555 timer 110402

Discussion in 'The Projects Forum' started by allenpitts, Apr 2, 2011.

  1. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    Good afternoon All About Circuits,

    Working with a motion detector (Quorum A-160 Passive Infrared Detector) that will turn on an LED display. The A-160 sends out a signal that lasts for two seconds but the display needs to stay on for a couple of minutes. So I built a 555 timer circuit to take the the second pulse from the A-160 and activate the display.

    I built a circuit from Ramsey called UT5. The schematic is at
    I modified it
    The the LED with resistor marked R7 represents the LED display.
    The nine volt battery with momentary switch would be replaced by the
    pulse from the A-160.

    Can't seem to get it work. Any ideas?


    Allen In Dallas
  2. SgtWookie


    Jul 17, 2007
    56k for the resistor R7 is far too large.

    Drop it down to ~1k. That should be safe for most any LEDs; about 10 to 14mA current.
  3. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    Good afternoon All About Circuits,

    Working on 555 timer circuit to operate relay.
    When I get it all together as shown the LED glows constantly.
    So I started deconnecting jumpers to see what was shorting the
    circuit. I replaced the 555 and tested the switch (normally open)
    Result: normal.
    I got the the circuit so it looks like
    With all the components stripped out except those shown in the dev circuit LED still glows.
    Can't see how adding the components, like the switch, back will
    improve the operation of the circuit since the purpose of the components is to turn the LED on and off if the LED stays on all the time.
    What is wrong with the circuit as designed?
    What is an approach for trouble shooting?
    Allen in Dallas
  4. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    Good afternoon All About Circuits,
    Built 555 monostable circuit shown at
    in the figure marked 'ILLUSTRATION':
    On the figure marked ILLUSTRATION there is a red jumper wire.
    When that jumper is connected between R4 (in the connection marked 'B')
    and the connection marked 'Gnd' the green LED glows. When the red jumper
    is removed, the red LED glows. When the red jumper is placed between the
    Gnd connection and the connection marked 'A' the red LED glows but that is the same as when the red jumper is not connected so not sure what that proves.

    Since the circuit is titled '555 Monostable Multivibrator' it was expected
    that it would light one of the LEDs for a certain amount of time and then
    go off. But I seem to have missed in the exposition an explanation of how
    the circuit should operate. Perhaps my understanding is so modest that the
    annotation is aimed at readers with greater knowledge. Before a basic
    perception of how the circuit operates and the logic behind is provided in the elucidation the breakdown jumps into 'illegal conditions' and 'edge only triggering'.
    Later in the article it is stated that the circuit is designed 'for 3 seconds timing duration'.
    Since the circuit would not work for me it is wondered which LED was supposed to come on for three seconds, the green or the red?
    Any recommendations on troubleshooting to try and get the three second duration to occur?
    Allen in Dallas