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  1. AnalogKid

    How to build a 455khz oscillator for a signal generator..

    Without a crystal, how will you know the output frequency is accurate? ak
  2. AnalogKid

    High Power Exclusive OR Gate Help

    I think you misunderstand the term "exclusive" as used in combinatorial logic parts. Of the six basic logic gate configurations, the exclusive-or and exclusive-nor are the only ones that will *not* do what you want. If one input to an XOR or XNOR gate is a pulse train such as a PWM signal, the...
  3. AnalogKid

    Modify circuit

    The schematic does not show the 5.1 V zener diode across the IC pins 1 and 8. This suggests there might be other schematic errors. ak
  4. AnalogKid

    Voltage Divider vs Series Resistor

    Since a series resistor is by definition in series with something, it is one half of a voltage divider. One way to address this is to separate the universe into two types of electrical energies - signals and power. Signals are low power and don't deliver significant energy to wherever they are...
  5. AnalogKid

    repairing an amplifier

    According to the schematic the transformer has a center-tapped secondary. This might in fact be two independent secondaries. If so, the two red wires might be the junction of the two secondaries to form the center-tap. Yes, a diode can fail pseudo-short after heating up enough to fracture the...
  6. AnalogKid

    Problem with new microphone preamp.

    Not quite. What you did will reduce the gain of the first transistor stage, but it also will shift significantly the DC operating point, probably leading to asymmetrical distortion. Referring to the schematic in post 1, break the connection between the first transistor emitter and the 1K/100u...
  7. AnalogKid

    LED Signal Dummy load

    1. 300 mA at 110 Vac is a *lot* of relay coil power. 33 W. Unless the relay is the size of a baseball, this probably is not correct. One option is that the relay coil takes 300 mA at something like 12 V. 2. The LEDs in your drawing will act as half-wave rectifiers, altering the relay coil...
  8. AnalogKid

    Creating a variable wien bridge oscillator

    In the davenn circuit, each capacitor range is 10:1, so covering the span from 100 Hz to 18 kHz would take two full ranges and part of a third. If you want the full 100 Hz - 18 kHz span in a single adjustment range, that is a range of 180:1. I suppose that circuit values can be found to do...
  9. AnalogKid

    Creating a variable wien bridge oscillator

    What is the overall frequency range you want out of the circuit? Separate from that, what is the frequency range you want with one turn of the adjustment pot? ak
  10. AnalogKid

    Creating a variable wien bridge oscillator

    The LM386 is not a general purpose opamp, and will not work in typical opamp circuits. It has a minimum forward gain of 20 and a fixed output DC offset. It will not work as the Wein oscillator component. It will work as a separate speaker amplifier stage. ak
  11. AnalogKid

    pull down resistor for npn relay driver

    Yes, it is all about loading effects. A perfect voltage source (power supply, opamp output, PIC output pin, etc.) has a zero ohm output impedance. The output voltage does not change even if the load resistance varies by 1000, or 1,000,000 to 1. It is a perfect voltage source in series with a...
  12. AnalogKid

    pull down resistor for npn relay driver

    If the PIC output pin is active (either high or low) at all times, you do not need an additional pull-down resistor from the base to GND. If the PIC output pin might go to an input mode or tri-state (high output impedance), then a pull-down resistor will decrease the circuit's sensitivity to...
  13. AnalogKid

    Differential Configuration for a Floating Signal

    The article explains in three places why it is necessary. Which parts do you not understand? ak
  14. AnalogKid

    Question about current

    Clamp on amp meters work well with AC - *real* AC. Continuous sine wave, not very distorted, symmetrical about ground, a firm mathematical relationship between the peak and RMS values, etc. The pulses to a stepper motor are none of those things, so the meter will indicate accurately only that...
  15. AnalogKid

    What is this

    Layout was done manually in PADS by someone with relatively little experience. Poor power routing, very poor ground management, no copper pours, etc. Inductors are interesting... ak
  16. AnalogKid

    Need help with amplifying a light

    The device you have is not the right one for your application. While lotsa people on this forum can guide you through designing add-on stuff to compensate, another approach is to fix the problem at the source. Have you looked into the Telephone Signaler 082 from the same company...
  17. AnalogKid

    What is this

    Could be a serial port expander or multi-modem expander. On the rear panel below the IEC AC power input connector is what looks like a manufacturer's ID label. Voltage, Current, Model Number, etc. What does it say? Or please provide a photo. ak
  18. AnalogKid

    Need help with amplifying a light

    Is the goal to have the light flash when the phone rings? ak
  19. AnalogKid

    9 Volt Phantom Power Circuit

    As a phantom power inserter, the two 10K resistors are a bit large; but the circuit looks fine to me. If the mic is a standard 2-pin electret element, there needs to be a bit more stuff at the mic end, but that varies based on the characteristics of the mic. ak
  20. AnalogKid

    Thermostat with lm35: help finding out the error

    As noted, a 393 has an open collector output that can only sink current, not source it. 1. IF (big if) the relay coil current is less than 50% of the 393 max rated output current, then it is safe to have the 393 drive the relay coil directly (with an added suppression diode across the coil...
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