Circuit only works when you touch the ground

Discussion in 'General Electronics Chat' started by BackyardBrains, Aug 26, 2010.

  1. BackyardBrains

    Thread Starter Member

    Aug 26, 2010
    Has anyone every seen this before? I have a circuit that seems to work well, but only if I touch the board with my finger.


    The part I have to touch is the "virtual ground". I divide the 9v battery into +/-4.5v with a voltage divider. I'm just curious if you have ever seen this before, and where I should start looking.

    FYI. This circuit is connected to a leg of a cockroach. I am amplifying the spikes from it's neurons in the leg.
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
    It's been my experience that circuits which won't work unless the board is touched somewhere has a device that's just on the edge of triggering, touching the board injects enough stray AC hum to get the device biased on.
    BackyardBrains likes this.
  3. SgtWookie


    Jul 17, 2007
    Your O-scope is connected to earth ground through it's power plug.

    Your circuit is "floating".

    If you connect the ground clip from your O'scope probe to what is supposed to be the project board 0v reference, aka ground, your problems should go away.
    BackyardBrains likes this.
  4. BackyardBrains

    Thread Starter Member

    Aug 26, 2010
    Thanks, guys. I suspect it something along the lines of the "floating" issue that SgtWookie is referencing.

    The circuit is battery powered, and in use should never have to access an external ground. The device records from the leg of a cockroach, and amplifies the electrical "spikes" and sends it to a speaker. I've attached the schematic.

    There is an audio out jack that allows one to connect our device to an iPad/iPhone... When this device is connected, the circuit seems to work. Perhaps this is the equivalent of what SgtWookie suggested?

    See video of iPad working:

    Beyond the audio out, the signal gets amplified 200x more and sent to a speaker using the LM386. It is in this mode that I have to touch the virtual ground.

    Any more suggestions would be welcomed.