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OT Need simple (I think) circuit

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  • #31
    Actually, the venerable NE555 would have worked well for this. It has a comparator with adjustable high and low threshold, and an output driver capable of 100 mA or so to drive a relay. But I think some of us had difficulty visualizing the actual circuit(s) and the parameters needed to make the best suggestions.
    http://pauleschoen.com/pix/PM08_P76_P54.png
    Paul , P S Technology, Inc. and MrTibbs
    USA Maryland 21030

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    • #32
      Nice "off-data sheet" application of 555. It literally had not occurred to me to look at that part for this. But I seldom use the 555 any more. IIRC the trip point is fixed at 1/3 of the supply, might not be where wanted and needs external parts to change it.
      CNC machines only go through the motions

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      • #33
        Originally posted by PStechPaul View Post
        venerable NE555
        Nice pull Paul. The ol 555 is very versatile. Thats what happens when a design over exceeds it original need.

        How old is that design? More than 30yr that I know of. JR

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        • #34
          The 555 is quite old. What actually came to my mind was a chip that was almost as old, the LM339. I have used them in the past and they work well. But they are obsolete now. I don't know if you can get them any more.

          Actually, virtually any op-amp IC can be used as a Voltage comparator. But they are not optimized for that use.

          I believe the 555 is still available. I have a bunch of them as well as the dual version, the 556. But there are a lot of pin-for-pin replacements for them, probably due to it's usefulness and popularity. Some are CMOS technology which makes them a much better choice.
          Paul A.
          SE Texas

          And if you look REAL close at an analog signal,
          You will find that it has discrete steps.

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          • #35
            You can change the threshold and trigger points by adding a resistor from pin 5 (CONTROL) to GND. I think the internal voltage divider consists of three 5k resistors. So a 1k resistor from pin 5 to GND would provide trip points of about 16% and 8% of Vcc supply.

            https://www.codrey.com/555-timer/pin...an-abridgment/

            More detailed information, and a claim that the 555 was so named because of its three 5k resistors:

            https://electronics.stackexchange.co...l-pin-of-a-555
            Last edited by PStechPaul; 11-21-2020, 12:52 AM.
            http://pauleschoen.com/pix/PM08_P76_P54.png
            Paul , P S Technology, Inc. and MrTibbs
            USA Maryland 21030

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            • #36
              Yes, that is so. And the output driver on the part is generally good for the 100 mA you suggested, although some of the CMOS may or may not have quite as high a rating, and at least the TI version is rated 200mA, source and sink.

              Should work with one resistor and one diode, plus a bypass capacitor.

              A comparator would appear to require at least three resistors, another transistor, and one diode.

              https://www.ti.com/lit/ds/symlink/ne555.pdf
              CNC machines only go through the motions

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