LED sequencer from relay

Discussion in 'The Projects Forum' started by allenpitts, Mar 26, 2015.

  1. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    Hello ACC forum,

    Experimenting with LED displays and sequencers.
    Bread boarded this sequencer:
    I believe this is the design that drives circuits like the Chaney 4431
    and others like it. The breadboard works well:

    Then the PIR sensor to relay circuit:
    was used to make the relay open and close the connection between the voltage source and the 555/4017
    sequencer. Basically the relay closes an opening created in the voltage connection
    in the schematic between the point where it says "+6v" and the connection to C1 10uF.

    When the PIR fired the sequencer, which was taking about 3 seconds to go thru the sequence
    before the relay was introduced,
    now goes through the sequence in about a quarter of a second, hangs for about a half second on
    output 0, then loops again. It looks like this:

    But it was expected that the relay would just act like a switch and turn the display on.
    Why does closing the connection to voltage thru the relay give different results from
    just plugging the red wire from the battery into the bread board voltage rail?


    Allen in Dallas
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
    At least part of your problem is electrical noise being generated by the 555 and/or the relay. Add a .1uF cap across the power pins on both IC's. Add a .01uF cap from pin 5 of the 555 to ground. In addition, you may have some relay contact bounce.
  3. Reloadron

    Well-Known Member

    Jan 15, 2015
    Noise and some contact switch bounce is also my guess.

  4. nerdegutta


    Dec 15, 2009
    Not related to your problem, but which PIR-module do you have?
  5. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    Hello Forum and Nerdegutta,

    Have used PIRs by SparkFun, Parallax, Radio Shack and AdaFruit. The documentation of the pin outs of the SparkFun PIR is inconsistent and confusing. The Radio Shack and the Parallax units work well and are responsive and well documented. The best of the four is the AdaFruit. By studying the AdaFruit
    unit it was discovered that the AdaFruit unit is an HC-SR501.
    The HC-SR501 is superior because it has two trim pots that allow for adjustment. One pot changes the time that the PIR sends a signal. The other pot changes the distance that the sensor detects a change in the infrared response. The PDF data sheet on the HC-SR501 is attached herewith.
    Some of the HC-SR501, dependent on the supplier, have a jumper pin switch that allows one to change the triggering function from repeatable to non- repeatable. In non-repeatable the sensor waits until the signal stops sending before it can be triggered again. In repeatable the sensor will begin a second signal during the first signal being sent if the sensor detects motion before the first signal is complete.

    Also all four distributors mentioned sell their PIRs for about $10. HC-SR501 can be purchased online through Amazon for $2. But purchasing
    at the lower price takes a couple of weeks, probably because they are coming direct from overseas.

    Finally, the Parallax unit has a red LED that glows underneath the Fresnel lens and makes the lens glow pink.

    Allen Pitts, Dallas Texas
    Last edited: Mar 27, 2015
  6. nerdegutta


    Dec 15, 2009
    Thank you for a good answer.

    I have the HC-SR501 myself, and I've just started to play with it. I bought them on eBay. It do not have the jumper installed. In fact the jumper pins are not there, but the solderpads are. This means I can short them to my likings.

    Thanks for adding the datasheet. I've been lokking for some pdf documentation.