), using the deepest sleep mode of the nrf24l01 and by pulling down any unnecessary microcontrollers pins, the current consumption goes down to 27 uA. Further, we don't really need a temperature reading every 8 seconds (longest WDT range), so the final revision of the program reads the temperature in one interrupt cycle, puts the MCU to sleep already during the 250 ms temperature conversion of the DS18B20 and sends the result it in another cycle. That gives one temperature reading per 16 seconds, which is still well enough, but drops the average current consumption to 13_uA.

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Download files:

Proteus 7.7 schematic & layout files (zip)

Atmel Studio 6.1 project / source code (zip)

As there is no step-up converter / energy harvesting IC, the open circuit voltage of the solar cell should go quite easily to 5 volts even in mediocre lighting, so the voltage would be higher than capacitor's voltage to allow the charging. The series schottky diode prevents the current from going in wrong direction when there is not enough light for charging. When there is lots of light, the charging voltage needs to be limited to 5.5 V, so the super cap won't blow up. That's done by using a voltage supervisor IC, which controls the mosfet and shorts the solar cell to ground through the 220 Ω resistor, if the voltage goes too high. Zener diode would have been a "single component solution" but they tend to be too leaky in non-conductive region, so it wasn't an option.

Click to enlarge. Sensor module with 2300 uF capacitor(s) as a power supply. Low voltage detection off vs. on

The recommended minimum input voltage range of 3.0 V LDO regulator is ~3.1 volts, but at that low currents, that specific LDO seems to works well above this, although the output voltage obviously starts to follow the input voltage when it goes above 3.0 volts. ATtiny2313A doesn't have an ADC which could be used for voltage monitoring, and the Brown Out Detector would be a bit too extreme, since there's no other option than resetting the whole MCU when it triggers. But there is an analog comparator, which can be used by connecting the internal bandgap reference (1.1 V) and comparing it to the operating voltage, divided with two resistors. I used 100k and 75k resistors, which makes 1.29_V when the operating voltage is 3.0 V. When the operating voltage dips below 2.56 V, the output of the resistor divider goes below 1.1 V and triggers the analog comparator. However, bandgap reference consumes a lot of current (~15 uA) so it's turned on only during the voltage test for a short period of time. If the voltage reaches that point, the DS18B20 doesn't work properly anymore, and measuring the temperature & sending the results are stopped to save some current. At that point we can also send a low voltage warning to the BS. There's also a routine which resetes the system occasionally if the low voltage is detected, so the memory of the MCU, RF module or the digital temperature sensor is refreshed to prevent corruption of the memory and malfunctioning of the system.

Currently I'm logging the temperature readings to my RPi 3 server (just for fun), and the module seems to work very well even in indoor lighting, without a direct sunlight. Next I'm going to put this outdoors when I'll find a good case for it.

Raspberry Pi UPS from junk box

I've been using the Raspberry Pi 1 as an ssh shell for Irssi and as a web server for my IP cameras and self-made IoT gadgets. Maybe I had just a bad luck with the corrupting SD cards, but I ended up to put the Arch Linux and other files to 120 GB SSD (although the SD card is still needed for bootloader), which was connected to RPi via 2.5" USB HDD box. Some time ago the SSD died and I got a good reason to upgrade the whole system, so I bought the RPi 3 and replaced the SSD with an 250 GB Samsung 840 basic. The extra performance (1x 700 MHz vs. 4x 1.2 GHz ARM cores) is nice especially when rendering timelapse videos from my IP camera photos.

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To get some extra reliability, I thought that some kind of UPS could be nice. There's plenty of complete kits on ebay, but I happened to have some suitable parts in my junk box, so I didn't bother to wait 2 weeks. It wouldn't need any fancy features, but obviously should have at least an overcharging protection and under-voltage cutoff if the power outage is long enough. So there's the plan:

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To keep it simple, there's no separate charging circuit for the 12 V sealed lead acid battery. I had couple of 12 V power supplies so it could charge the battery up to 12 volts. However fully charged battery would sit around 14.2 volts, so charging it only to 12 volts would waste about half of it's full capacity (2.9 Ah). For me, that's not a problem because the RPi would stay on more than long enough even with 1.45 amp-hours. We can limit the charge current of the battery with the 47R resistor, but allow higher discharge currents through the schottky diode, when the primary power supply cuts off. Theres also a protection diode for the 12 V power brick, so it cannot draw current from the battery when the main PSU is down. At these voltages we don't have to care about overcharging the battery.

RPi requires 5 volts, so there's also a regulator. Because the voltage drop from 12 V to 5 V is quite large when considering the 400-1000 mA operating current, I chose a buck switch mode power supply module instead of regular LM7805 linear regulator. That Murata module is pin-compatible with LM7805 but the efficiency is a lot better as it's an SMPS.

