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  1. hevans1944

    Controlling Separate Source with Low Power Signal

    This could be a very viable and inexpensive approach, depending on what kind of electrical response time is needed. In our bearing test lab we have two variable speed motor drives used for bearing load testing (I don't know if they are testing the bearings or testing the lubricant). A...
  2. hevans1944

    Controlling Separate Source with Low Power Signal

    I agree. High power unity gain buffers, or buffers with gain, are readily available but pricy. Check out this web site for Apex Microelectronics. We use their PA-12 hybrid power op-amps to drive coils and high voltage transformers for ion-beam scanning. They can be configured with gain, so a 0...
  3. hevans1944

    Heatsinking using mica insulator and thermal compound

    Or just use a 4-40 x 3/8 nylon screw, 4-40 nylon nut, and a #4 flat nylon washer. No need for the stepped nylon washer (although one probably came with the mica insulator) with all-nylon hardware.
  4. hevans1944

    ATX PSU Power Toggle Circuit

    My advice above was from many years experience interfacing real world devices with TTL circuits. TTL is pretty much unforgiving when it comes to enforcing rise/fall times, setup/hold times, and logic levels. Schmitt-trigger inverters, when they became available as TTL components, were a...
  5. hevans1944

    ATX PSU Power Toggle Circuit

    I think you should de-bounce and clean up the rise and fall times of the signal applied by S1 to the CLK input of the 4027. Otherwise, you may get multiple "clock" pulses each time the switch is actuated and then deactuated to create a positive-going clock. If this occurs, each of the extra...
  6. hevans1944

    this close to setting the perfboard on fire, 2013 darlington array

    Yes, remove resistor R3 and follow the instructions below. The resistor you use in each string of three LEDs depends on the forward voltage drop across the string at the current you intend to operate each string. The value will be the voltage from pin 3 of the 555 IC minus the forward voltage...
  7. hevans1944

    and/or gates with 120 and 240 volts

    Old-school electromechancal relays are definitely what I would choose. I once did a ridiculously complicated design using DC relays with steering diodes to implement a raster scan of a motor-driven X-Y table. The X-Y table had a 2.5 cm diameter ZnSe infrared window (or other infrared...
  8. hevans1944

    Scratches/scoring on photo etched PCB

    Those scratches are bad news and will definitely result in open traces. Assuming normal care in making the exposure through the transparency followed by developing and drying, those scratches were probably on the un-exposed photoresist when you received them from Jameco. Did you carefully...
  9. hevans1944

    74HCT08: an IC beginning

    Oops! I have a two 35 mm film cans filled with both, but I always keep a data sheet handy when breadboarding with them. Thank you for the correction.
  10. hevans1944

    Is there any way to fix transistor voltage drop?

    I've been more or less lurking here waiting for more information on what the "DEVICES" are. It appears that they present two terminals with 12 to 24 volts DC, available from "something" whenever an LED on the DEVICE (or somewhere else) illuminates. Or perhaps the voltage is always there. When...
  11. hevans1944

    74HCT08: an IC beginning

    Note: the chip you say you have is the SN74HCT08N. The T in the part number means it uses TTL inputs and should have a 5 V supply. It is still a CMOS circuit though, meaning low-power operation and typically low-current inputs and outputs. I would use the output to drive the base of a small NPN...
  12. hevans1944

    Is there any way to fix transistor voltage drop?

    IIRC, I first used a UJT sometime in the late 1950s to build a light dimmer using inverse-parallel connected SCRs. I pretty much quit using UJTs and PUTs when the NE555 and TTL monostable multivibrators became available for timing functions. I really liked the simplicity of the UJT. It sort of...
  13. hevans1944

    Antenna inductance

    Measuring Inductance can be a real pain compared to measuring capacitance. All the things you mentioned can affect the measurement, especially for low inductance values less than 100 μH. To the OP, what exactly did you use to measure inductance? My multimeter only does capacitance, just fine...
  14. hevans1944

    this close to setting the perfboard on fire, 2013 darlington array

    As @Colin Mitchell said, you need a current-limiting resistor is series with each group of three series-connected LEDs. The total current supplied from pin 3 of IC1 would then be distributed to the six sets of three LEDs through separate connections for each set to IC2. A current limiting...
  15. hevans1944

    The Blue LED - The 2014 Nobel Prize in Physics

    Hmmm. A direct transition from kerosene lamps to white-light LEDs for third-world developing countries, eh? Some young lady from the Philippines recently "invented" a white-light LED flashlight that operates on body heat. Interesting concept. In 1959 the Russians demonstrated thermocouple...
  16. hevans1944

    L.P and H.P circuits

    Yes. Yes. See my post #4 above.
  17. hevans1944

    L.P and H.P circuits

    Vo(0+) occurs after the input voltage, Vi, transitions from 0 to E. At time Vo(0+), Vi = E. Vo(0+) will then be Vi R1/(R1+R2), assuming C1 is fully discharged at Vo(0). So the first part of your answer is correct. Vo will rise exponentially to toward Vi = E until at Vo(∞) = Vi = E. So the...
  18. hevans1944

    Antenna inductance

    It is unfortunate you sent the bad ones back. Hard to troubleshoot the problem when the problem isn't at hand. I suspect there may have been some shorted turns in the bad antennas, maybe because the insulation deteriorated over time or wasn't very good to begin with. Did you purchase new...
  19. hevans1944

    74LS193 counter issue - Please help

    Remove the load to ground connection after the counter is loaded. See @Gryd3 post #6. It is a momentary connection.
  20. hevans1944

    Is there any way to fix transistor voltage drop?

    So why not, instead of a resistor in the collector of the transistor, use a relay coil, with a diode across the coil to suppress voltage spikes when the relay de-energizes? When closed, the normally-open contacts on an energized relay are pretty close to a dead short. I would also add the...
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