Category Archives: Electronics

Official Raspberry Pi 7″ Touch Screen Display – Anytime now.

The long awaited official Raspberry Pi 7″ Touch screen display is to be released any time now. I am very excited! Distributor Farnell / Element14 has been preparing marketing material for it.

Highlights are:

  • 7″ Touch screen display
  • 800 x 480 pixels resolution
  • 10 finger capacitive touch
  • Connects to the Raspberry Pi DSI port via an adapter board.

Official Raspberry Pi Display
Here is the datasheet from Element14. The product page is not quite up yet.

Raspberry Pi B+ Power Protection Circuit

I just bought the newly relased Raspberry Pi (RPI) model B+. In comparing the schematic with the older model B, I noticed they have updated the power protection circuit.

The circuit functionality is mostly obvious. There is polyswitch F1, a self-resetting fuse which protects the RPI from overcurrent condition. There is the Transient Voltage Suppressor (TVS) diode D5 which protects the RPI against static discharge and possibly overvoltage condition. However, the function of mosfet Q3 together with the matched PNP transistors U14 is not so obvious. A mosfet is usually used as a reverse polarity protection, but such protection is not needed for the RPI because of the polarized USB connector used.

“RPI B+ Power Protection Circuit”

“Reverse polarity protection with mosfet”

I wasn’t able to work out the PNP pair functionality by searching the Internet. Adafruit also noticed this transistor pair but did not offer useful explanation.

It turns out that the matched PNP transistors are used as voltage comparator. When Vin > Vout, the PNPs are biased in such a way that the mosfet is switched on. When Vin < Vout, the mosfet is switched off. In another word, the circuit acts as a diode letting current to flow only in the forward direction. The advantage of this "diode" over silicon or even schottky diode is the much lower voltage drop. This diode functionality is important because when the RPI is connected to a computer USB port, the RPI must never source current upstream into VBus. Without this PNP circuit, if there is voltage present on Vout, the mosfet will be turned on. Voltage on Vout will be present if the RPI is powered through the GPIO expansion port. If this applied voltage happens to be higher than VBus, then current will flow into VBus, potentially damaging the computer. The way the circuit works is this: if Vin=Vout, lets say 5.0V, the PNP transistors are biased in a current mirror configuration. Vb of transistors U14 will be 5-0.6=4.4V, the mirroring configuration means that Vg will also be at 4.4V. Vgs is 4.4-5.0=-0.6V. The mosfet will be in the off state (almost turned on). If Vout is slightly lower than Vin, lets say 4.9V, U14a will still be biased as before, so Vb is still 4.4. Vbe of U14b will now be 4.4-4.9=-0.5, U14b will be off and Vg will be 0V since no current flows through R3. Vgs will be 0-4.9 = -4.9V, and the mosfet will be fully switched on. If Vout is slightly higher that Vin, lets say 5.1V. U14b will be turned on via the biasing resistor R2. Transistor U14b will be in saturation mode and Vg will be 0.2 V lower than Vout, Vgs = -0.2, and the mosfet will be off. Conclusion
The protection circuit in the RPI model B+ functions as an ideal power diode.

bladeRF enclosure – made from laser cut acrylic

I made an enclosure for my bladeRF board from laser-cut acrylic (also known as “Perspex”). This is the first time I experimented with custom laser cutting. The laser cutting service is provided by SeeedStudio. I’m very pleased with the result, see photos below.

bladeRF acrylic enclosure

bladeRF acrylic enclosure

The acrylic used is 2.55 mm thick. The PCB is mounted to the bottom plate using eight M2.5 hex spacers. The top plate and the bottom plate are held together by four M4 hex posts.

Laser-cut acrylic is good for making a prototype enclosure. It is quick and easy to design. It requires no effort in producing them. It also has a nice distinctive look.

ViewRF software – RTL-SDR Spectrum Analyzer for BeagleBone Black

In my previous blog entry I demonstrated my RTL-SDR based spectrum analyzer running on the BeagleBone Black (BBB) with a 7-inch touchscreen LCD.

In this entry I’ve provided instructions on how you can run this software on your own BeagleBone. I’ve released the software – ViewRF – under the GPL license. You can download the source code hosted on GitHub.

