Tech Forum

The maximum reverse voltage across the diode will be 36 volts. A conservative rule of thumb is to double that, so choose a diode with a rating of at least 72 volts. The maximum current will be whatever your PWM controller delivers to the motor. When the PWM controller shuts off at the end of each cycle, the voltage across the motor inductance will instantaneously reverse polarity and begin to flow through the diode. Using the same rule of thumb, pick a diode that is rated for twice this current. You also need to be sure the diode can dissipate the power while keeping its junction temperature below TJ max (150 deg C is typical). The power dissipated is 1/2 L I^2 (the energy stored in the motor inductance) multiplied by the frequency of your PWM controller. Depending on this result, you may need to put the diode on a heatsink. Once you know how much power you need to dissipate, check the diode data sheet to get the thermal resistance Rth(j-c) in degrees C/W. Multiplying your calculated watts by Rth tells you how much the junction temperature will rise above its case temperature. There will also be a rise from the case to whatever the case is mounted on (probably a heat sink) but this will be small so you can probably ignore it if you follow the diode manufacturer’s recommendations on how to mount the diode. There will also be a temperature rise of the heat sink above the ambient air. Calculate this using the heat sink data sheet and your calculated power dissipation. Add all of the temperature rises to whatever your maximum ambient temperature is expected to be and if this is less than Tj max, you are OK thermally.

I did an experiment a few months ago using a serial port as a light meter that can log brightness data to a file. I posted it at www.cedarlakeinstruments.com/blog/?p=10. The software I wrote to display data isn't online, but I can email it to you if you'd like. Just contact me at www.cedarlakeinstruments.com. Here is the posting:

The lowest cost way I could think of was to avoid a microcontroller or other logic component and use whatever was already on a modern PC. My idea was to use a Cadmium Sulphide (CdS) photocell. CdS cells are used in many camera light meters and work by changing resistance in response to ambient light. By using the current through a photocell to charge a capacitor, we can tell the light level by measuring the time taken to charge. In days of old, we could have used a joystick port to do this, but those are long gone, now replaced by USB-connected joysticks.

On PCs without joystick ports, we could have done the same thing with the printer port. However, those are also a thing of the past.

So far, most PCs still come with serial ports, so we'll try those. The Clear To Send (CTS) and Data Terminal Ready (DTR) lines can be controlled from a Win32 or .NET application. There is some concern about the responsiveness of Windows, but as long as we avoid very short time constants, we can get usable data.

The concept is to build an RC circuit from a photocell and a 200 mF capacitor. The photocell is driven from the DTR line, and the junction of the capacitor and photocell is read by the CTS input. Here's the schematic; it's pretty simple:

The results are surprisingly useful. Discharging the circuit for two seconds and then charging it while polling the CTS line every five milliseconds shows a clear difference as the photocell is pointed at various areas of a lighted room.

All you need is a small solar cell mounted in the end of a "toilet paper" tube. Make sure the cell's face faces inward and you seal the tube end containing the cell so it's (more or less) "light-proof".  If you wish, paint the inside of the tube FLAT BLACK before mounting the solar cell (reduce any "internal reflections, etc." of the tube).

To use: Connect the solar cell's leads to the DMM (polarity is not important: all you want is the "absolute" display value) and set the DMM to the 2V scale.  Darken the room and turn on the CFL you want to "measure".  Wait a few minutes to allow the bulb to fully "warm up" and point the open end of the tube at the glowing CFL.  The voltage reading on the DMM display will give you the "relative brightness" indication you're looking for (i.e., "higher" voltage reading = brighter lamp). IMPORTANT: Make sure you use the same "measuring distance" (i.e., 1 inch) between the open end of the tube and each CFL you're measuring for best accuracy! Repeat for all CFL's you want to measure.

Hope this helps.

Chances are the magnets are OK on your stylus, the problem is most likely with the coil system in the cartridge body. The coils are not to be magnetized, their job is to generate a voltage from the mechanical movement of the magnets on the stylus shank when playing a record. My experience with the AKG P8 is that it does not age well, so rather than fight a losing battle I suggest replacing it with something new. I just purchased a Shure M97xe and I have no complaints.

The Wand board will run Android and is very similar to the raspberry pi. www.wandboard.org

While it is not exactly a wall mounted product, the Arduino ADK (Android Development Kit) is built to interface with the Android platform. I hear that the Raspberry Pi is close to being able to run Android and that might be an interesting alternative.

Look at www.tubesandmore.com They have both local oscillator coils and i.f. transformers for am tube radios listed. P-C208B, P-C70-OSC, and P-C78 look like the ones.

There was a thread on a Microchip forum that may help. www.microchip.com/forums/tm.aspx?high=&m=697725&mpage=1