A Simple TX for Experimentation 2N7002 project

OK, so back to practical stuff on the bench. I breadboarded a simple push-pull power-oscillator using a pair of 2N7000 MOSFETs, operating in the HF region. (I've attached an LTSpice model if you want to tinker with it, I used a 2N7002 model and asymmetric bias resistors to keep spice happy, the real circuit uses 2N7000s and starts just fine with 4k7 bias resistors on both sides.)

The "180p" capacitor tunes the tapped coil to the frequency of operation, select or make it variable as desired. In one implementation I put 6v8 zener diodes on the MOSFET gates to protect them against over-voltage destruction, at low powers this is not strictly needed, but at higher powers you may need them. Similarly the pair of 33 pF feedback capacitors need to be selected with the frequency of operation in mind. The MOSFET drain breakdown voltage is also important if you are trying to scale up this circuit. While simple, other approaches are probably better for high powers, the MOSFETs are spending a lot of time in their transition regions, dissipating a lot of power. A purely switching class-E approach is obviously better, but suffers from sensitivity of tuning to load impedance in my brief experiments with it (using an IRF510 device). (I've attached another spice model attempting to show the class-E TX approach, I started with values derived using my class-E power amplifier design calculator, as shown it is not perfectly tuned. The practical circuit tunes up nicely and is quite efficient > 70%.) The breadboard TX in the video above is a class-C version with weak capacitive coupling to the tank to optimise its Q. Yet another approach is half or H-bridges, these show great promise, perhaps driving a magnetically coupled "link" winding rather than the tank directly, allowing the tank to float, and facilitating easy variation of coupling to it to optimise its Q... A subject for more detailed investigation at a later date perhaps.

C-52 Evaluation Board 62256

A circuit diagram of the C-52 EVB is depicted in Figure 1. See at EA pinfirst, I put EA to Vcc configuring the 89C52 started internal code executionwhen reset. The first 8kB code space, 0000H-1FFFH is then be a monitorprogram, i.e., PAULMON2. A 32kB SRAM 62256 uses 15 lines address, A0-A14,while A15 of the 89C52 connects inverter gate, 74HC00, to CE pin. Thismakes the address space of the SRAM to be 8000H-FFFFH, i.e., A15 must be'1' to enable 62256. See OE pin, RD and PSEN are tied together with ANDgate made by two NAND gates. This makes the address space 8000H-FFFFH seenby 89C52 can be external code and data memory. Thus during in monitor modethat runs under PAULMON2, user may write hex code or download intel HEXfile to 62256. When jump from PAULMON2 to user program and run user program,this space is then be seen by 89C52 as a code space. Since P0 and P2 areused for connecting external RAM, left P1 and P3 for experimenting withreal world interfacing through input/output port. Nowadays there are manyperipheral chips that use serial protocol, say I2C, SPI. Thus only twoport is surely enough.

An Arduino Compatible - CP2102

Standard Arduino boards use FTDI’s FT232RL to interface with computer’s USB port. Since FT232R is just a USB to UART converter, it is possible to build an Arduino compatible USB interface using other USB to UART chips.

One such alternative is Silicon Labs‘ CP2102. I particularly like this USB to UART transceiver because very few extra components are required for it to work. As an added benefit, this chip is also cheaper than the ubiquitous FT232R. Of course, there are also a few trade offs. First of all, CP2102 does not provide a bit bang interface (the X3 pins on the Arduino board on the other hand can be used for bit bang operations, but the X3 pins are not soldered with header pins by default and thus for the average users no bit bang support should not be an issue). Secondly, CP2102 does not have the configurable general purpose I/O pins to drive the TX/RX LEDs. There are other minor differences as well (for instance the maximum transmission speed for FT232R is 3Mbps while CP2102 tops at 1Mbps. Both chips are more than adequate for the maximum 115,200 baud rate supported in Arduino environment), but they do not affect the performance in our application of interfacing with Arduino.

Here is the schematics for using CP2102 with ATmega328p (the circuit below is compatible with the Arduino IDE):

if you compare the above circuit with the official Arduino Duemilanove board you will see that the interfacing portions (RXD, TXD and TDR) are virtually identical.
Since CP2102 comes only in QFN-28 packaging, some people might find it slightly harder to deal with than TSSOP. Using the prototyping method I mentioned a few months back though, it is fairly straightforward to use the chip on a standard perf-board nevertheless. No special tools or stencils are needed. The following picture shows the USB to UART converter portion of the Arduino, which can be used to replace the FT232 break out board. I chose to break out the converter so that I could use it in other projects that require serial connections.



If you are running Linux, you do not need any third-party device drivers. All recent Linux kernels support CP210x via the usbserial kernel module. Once connected, you should be able to use dmesg and see these messages:

    [ 8333.572512] usb 8-2: new full speed USB device using uhci_hcd and address 3
    [ 8333.744748] usb 8-2: configuration #1 chosen from 1 choice
    [ 8333.785114] usbcore: registered new interface driver usbserial
    [ 8333.785161] USB Serial support registered for generic
    [ 8333.785221] usbcore: registered new interface driver usbserial_generic
    [ 8333.785222] usbserial: USB Serial Driver core
    [ 8333.792419] USB Serial support registered for cp210x
    [ 8333.792460] cp210x 8-2:1.0: cp210x converter detected
    [ 8333.920011] usb 8-2: reset full speed USB device using uhci_hcd and address 3
    [ 8334.076745] usb 8-2: cp210x converter now attached to ttyUSB0
    [ 8334.076760] usbcore: registered new interface driver cp210x
    [ 8334.076762] cp210x: v0.09:Silicon Labs CP210x RS232 serial adaptor driver

If you are running Windows, you will need to install the royalty-free driver from Silicon Labs directly.

Under Linux, CP210x shows up as a a ttyUSB device. You can use the Arduino IDE to program your ATmega328p’s just as you would with an official Arduino. Serial communication via the serial monitor works the same way as well. Like the official Arduino, the above circuit also automatically resets whenever you upload a program.
 

6 Vintage Nixie tubes MC34063

I recently purchased a set of 6 Vintage Nixie tubes from ebay. The gentleman was very kind to offer me a good deal. I was looking for at least 4 numeric tubes but he only had two. I ended up buying 2 x IN-4, 2 x IN-15A and 2 x IN-15B. Nixie tubes are pretty cool, these were used long before the invention of Led’s or LCD’s. Here is what Wikipedia has to say about them.

To test drive these I used the MC34063 DC-DC converter as it is cheap to use and that is all I had to hand. What we need is at least 170 volts to drive one of these. AN920-D Page 28 has a step up converter schematic which suites this application. The circuit redrawn looks like:

There are many circuits on the internet but most have not mentioned the series 47k resistor, this is very important as it limits the current drawn. Without this the tube will appear brighter but can burn out very soon. The transformer details can be found in the application note mentioned above.

A female D connector was sacrificed on a bench wise for the sake of obtaining the pins that would perfectly fit the electrodes of the Nixie tubes. I don’t think soldering directly to these tubes is a good idea. Connectors can be purchased from ebay but they are expensive.