• After building a flyback high voltage supply, which I will describe in a later post, I was experimenting with putting the electrodes into thin glass tubes. This produced much longer arcs due to the concentration of the ionized air.

    To get an even better result, I tried evacuating the tubes with a syringe.  To seal one end of the tube I placed a thick wire in one end and put a drop of hot glue around it.  Over the other end I put a piece of PVC hose and pierced the wire through it into the tube. The entryhole was also sealed with hot glue. To ensure a tight seal some air is sucked out of the tube while the hot glue is still liquid.  This way the glue fills the holes.

    Tube with sealed Electrodes

    Tube with sealed-in Electrodes

    As the glass tubes only have a volume of about 5ml and the syringe 65ml, one pull reduces the pressure inside the tube from 1bar to about 0.05bar. (p*V = const.)

    When the voltage (between 20kV and 30kV) is applied to the electrodes, a continuous channel of violet (the photo does not reproduce the color correctly)  nitrogen plasma is formed inside the tube. The plasma quickly heats the glass and the hot glue, so don’t apply the voltage too long or the seal will break. Magnetic fields will deflect the plasma channel.

    Plasma Channel deflected by magnetic Field

    Plasma Channel deflected by magnetic Field

    These small tubes quickly got boring, so I decided to make a bigger version with a Ø30mm L=100mm plexiglas tube. To seal of the ends, I turned caps on the lathe. The caps were then fitted with a hole for the electrodes and a hose connection. The holes around the electrodes are again sealed with hot glue. To get the caps airtight, o-rings were used.

    To get a low enough pressure inside the tube, the syringe has to be pulled several times. Between each pull, the tube has to be closed. This is achieved with an electric valve.

    Cap with O-Ring

    Cap with O-Ring

    Tube with Cap and Hose

    Tube with Cap and Hose

    When the pressure inside the tube is low enough and the voltage high enough, a glowing channel of plasma will form. Due to the heat it generates in the residual air, it will bulge upwards. This will heat the tube and slowly melt the plexiglas. The plasma stream can be deflected with strong magnets or by influencing the electrostatic field around the tube, i.e. by placing your fingers near or on the tube.

    Plasma Channel in Discharge Tube

    Plasma Channel in Discharge Tube

    Plasma Channel in Discharge Tube

    Plasma Channel in Discharge Tube

    With one of the smaller tubes I put a drop of ethanol (C2H5OH) inside the syringe before pulling the air out. This produced a thin ethanol vapor inside the tube and caused the plasma to glow blueish-white instead of violet.

    Blueish Glow from Ethanol Atmosphere inside the Tube

    Blueish Glow from Ethanol Atmosphere inside the Tube

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  • Due to university and work it has been a while since the last post. But I just completed a little project that’s worth posting.

    32kHz Oscillator

    32kHz Oscillator

    Several old 27C256 EPROMS were lying around unused. So I thought about a purpose for them. As I also had some 8×8 LED matrices, a little animated display came to mind.
    With each frame consisting of 8×8 pixels the 32kByte EPROM can hold 4096 frames. Each byte holds one line of the display, eight bytes one frame.

    The lower 3 addressbits of the EPROM have to by switched synchronously with the corresponding line on the display. This is achieved by wiring them to a 3-to-8-decoder (74*238) which in turn switches the lines. As up to eight LEDs can light up at once per line. To handle the current an ULN2308A darlington driver is used.

    The columns are directly controlled by the data-output of the EPROM. To drive the LEDs a 2N2907 transistor is used.

    The clock is generated by a crystal oscillator circuit consisting of a 32768 Hz crystal and an inverter gate.

    The 32kHz squarewave from the oscillator is then divided by a 12-stage ripple counter (4040). The seventh to ninth stage are used for the line-addressing and are wired to the A0-A2 inputs of the EPROM and to the A,B,C inputs of the 74*238.

    The next three stages are connected to a DIP-switch. The output of the switch leads to the clock input of another 4040. This way the frame-rate is selectable from 32fps, 16fps and 8fps.
    Only fourteen addresspins of the EPROM are used, the fifteenth can be set to high or low via a jumper.

    Logic Section of the Display Schematic

    Logic Section of the Display Schematic

    I wanted the whole circuit to fit under the LED matrix, but unfortunately the EPROM is slightly bigger, so the circuitboard protrudes about 3mm on the left and the right.
    Apart from the LEDmatrix and the EPROM only SMD components were used on the two-sided PCB. The layout is rather dense and 0.3mm vias were used. Originally i wanted to etch the PCB myself, but quickly gave up that plan when the opportunity to let it manufacture for free together with other boards arose.

    EPROM Display PCB

    EPROM Display PCB

    EPROM Display PCB with EPROM

    EPROM Display PCB with EPROM

    EPROM Display LED Matrix

    EPROM Display LED Matrix

    In the pictures above you can the bottom side of the populated PCB with one of the 4040’s, the 74*238, the crystal (the little golden thing), the DIP-switch and the EPROM-socket. In the next picture, the EPROM is inserted and on the last picture the LED-Matrix is lighted with some random data that was stored in one of the old EPROMs.

    You can download the schematic and the board-layout (for EAGLE) here.

    EPROM Display Program

    EPROM Display Program

    To easily generate data for the display, a small program was written in Delphi. You can draw each image on the 8×8 field and save the sequence of images to a binary file that can be directly programmed into the EPROM.

    The source-code can be downloaded here.

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