The hardware you need (other than the BeagleBone itself) is the RTL2832U based DVB-T (digital TV) USB dongle. See RTL-SDR wiki for a list of supported hardware. I’ve tested the software with dongles based on the Elonics E4000 tuner and also the Rafael Micro R820T tuner. These dongles are available from various online shops.

The software has been designed to run on a 800×480 resolution screen, ie the resolution of the 7 inch LCD cape.


  1. Download and unzip (using 7-zip) this microSD image:
    Angstrom-Cloud9-IDE-GNOME-eglibc-ipk-v2012.12-beaglebone-2013.06.20.img.xz. Then copy this unzipped image using Win32 Disk Imager to a microSD card.
  2. Insert the microSD card to your BBB and power it on.
  3. After you are logged in, open a terminal, then disable X Windows autostart using the command:
    systemctl disable gdm
  4. Reboot:
    /sbin/shutdown -r now
  5. At the console, login as root, then calibrate the touchscreen
  6. Install these Angstrom packages:
    opkg update
    opkg install qt4-embedded --force-depends
    opkg install libqtsvge-dev --force-depends
    opkg install libqtguie-dev --force-depends
    opkg install qt4-embedded-dev --force-depends
  7. Set the following environment variables:
    export OE_QMAKE_CXX=/usr/bin/gcc
    export OE_QMAKE_CXX=/usr/bin/g++
    export OE_QMAKE_LINK=/usr/bin/gcc
    export OE_QMAKE_INCDIR_QT=/usr/include/qtopia
    export OE_QMAKE_MOC=/usr/bin/moc
  8. Download, compile and install QWT:
    tar -xjvf qwt-6.1.0.tar.bz2
    cd qwt-6.1.0

    Edit qwtconfig.pri, remove QwtOpenGL, QwtDesigner
    Edit ./src/, change TARGET to:
    TARGET = $$qwtLibraryTarget(qwt$$QT_LIBINFIX)
    Continue with the compilation process:
    make install
  9. Download, compile and install RTL-SDR library:
    git clone git://
    cd rtl-sdr
    autoreconf -i
    ./configure --enable-driver-detach
    make install
    make install-udev-rules
  10. Edit /etc/profile, add the line:
    export LD_LIBRARY_PATH=/usr/local/lib:/usr/local/qwt-6.1.0/lib
  11. Edit /etc/profile.d/, if using touchscreen add the line:
    export QWS_MOUSE_PROTO=LinuxInput:/dev/input/touchscreen0
    or if using a mouse, add the line:
    export QWS_MOUSE_PROTO=MouseMan:/dev/psaux
    Logout, then log back in again.
  12. Download and run ViewRF software
    git clone git://
    cd ViewRF
  13. To run the pre-built binary (don’t run the git checkout command if you are building from source)
    git checkout 9b4d425331
    ./ViewRF -qws

I hope this software is useful to you. Let me know what you do with it. Feel free to donate :), link on the sidebar.

Demonstration of my Spectrum Analyzer project

A demonstration of my Spectrum Analyzer project built using the Beaglebone Black and a USB DVB-T digital television receiver. The receiver is based on Realtek RTL2832U accessed using RTL-SDR library. The demonstration shows the RF spectrum of various sources.

If you are interested in this project, I’ll be releasing the application and source code in GitHub so you can build your own. Probably in a month or two, or sooner, or later.
You can now run this software – ViewRF – on your own hardware. See

Electronics pliers and side cutters


Small precision pliers and side cutters are essential for electronics work. I’ve owned and used the pliers and cutter pictured above for a few months now. I found them to be really good.

The cutter is a Knipex 77 32 115 ESD, and the pliers are CK Tools 3772-1D-120. Both feel really good in the hand, have nice spring, top quality construction, have ESD handles and they both use the highly durable box joint construction.

I chose the version of pliers with serrated jaws as they give better grip comapred to the smooth one.

The cutter is rated for cutting copper wire of diameter 0.3 to 1.3 mm.

BeagleBone Black (BBB)

BeagleBone Black

This is my new BeagleBone Black (BBB). It is an ultra low cost ($45) ARM computer that readily runs Operating Systems such as Linux or Android. It is based on Texas Instrument Sitara Cortex A8 1 GHz System-on-Chip. I won’t attempt to describe the board in detail as there are already many good descriptions on the Internet.

