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Mains hum 555 timer circuit problems

Discussion in 'Electronic Design' started by Midge, Sep 26, 2007.

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  1. Midge

    Midge Guest

    I found a circuit in an old book of 555 timer projects for a non-latching
    mains hum single plate touch switch, but I just can't get it to work!

    The touch plate is connected to pin 2 of the 555 via a 0.1uF capacitor.

    Pin 2 is also connected to the cathode of an IN4001 diode, and the anode of
    that diode is connected to 0v

    Pin 2 is also connected to the anode of a second IN4001 diode, and cathode
    of that diode connects to Vcc. A resistor which supposedly sets the
    sensitivity of the circuit is connected in parallel with this diode between
    pin 2 of the 555 and Vcc - suggested values "in the range 47K-10M"

    According to the book, the 555 should trigger permanently when the resistor
    value gets as high as 10M. It doesn't, nor at 20M, nor 30M. This suggest to
    me there is a flaw in the design.

    I can get it to work with some stability (but not enough!) by simply
    connecting pin 2 to the touch plate via a longish wire. I suspect this is
    because the wire is picking up background noise sufficient to stop the 555
    triggering, and my finger is then dragging the signal down a bit, and the
    555 then triggers.

    I can also get it to work by going for a leakage based (i.e. two contact
    pad) solution of connecting pin 2 to Vcc via a 10M resistor and one pad to
    pin 2 and a second to 0v - but it's not that sensitive + I really could do
    with a one-pad solution.

    Other info about the circuit:

    Supply is a 9v battery. There is no indication the circuit needs to be
    Pin 6/7 are connected to 0V via a 1uF capacitor, and to Vcc via an 820K.
    Pin 5 connects to 0v via a 01.uF capacitor.
    I'm operating it in the house where there should be plenty of mains hum to
    drive the circuit
    Pin 3 drives a 9V relay via another IN2001 diode (another IN4001 diode + 1uF
    capacitor sit across the coil of the relay)

    So if anyone has:

    1. Any ideas for getting this circuit to work properly and reliably
    2. A really good proven one pad non-latching circuit design which can
    preferably run from battery and doesn't need to be grounded
    3. Any other proven one-pad non-latching design. I won't rule out a solution
    that needs to be grounded and/or mains powered., but its just not as


  2. GPG

    GPG Guest

    Replace the 1Nxxx diodes with 1N4148. Use a CMOS 555
  3. hi,
    dont worry ... i have designed a circuit on 555 timer send mail
    kaushik899 @ gmail .com
    i will send .

  4. John Fields

    John Fields Guest

    Try this: (View in Courier)

    | |K | |8 |
    | [1N4148] [820K] +---+---+ |
    | | | 2|_ Vcc _|3 |
    IN>---[0.1µF]--[1M]<----+-------|----O|T R|--|--> OUT TO RELAY
    | |K | 6| | |
    | [1N4148] +-----|TH | |
    | | | 7|_ _|4 |
    | | +----O|D R|O-+
    | | | | GND |
    | | |+ +---+---+
    | | [1µF] |1
    | | | |
    | | | |

    With nothing on IN, rotate the 1 megohm pot until OUT goes high,
    then back the pot off until OUT goes low.

    Now touch IN, and OUT should go high for as long as you keep
    touching IN.

    If the sensitivity is too high, back off R1 until you get the
    sensitivity you need.

    BTW, how reliable does the circuit have to be?
  5. default

    default Guest

    I have a circuit just like that. Mine is a "lightening detector"
    (spark) detector. It has a pot to set the sensitivity and a single
    common npn transistor to amplify the signal into pin 2. (pulse is
    capacitively coupled into pin two like it shows in the application
    notes - 10 K pullup on 2 with a .1 ufd cap to take the negative
    transition into it).

    2N2222A has a 10K pull up on its collector and the base goes to a wire
    that detects the static with a crude high pass filter, since I don't
    really want it triggering on hum. If I touch the antenna it triggers
    very reliably, so I'm thinking if you add an npn transistor to amplify
    the touch plate and use AC coupling for the trigger you should have
    what you want.

    My device also triggers with one of those piezo electric igniter used
    for lighting propane grills from about four feet away. Antenna is a
    1' piece of wire.
    Hash: SHA1
    FWIW, you can buy a really, really cheap chip
    ( for less than the cost in
    parts for your circuit (yeah I know it's not as fun though). They just
    require a resistor & capacitor to work. I've used these chips a number
    of times and they work perfectly. There's no calibration involved and
    they draw next to nothing from Vcc.

    - --
    Brendan Gillatt
    brendan {at} brendangillatt {dot} co {dot} uk
    PGP Key:
    Version: GnuPG v1.4.3 (MingW32)

    -----END PGP SIGNATURE-----
  7. Midge

    Midge Guest

    Just a bit of background.....

    I'm setting up the switches to help someone who is nearly paralysed with MS
    to control a few things around her - hense the need for a one pad solution
    for ease of use.

    Switching does need to be pretty reliable so I'll try some of your ideas and
    see how I get on.

    Thanks for the prompt help.

  8. hey midge ..did my circuit helped you .... ???
  9. neon


    Oct 21, 2006
    pin 2 requires 50 micro to trigger so now we need a pulse to go negative tie a pullup to the rail from pin 2
    add a cap in series with pin 2 now if you touch ground with your finger it should discharge the cap making pin 2 spike on down. that should trigger it however once triggered it will stay latched until time out
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