Driving a 32Khz ultrasonic transducer with the Prop...



  • Tracy AllenTracy Allen Posts: 6,479
    edited 2019-05-23 - 18:29:52
    denno wrote: »
    First, I decoupled the outputs OUTA and OUTB of both 4427's with a 10N capacitor to ground.

    Really, the outputs? The decoupling should be from the power supply to ground. They suggest 0.1µF in parallel with 4.7µF.
    The 1nF capacitors on the outputs in the test circuit are not bypass; they are there to simulate a capacitive load (the piezo). If you really have 10nF from the outputs to ground, that is simply a heavier load that requires heavier power supply bypassing.

    In my last post, the PropBASIC syntax should have the mode for logic never as the first parameter, not the last.
    COUNTERA 128, ultra_1,ultra_2, 0             'logic never, output low from counter logic
    COUNTERB 128, ultra_3,ultra_4, 0             
    In NCO mode 40 as you had it, I believe a zero frequency will leave the Prop in whatever phase it happened to have when it hit the instruction. Could be A high B low, or the opposite. That in itself should be okay and not cause any overheating. So I take back the comment about the counter being disabled. With mode 40 you still had it operating with outputs enabled at zero frequency.

    Chips not getting hot as youi have it now?
  • Then you should be able to see it on the oscilloscope, but remember when you hook up the 'scope that neither side of the piezo is at ground, so the power supply for the Prop has to be isolated from the ground on the oscilloscope. If not that could be bad for the output pins!

    Good advice!

    Also, some scopes and PCs connect earth ground to logic ground. So, even if your power supply is isolated, having your prop connected to the PC at the same time as the scope is, isolation will be lost. If this is the case, and if your scope has two channels, you can also connect each signal wire to an output pin, leaving the ground leads disconnected. Then set the scope for difference input on the two pins.

    Simplest of all with a dual channel scope is just to display both channels, one above the other, and witness that they're 180 degrees out of phase.


    Something one only needed to connect scope probe ground to TV Hot chasis ground for a memorable life / object lesson.
  • First of all, I do understand about the isolation of the grounds, etc. I am running my lap top on it's battery, when ever I program anything, and the test circuit is being supplied by a separate 12 volt small car battery, with the charger disconnected. So the only ground brought into play is the scope...

    In answer to your question Tracy, the chips will warm up, according to my temperature probe positioned under the chip in the slot of the breadboard. Under continue operation, will start out at ambient air temp...in this case around 65 degrees F, and warm up to around 87 degrees F. This will happen with the decoupling capacitors in place or removed. The capacitive reactance of the .001u at 20Khz is around 8K ohms..not much of a load. I even put in .01u and there was no change on temperation variation. That reactance is around 800 ohms.
    Seems OK on this temperature, as before they would start to burn my finger after only a few seconds...almost not enough time to disconnect the power before I smell that burnt IC smell that we all know.

    In measuring the current draw of only both of the 4427's there is .127A..127mA...the 4427's are not going to run continuously, as they are triggered by PIR's, and when the heat/movement source goes away (aka deer) they are turned off, by putting the Prop into the condition that Tracy suggested. COUNTERA 128, ultra_1, ultra_2, 0..

    And, yes, this is just an experiment, defending the garden. And, yes, I know you can buy these things on Amazon, but there is more pleasure in building your own...it's there?

    Thanks again...to all...DennO
  • Tracy, I sent you a private message, I think, about a non related Prop question involving a circuit that does not use a Parallax product. Not sure I did it right, however..DenO
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