This board is often compared to the Raspberry PI (RPI). This site has good comparison: . The two boards are similar in many way, but there are differences. The RPI is better suited for video playback, and the BBB is better suited for hardware development or hacking.

This BBB has much potential. With a microcontroller (eg. PIC or AVR or Cortex Mx), it is a lot of work to implement high-level functions such as tcp/ip, graphics, file-systems, threading, webservers, etc. There are libraries available, but are usually costly and feature limited. High-level libraries are available for the BBB for free as part of the Linux operating system.

With computers running desktop operating systems, interface to hardware peripheral is usually limited to USB or RS232. The latency of such interface is usually high and not guaranteed. I’m hopeful that the BBB can overcome this limitation. The Sitara System-on-Chip contains two Programmable Real-time Units (PRU). A PRU is a separate 200 MHz CPU core that is meant to implement “soft” peripherals, peripherals with tight real-time constraint.

Frequency calibration of my Rubidium Frequency Standard

In my last entry I compared the frequencies of my two 10 MHz frequency references. One is from FEI FE-5680A Rubidium Frequency Standard (RFS) and the other one is from Trimble Thunderbolt GPS Disciplined Oscillator (GPSDO).

The frequency accuracy of RFS drifts over time. The manufacturer specifies drift of less than 2×10-9 per year, which for 10 MHz reference is 0.02 Hz . I don’t understand the drift mechanism, but in any case the frequency offset of this RFS needs to be measured and corrected. (Why? because I could.)

The frequency accuracy of GPSDO, on the other hand, does not drift with time. This is because the guys at the U.S. Naval Observatory (USNO) continuously monitor and correct the clocks in their GPS satellites to track UTC(USNO). The UTC(USNO) in turns tracks the Coordinated Universal Time (UTC) time maintained by BIPM in Paris.

In my last entry, the way I compared the two frequencies was by comparing the phase of the two waveforms on an oscilloscope. In that short time comparison, the two frequencies were virtually identical. The two waveforms were virtually stationary with respect to each other. In order to measure the difference between these two nearly identical frequencies, I need to track the phase difference over a much longer period of time. Here is the block diagram of the circuit that helps me do this:


For convenience, both 10 MHz references were first converted to 1 pulse-per-second (PPS) signals. One PPS drives the “start” signal of a Time Interval Counter (TIC) and the other PPS drives the “stop” signal of the same TIC. The TIC measures the time interval between the “start” and the “stop” signal. The TIC is build using a digital counter clocked at 50 MHz which gives 20 ns resolution. The interval values were then sent to a PC for data-logging. And here is the result:

The top graph shows the time interval measured. The horizontal axis is the elapsed time. Each sample point correspond to 1 second. Over a period of 8.3 hours, the PPS signal of the RFS drifted by 320 ns. This drift corresponds to an error of 1.1×10-11 or 0.00011 Hz.

The bottom graph plots the self-monitoring performance of the GPSDO. It shows that the GPSDO is functioning correctly during the duration of the experiment.

The frequency error of 0.00011 Hz is pretty impressive. This RFS was calibrated about a month ago. This hasn’t always been the case. When I first acquire the RFS, its frequency error was 0.072 Hz. It wasn’t bad, it was good, but it is impressive now 🙂

Next: improve the resolution of my counter. Short of buying Stanford Research SR620 counter, I’m planning to add a linear interpolator to my digital counter. Basically it will use the rise time of an RC circuit to measure time in-between the counter oscillator “ticks”.

Next: Understand time stablitity measurement, also known as Allan Deviation.

Fluke 17B Review – Part 2

This is the second part of my Fluke 17B multimeter review. If you haven’t seen the first part, read it here.

Fluke 17B

A Bit of Background
The two things I’ll consider in this review are safety and measurement confidence. Other aspects of the meter such as features and accuracy can be easily compared by reading the meter’s specification. Safety refers to how safely the meter fails. This can be due to an overvoltage or due to putting the meter in the wrong mode (eg measuring Voltage in the Current range). Measurement confidence refers to how much you can trust the meter reading.
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