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  1. K

    LED power source

    You'll need a power supply with enough current to drive 300 LEDs plus the supporting circuitry. At 20 mA per LED, if all 300 need to be lit at once that's 0.02 * 300 = 6 amps, plus whatever the Raspberry Pi and the ICs driving the LED draw. You should probably use an 8-10 amp power supply to...
  2. K

    LED power source

    To light 300 LEDs with individual control, you'd need separate outputs from the R.Pi to each LED. Of course, no microcontroller has 300 I/O lines, so another solution is needed. One way is to multiplex the LEDs. If you take 300 LEDs, you can arrange them into an 17x18 matrix (electrically...
  3. K

    How to fade cycle PC powerled when it's in sleep mode

    The concept is sound, but to make the LED fade on and off you'll need a much larger capacitor. I would eliminate R4 as well, as R1 will control the charging rate (fade-in) and R2 will control the discharging rate as well as limiting LED current. Also, depending on how the mobo drives the LED...
  4. K

    LED power source

    The Pi won't be able to power that many LEDs by itself, you'll need driver circuits of some sort (transistors etc.) and a power supply. Also, you say 200-300 LEDs. All individually controllable, or in groups? What's the application? Is it a matrix of LEDs or a single, long row of them?
  5. K

    Is it possible to modify a Technics turntable for remote control with donor parts from another?

    Is the turntable fully automatic? Meaning that, with the push of a button, it will turn on, move the tonearm to the start of the record, lower it, play, then lift and return the arm to the park position at the end of the record? If it is, it should be possible to retrofit a remote control...
  6. K

    Trying to figure out what this is..

    X3.. we need component markings or a schematic. The little black things look like transistors, but what kind? PNP, NPN, N-channel or P-channel FETs, etc. No way to know without markings. There's also an 8-pin IC (get the number off that if you can) and a diode. Some better quality pictures...
  7. K

    Old RS-232 compatibility with new computers?

    RS232 ports in the past (when they were integrated on motherboards or on expansion cards) would use the 12V rails to provide the output signals at +/- 12V. USB-Serial adapters use MAX232 or similar ICs that double and invert the 5V power to derive (approximately) +/- 10V, though in reality it's...
  8. K

    Adders

    The simplest implementation of a half-adder is an XOR gate and an AND gate. It's possible to make an XOR gate out of 4 NAND gates or 5 NOR gates, or a NAND, OR and AND gate (according to Wikipedia). If you only have AND and OR gates, you'll need inverters to make them into NAND or NOR gates.
  9. K

    Have you ever gotten "lucky?" (when repairing electronics)

    I know, get your minds out of the gutter... :) When troubleshooting or repairing electronics, have you ever "lucked out" and found the faulty component out of sheer luck (or intuition) relatively quickly? I did last weekend. I have a Zenith Pro895x CRT projector which I'm fixing up for a...
  10. K

    Crosley Director model phono/cd

    That's not wax, it's hot glue. Probably put there to hold the heatsink in place before it was soldered down and bolted to the transistor. Or maybe just to hold the heatsink in place entirely, if it doesn't have solder tabs. Looks like that resistor got hot and melted the glue around it.
  11. K

    Simple Amplifier

    In your schematic, the MOSFET's source is on the bottom, going to ground, the drain is on top where it says 4.5V, and the gate is on the left, going to the pickup. You'll have to look at the datasheet for the actual transistor and its package you're using to get the correct pinout. Some are...
  12. K

    Rest in Peace, Kris

    I haven't been over here much lately but I am saddened to hear of Kris's passing. I always enjoyed his posts and he always made a point to make new members feel welcome here. A few years ago I created a musical piece I always like to share in these circumstances. I call it White Light, an MP3...
  13. K

    Damaged dc power jack traces

    Sometimes it's easier, and more reliable, to solder a jumper wire to bypass a damaged trace. Find a point on the board that the damaged trace connects to, and solder a wire between the power jack and that point.
  14. K

    Stumped... can someone identify this surface mount component?

    Can you post a decent resolution pic of the entire board? Or at least the area of the board where the mystery part is? What is the board from? What does it do?
  15. K

    Comparators with diodes on the output why?

    As davenn explained, an op-amp or comparator IC can be designed to source current (PNP transistor output, so a positive current flows from the V+ rail to the output), or sink current (NPN transistor output, so a current flows from the output to GND). An op-amp IC typically has both the PNP and...
  16. K

    Finally got me a 'scope - Have some questions

    Noise can vary from location to location depending on where live electrical wires are relative to your scope and body, presence of electronics such as compact fluorescent lights or computers, etc. In some spots two electrical lines could augment 60Hz noise, and in other spots cancel one another...
  17. K

    Comparators with diodes on the output why?

    The diodes are needed since 741s can both source and sink current, meaning if one is "high" and one is "low" current would flow between them in a nearly short circuit. If comparators with open-collector outputs like the LM339 were used, the diodes wouldn't be needed, since these devices only...
  18. K

    Finally got me a 'scope - Have some questions

    The noise when you touch the calibration trimmer with your metal screwdriver is just RFI/EMI your body is transmitting into the probe via the metal screwdriver. A lot of it is 60Hz from the electrical wires in your room. Higher frequencies could be from electronic devices like TVs, computers...
  19. K

    Hot air soldering experience

    The main use of hot air soldering is for soldering or unsoldering surface-mounted parts. A regular iron is easier to use with through-hole components. If your hot air station has a temperature control, you could set it to a lower temperature to reflow wax.
  20. K

    No Color. Black and White Picture

    Did you check the Color control? If it's turned all the way down (or not working) it could lead to a B&W picture.
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