The undervoltage cutoff prevents the output of 5 V SMPS module to fall down slowly by causing all kind of unwanted effects for the RPi. When the battery voltage reaches ~6 V, the relay disconnects the battery, which is connected again when the main 12 V supply returns. The coil of the relay is rated for 5 volts, so there must be a series resistor to limit the current. The value of the resistor sets the cutoff voltage. But why there's a capacitor in series with relays coil? I noticed that when the resistor value would be good in terms of cutoff voltage, the relay didn't switch on even at 12 volts. So the capacitor works as a high-pass filter which bypasses the 270R resistor and provides the "kick" to the relay coil when the main PSU turns on, but after that, the current passes only via the resistor, as the capacitor cannot pass DC current. After the transition, everything abouve 6 volts is enough to keep the relay switched on.

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* The picture above still with the RPi 1 model B.

30 MHz DIY Signal Generator (part 1)

A year ago I started this simple, microcontroller based DIY signal generator project which was capable for approximately 60_kHz sampling rate. However, that ended up to be quite boring becase I need test signals with higher bandwidth in my projects and there's already couple of similar MCU based signal generator projects on the internet. One idea leaded to another, and now I have dual channel, 90_MS/s, 30_MHz, 16-bit self-made arbitrary waveform generator (AWG). This won't be a complete step-by-step tutorial to build a similar one, but I'll try to explain enough details to give a good overview about the system. Also, the HW design and source code will be released, if someone wants to build one or even develop it further!

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The user interface (UI) is managed mostly by the Atmel ATmega microcontroller. The signal generation itself is done in FPGA, which controls also the LCD screen, and couple of other miscellaneous tasks. The digital signal goes from the FPGA to the high-performance DAC through the 2x 16-bit parallel bus, and the analog signal passes through the low-pass filter, to the high speed op-amp and output BNC connectors. The low-pass filter is needed to remove the unwanted alias frequencies.

Analog bandwidth__	30 MHz
Sample rate	90 MS/s
Channels	2 (+one internal modulation gen. / ch.)
Voltage swing	±8 V (±4 V to 50 Ω load)
Waveforms	Sine, Square, Triangle, Saw, Noise, Arbitrary__
Modulation	FM, AM, PM, Frequency sweep
Others	2.8" TFT touch screen, 3 rotary encoders, 10 MHz clock reference input (BNC), 2.5 PPM internal clock reference, USB remote control & SW update

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The first prototype was built by using a Lattice XO2 breakout board (~$25) and self-made ATmega328P development board. The DA converter was implemented with two 8-bit R2R ladder DACs. There was no LCD or control knobs attached yet, but all the commands was given via virtual COM port (FTDI FT232RL) to the MCU, which controls the FPGA via SPI bus.

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The next step added the LCD screen, control knobs and a frontend low-pass filter & amplifier (TI_THS3001) testing board to the prototype. The DAC component (AD9747) was not tested in this prototype, because the breakout board just for that would have been quite expensive due to the price of the DAC IC (~$30). Instead of physical breakout board, the DAC and the analog front-end was simulated by using Agilent Advanced Design System, and the simulations ended up to match pretty well to real-world performance tests.

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Download schematic:

PDF File (rev 1.0)

Proteus 7.7 Schematic file (rev 1.0)

As you may notice, there is no separate JTAG header for the FPGA, but there's a ISP header for flashing the ATmega MCU. After flashing the Chip45boot2 bootloader to the MCU, the ISP header isn't needed anymore. The Lattice XO2 breakout board has a FTDI FT2232H USB interface IC which is programmed to USB-JTAG bridge configuration. The same IC is utilized also in this design, but more of that, it has also a USB-UART bridge functionality, so both, the MCU and the FPGA can be programmed via the single USB connector. The USB-UART bridge also allows the debugging and remote control features for the MCU.

The schematic design of the front panel leds & knobs may look a bit strange at the beginning, but the idea was to use as few MCU pins as possible for controls. All of the signal wires are connected to ADC pins of the MCU, which switches these pins very rapidly between the input mode (reading the values of knobs) or the output mode (feeding current to the leds). Depending on the voltage value, the MCU deduces the position of the switches. More common way would have been to place an extra MCU to front panel PCB for knobs & leds and the communication would pass through I2C bus, but again, this solution was chosen to keep the design as simple as possible (and to try something new). As an after thought, it works quite ok after tuning the ADC treshold values, but in my next AWG project I'd use a separate MCU for controls.

