Shadow Detector help

Discussion in 'The Projects Forum' started by ofak, Jul 27, 2013.

  1. ofak

    Thread Starter New Member

    Jul 27, 2013
    Hey all, new to the forum and to electronics. I purchased the Radioshack electronics learning lab to try to educate myself with electronics. There is a circuit i built with a LDR that detects light/dark using a DPDT switch.

    I need to be able to change/adjust the beep duration using possibly a pot capacitor etc, but i don't even know where to begin. here is the schematic, any help would be great.
  2. JohnInTX


    Jun 26, 2012
    Use a 555 connected as a monostable multivibrator. Break your posted circuit at the B(ase) of the transistor, move R5 to pin 2 of the 339 and remove R4. Connect the 339pin2/R5node to trigger the 555. Add the supporting R-C components to the 555 to set the timing. Connect the buzzer to the output. Depending on the buzzer, you might get away with driving it without the transistor.

    Also see Bill Marsden's 555 stuff. All you need to know!

    Have fun.