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  1. hevans1944

    Testing Salvaged Motors

    Congratulations, Darren, on a job well done! No, signals are always represented by the presence of a voltage (or a current) between two points in space. Often one of these two points is called "ground" or "signal common" or "analog common" or some such name and is shared with multiple signal...
  2. hevans1944

    Testing Salvaged Motors

    It sounds like (literally) that you have discovered the wonderful, mysterious, tear-your-hair-out, world of ground loops. Entire volumes have been written and published on how ground loops are formed and methods to defeat them. As you have discovered, sometimes removing a "ground" connection...
  3. hevans1944

    Testing Salvaged Motors

    You are still placing the motor winding in series with the emitter of the TIP120 BJT. Try placing the motor winding between the collector and the +12 V DC supply. Because you are driving the motor with square waves, you need a diode in parallel with the motor winding to suppress the back emf...
  4. hevans1944

    Determining The Accuracy of Amp Meter

    Not true. The voltage of the power supply is divided across the resistor and the resistance of the ammeter connected in series. You don't know the resistance of the ammeter, but you do know that whatever current is flowing in the ammeter (as a result of the power supply voltage) is also the same...
  5. hevans1944

    Determining The Accuracy of Amp Meter

    Why two resistors? Just use one resistor with a variable power supply, placing the ammeter in series with the resistor and power supply. Measure the resistance of the resistor. Turn on and Increase the power supply output until the ammeter reads full scale. Measure the voltage drop across the...
  6. hevans1944

    Booting Raspberry Pi 3 B+ from USB Drive

    It would appear that everything you need to know can be found at a link on this page, the result of a Google search using the title of your post.
  7. hevans1944

    How do I build a underwater Infrasonic (10 -30 Hz) projector?

    There is nothing magical about a magnetostrictive rod. A nickel or a nickel alloy rod, or even a soft-iron rod should work. There are also rare-earth alloys, Terfenol-D for example, that work better, but the Chinese have pretty much cornered the market on rare-earth elements. Prices are...
  8. hevans1944

    Testing Salvaged Motors

    Sounds like you have the concept nailed. The PWM waveform, after low-pass filtering, should resemble the CV controlled synth output. Not sure what effect you are trying to create, but make sure the PWM frequency is much larger than the highest synth frequency. The motor response will be limited...
  9. hevans1944

    Testing Salvaged Motors

    Yes, 50 mA using the digital muiltimeter on its 10A range. ALL the specifications are important, not just the maximum collector current, or in the case of MOSFETs the maximum drain current. You should NOT be using MOSFETs or BJTs in their linear mode with motors. Either type work well as...
  10. hevans1944

    Fence flasher

    As per usual the OP is trying to tell us how he "solved" a problem but now needs to "fix" the solution. The OP should instead tell us WTF he is trying to DO! For example: I have this really powerful electric fence protecting my kraal from both four-legged and two-legged predators. I want to...
  11. hevans1944

    Question about voltage divider

    The four voltage-divider resistors are not ALL connected to ground. Only R4 is connected to ground. The other three resistors are in series with R4 and each other, hence the need to calculate their values based on the current through each of them and the voltage drop across each resistor. Go...
  12. hevans1944

    MPPT charge controller

    True "really soon now" if not already. But low-earth orbital velocity is around 28,000 km/h or nearly mach 23. That's why the railgun "barrel" needs a vacuum inside while the sabot carrying the picosatellite is accelerated. Probably will need considerable length too, compared to current military...
  13. hevans1944

    MPPT charge controller

    There are problems still to be solved in using railguns to orbit anything. Atmospheric friction is the big elephant in the tent. What goes up into orbit eventually comes back down, and re-entry into the Earth's atmosphere burns up unprotected objects as their kinetic energy is converted back to...
  14. hevans1944

    MPPT charge controller

    Seems like a reasonable requirement. My misunderstanding was a result of your earlier statement: With electronics becoming more and more compact and less massive, there is very little penalty involved with including persistent telemetry with the "mothership." The amateur radio community...
  15. hevans1944

    MPPT charge controller

    Perhaps you can convince someone on your satellite development team, someone who has electrical engineering expertise, to join Electronics Point and start a new thread exploring the power management problems associated with solar-powered electronics in low-earth orbit. I must assume this is a...
  16. hevans1944

    How do I build a underwater Infrasonic (10 -30 Hz) projector?

    Forget about using piezoelectric ceramic transducers for 10 to 30 Hz infrasonic sound waves in water. You need a much more powerful transducer capable of moving a significant mass of water under controlled excitation. A magnetostrictive transducer will do the job nicely. To build one you need...
  17. hevans1944

    Motion sensor that senses motion not the change in temp

    That is a good idea, but I am not sure it would work because of the way pyroelectric detectors respond to infrared radiation: their bulk temperature changes, which is what causes their embedded charge to change. An IR LED might not provide enough power to significantly raise the temperature of...
  18. hevans1944

    Motion sensor that senses motion not the change in temp

    You could mount a resistor on the lever arm, in view of the PIR sensor, and provide enough current to it to raise its surface temperature above ambient by a few degrees... amount necessary depends on sensitivity of the PIR. You may possibly not understand how a PIR sensor works. Typically there...
  19. hevans1944

    What's the "danger zone" for tesla coils?

    Here is an anecdotal story. Your mileage (or kilometers) may differ of course... Near the middle of the last century I was serving a four-year term of enlistment in the U.S. Air Force. Life was boring for a single airman stationed at a snowy, isolated, post in the northern peninsula of...
  20. hevans1944

    12V/5A from 19.5V/4.6A laptop charger

    You measure the current that actually flows when the rated voltage is applied. Do this with a multimeter using its 10 A DC current measuring function. Place the multimeter leads in series with one of the solenoid leads. Apply rated voltage between the free multimeter lead and the other solenoid...
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