As the XO2 FPGA has a quite nice clock manipulation features built-in, I used a temperature compensated crystal to provide an accurate clock reference for the signal generator core. However, if even that isn't accurate enough or the clock of the different devices needs to be synchronized, there's also a input BNC connector for the external 10 MHz clock reference, and the internal clock MUX of the FPGA selects the clock, that is selected from the UI menu.

To keep the design as simple as possible, there is no variable gain amplifier or even separate DAC for offset leveling. This compromises in the resolution of the AWG, since the smallest possible voltage step is 16_V_/_2^16_=_0.24_mV in any voltage or offset scale.

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The design was fitted into 50 x 100 mm 2-layer PCB, which was ordered from ITead Studio. There was only one bug, which was easily corrected with a piece of copper wire, so I decided not to order a new revision. Also the 5 V linear regulator, next to the power connector was replaced with SMPS module, which gives a lot better efficiency. The front-end op-amps still have linear regulator for the best possible noise performance. Power rails of the DAC are well filtered and regulated with LDO regulators, so the switching noise wouldn't pass to the signal outputs.

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The project box (Bahar BDA-40004-W200) was ordered from AliExpress, and modified for all the knobs, connectors, leds and the display. The layout was designed in 3D modelling software to make sure that everything fits nicely and the placement looks logical. Someone may like separate buttons, but I'm a fanboy of rotary encoders, so there's three of them! One controls the selected menu item, another the digit of the selected value and third knob controls the value of the selected menu item/digit.

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All the necessary pieces fitted into that project box. The main power supply is implemented by using IEC power connector with fuse holder & switch and 230 V to 2x15 V toroidal transformer. AC voltage goes to the PCB with fuses, MOVs (over-voltage protection), rectifier and bypass capacitors. +-21 VDC voltage passes to main PCB. The transformer with 2x 10-12 VAC outputs would be ideal, but I happened to have this 15 VAC model already, so it shall be good enough, althoug the output voltage is a bit high.

WARNING - Do not attempt construction of the power supply if you do not know how to wire mains equipment.

Well, that was quite broad description about the project. The Part 2 will include the PCB design files, source code, description about the program and performance measurements. Please, Tell me in the comments, what else you want to be included into Part 2!

Case mod: Fractal Design Define Nano S

Maybe not that much electronics related, but lately I've been doing this computer case mod project. Last years my computers have been in a ~105x40x25 cm closet with three Mobo & PSU mounts and four 120mm Noctua fans. Since my main computer is on 24/7 and I'm sleeping in the same room, the box is covered with sound absorbing material.

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That's a good solution as long as I don't have to move the computer. But having a LAN Parties twice a year have been a bit problematic, since I have to disassemble the computer to the cardboard boxes and assemble it again at the party place. Also, the TV stand / Computer closet may not come along to my new apartment in the future, so its a good time to invest into a proper computer case. I already have some mATX cases and a huge full tower, but neither of them wouldn't fit into my needs. I've always liked the look of Fractal Design Define series cases, but they are just too big to fit nicely in my apartment, or to be carried around even twice a year. Lately Fractal Design released this very good looking Define Nano S case, which would be perfect unless it wouldn't support ITX motherboards (170x170 mm) only.

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But wait, the Nano S case measures 203x330x400 mm so the ATX motherboard (305x244 mm) would fit into it after a bit of dremeling. Because the placement of the power supply was changed and the hole for motherboard's I/O connector had to be moved upwards a bit, the easiest way was to buy a 1.5 mm thick aluminium sheet and re-make whole back side of the case. The motherboard tray was taken off from a full ATX tower and cut down to fit into this chassis.

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Maybe the worst thing in Fractal Design's Define cases are the overly bright blue PWR & HDD leds, which were replaced with dimmer green & orange leds. These won't illuminate the entire room at night. I have a 250 GB SSD for the operating system and 3 TB HDD for everything else. There's also a custom made HDD mount, which have a ~3 mm thick sheet of rubber between the HDD mount and the case. This absorbs very efficiently the constant humming noise of the 7200 RPM HDD. The plate on top of is sized so that the whole thing can be mounted easily to the 120 mm fan slot.

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Although the new placement of the PSU won't allow big CPU coolers to be used, my Prolimatech Samuel 17 seems to manage with the Xeon E3-1230v3 CPU just fine (idle: 38°C, load: 65°C). There's a 120 mm CPU fan blowing towards the PSU, so the hot air goes out directly, through the PSU.

Until now, my backing up solution has been a bit fiddly, but now I decided to include a 3.5" hot-swap HDD mount in the back side of the case, so I can easily insert a HDD maybe twice a month, and put it somewhere else, so it's completely offline when not used.

There's also four timelapse videos about the modding process and the end result (end of the part 4):