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  1. AnalogKid

    Discharge time of 2 capacitors in parallel

    Since your question are not clear, can you post your schematic? It will save a lot of time. ak
  2. AnalogKid

    AD590

    Depends - what are you trying to sense the temperature of? And what is the temperature range? And what is the application? Is this for a high-low comparison, or linear measurement like a thermometer? Whenever possible, such as with a simple comparator or with the A/D input to a...
  3. AnalogKid

    Voltage multiplier

    Most TENS units have a small switching power supply. This can be a non-isolated boost circuit of a fully isolated flyback, but either way there will be magnetics involved and that means some bulk. Boosting 3.7 V to 100 V is a ratio of 27:1. At 90% efficiency (a very high value), that becomes...
  4. AnalogKid

    Differential amplifiers - why amplify the difference?

    Are you asking about a differential transistor stage, sometimes called a long-tailed pair, or the differential inputs to an opamp IC? The overall discussion is the same, but the explanation is different depending on the starting point. ak
  5. AnalogKid

    Multiple astable 555's

    Kids...
  6. AnalogKid

    Multiple astable 555's

    Maybe, but not well. Your single string of pots is connected to four other resistors and four timing capacitors, none of which are adjustable, or will track with temperature, or are close enough in their relative values not to be noticeably out of tune. This will introduce low frequency beat...
  7. AnalogKid

    Pocket Laptop (I'm new here...)

    Secret things...
  8. AnalogKid

    Help Changing A Switch

    Safer to leave the switch in place and solder the jumper between the pins on the bottom surface of the board. ak
  9. AnalogKid

    Help Changing A Switch

    That is a tact switch, SPST momentary. On the bottom side of the board, the two larger pins are the frame and have no electrical function. The two smaller pins are the switch contacts. Jumpering them together is the same as holding down the switch continuously. If you device doesn't mind...
  10. AnalogKid

    5000W high power mosfet

    Power dissipation in the transistors and power dissipation in the load are two very different things. Power dissipation in each transistor, assuming a nice square drive signal that can handle the transistor's gate charge, is it's on resistance (Rdson) times the square of the current through it...
  11. AnalogKid

    Hack Basement Watchdog Water Alarm

    Not an actual problem. Remember that when the sensor is not tripped, the chip has no GND connection because the FET is like an open switch (or a very high value resistor). The chip is essentially off, and floating above ground. In fact, it is the input impedance of the meter that is making...
  12. AnalogKid

    Hack Basement Watchdog Water Alarm

    What is your relay manufacturer/part number? Do you know its coil voltage rating, resistance, etc?
  13. AnalogKid

    noisy differential signal

    Could be scope probe compensation, too long a ground clip lead, or some other inductive high-freq effect. Does the scope have a calibration waveform output? If so, how does it look? ak
  14. AnalogKid

    Hack Basement Watchdog Water Alarm

    Note that the 2N7000 is installed backwards according to the legend. This is not a problem because the legend is wrong. Also, the FET is not doing a "normal" thing. My snap judgement in post #7 is incorrect. The FET is installed between the 4069's Vss pin and circuit GND. It does not gate...
  15. AnalogKid

    Hack Basement Watchdog Water Alarm

    When there is a water event, does the alarm beep intermittently or make a continuous tone? If it beeps, then: The TI chip is a hex inverter. Multiple gates are connected in parallel to increase the current drive capability into the piezo beeper. This forma a crystal oscillator, with the...
  16. AnalogKid

    Parasit effect at the input port of a microcontroller

    That is a start, but to be sure we need a schematic of the circuit generating the normal signal into the port. For almost any electronic device or circuit, there is a current surge, called inrush current, when it is attached to a power source. Depending on how fast the surge starts, it can...
  17. AnalogKid

    Black patch (spot) in center of red led light

    Might be the point where the bonding wire is attached. https://en.wikipedia.org/wiki/Light-emitting_diode#/media/File:LED,_5mm,_green_%28en%29.svg ak
  18. AnalogKid

    Help me identifying the multiple leads coming off this pcb

    A very long time ago I inherited an interesting collection of electronics from a mentor, and a versions of that amp were in it. I still have 1 or 2, waiting for the right project. ak
  19. AnalogKid

    Help me identifying the multiple leads coming off this pcb

    Here are three variations on the theme. The last one, "transistor-topics", is the closest to what you have, with PNP transistors and a "positive ground". ak
  20. AnalogKid

    Help me identifying the multiple leads coming off this pcb

    The amp you have is from back when transistor circuits were patterned after tube circuits. There is an input amplifier stage (not on the google schematic), a driver for the interstage transformer (the small one), and a split push-pull class B output stage driving an output transformer (the...
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