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  1. AnalogKid

    why 74hc inputs are high by default?

    I looked at several HC datasheets, and don't see anything that indicates that the inputs have a default state. Given the extremely high input impedance, you might be seeing the effect of a charge on the input capacitance. In any case, it is .n.e.v.e.r. ok to leave logic circuit inputs...
  2. AnalogKid

    Old transistor stereo problem

    That could be a simple resistor network (but with several different resistor values, unlike standard DIP and SIP parts). Or, if the back side is not flat, it might include capacitors. For example, it could have all of the Rs and Cs for a phono/tape preamp or tone control circuit. Your...
  3. AnalogKid

    Help identifying resistor

    What are the length and diameter of the resistor body? This will help us guess at its power rating. Measure the max diameter at the end caps. Also, what is the hole spacing on the pc board? ak
  4. AnalogKid

    Capacitor?

    You need to know three things: 1. The current drawn by the control board. 2. The lowest power source voltage that can still run the control board. IOW, the control board input voltage range. 3. The amount of time the capacitor has to supply charge before the normal power source is...
  5. AnalogKid

    Automotive reverse polarity

    It's hard to say without seeing your schematic. ak
  6. AnalogKid

    New Format

    Both: (old) Win7 plus Firefox (new) Win10 plus Chrome Only: Report Quote Reply ak
  7. AnalogKid

    Detection of electronic device

    Why? Why do you think you are being targeted? Also, please provide a link to the type of device you are concerned about, so we have some context for the tech details. "Ultrasonic" is not enough. ak
  8. AnalogKid

    Buzzer Circuit

    Disagree. As an emitter follower, U3 in your schematic decreases the voltage available to the buzzer by over 10% in a 5 v circuit, and by over 18% in a 3.3 V circuit. Besides the decrease in volume - if the "buzzer" is in fact a piezo beeper, that could be below its specified operating voltage...
  9. AnalogKid

    Buzzer Circuit

    The circuit concept as shown will work, but not very well. This is because when the buzzer is on, its current comes through R2. A better way is to eliminate R2, connect one end of the buzzer directly to Vcc, and connect the other end to the U2 collector. In this way, the transistor acts as a...
  10. AnalogKid

    Opto Coupled Tunable Coil Circuit - Comments Please

    What does this mean?
  11. AnalogKid

    Opto Coupled Tunable Coil Circuit - Comments Please

    There is no need for impedance matching into a typical audio amplifier. Also, an optocoupler is a very bad choice for a buffer stage because it is so non-linear. I would say an opamp or even a simple emitter follower would be better, but we still so not know the max and min frequencies of...
  12. AnalogKid

    Opto Coupled Tunable Coil Circuit - Comments Please

    I understand that transformer. It is an audio output transformer, as in old 6-transistor radios, to step up the current into an 8 ohm speaker. What I don't understand is the optoisolator. Operating in its linear range, it will severely distort the audio signal (assuming it is not a...
  13. AnalogKid

    Opto Coupled Tunable Coil Circuit - Comments Please

    Many of us here have a pretty good handle on the concept of tuning. BUT - 1. Please explain what *you* mean by "tuning". Are you trying to select or extract or respond to only one frequency (or a narrow band of frequencies) from an audio signal that has many kilohertz of bandwidth? 2. What...
  14. AnalogKid

    how to create autoswitching circuit for 12v ups for router

    A relay will work if you add a large capacitor. Measure the current drawn by the router when it is operating normally. This will be less than the input current rating on its label, and determines the size of the capacitor. ak
  15. AnalogKid

    Simple Waveform Generator Help

    A more common approach from the 70's is to use an 8-bit shift register instead of the Johnson counter. ak
  16. AnalogKid

    Simple offset voltage question

    There are several online simulation sites that do not require any software to be downloaded and installed, but I've found them to have relatively low performance and limited capabilities; to the point that I no longer track them to see if they have improved. Linear Technology Spice (LTS) is the...
  17. AnalogKid

    What is the component

    know where this symbol came from. Please post a larger section of the schematic. Absent that, I vote with HK - a pcb-mount ferrite bead with an impedance of 120 ohms at some specified frequency (typically 100 MHz). ak
  18. AnalogKid

    Simple Waveform Generator Help

    First, the bad news. He shows the circuit running on a single 9 V battery. It will not. It needs both plus and minus supplies. This can be two 9 V batteries in series, with the common point as the circuit ground. Note that there is no ground symbol anywhere in the schematic. This tells us...
  19. AnalogKid

    Generating two complementary signals on 12 V supply on/off

    Ummm ... no.
  20. AnalogKid

    Generating two complementary signals on 12 V supply on/off

    Something like this. Signal flow is left-to-right and monotonic, as opposed to the 12 V ON signal reversing direction twice. Also, R1 is shown more clearly as the series leg of a 2-resistor attenuator. Note that this is clipped out of a standard A-size schematic page with a zone id border...
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