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Building a door knob alarm need help with some simple questions

  1. Dec 6, 2011 #1
    Hi. I am building a door knob alarm as depicted in the circuit here The circuit is supposed to signal an alarm when a door knob is touched. This is a very straightforward circuit, as everything to the left of SW1 is simply an oscillator that gets heavily damped when a hand comes in contact with a doorknob. The capacitance of the hand damps the oscillation, switches off biasing to an accompanying transistor, and an LED lights up. When the hand is removed, the oscillation is undamped, and the LED goes off. Simple enough huh? Everything to the right of SW1 is simply a series of latches that triggers an alarm when a hand touches the doorknob. The alarm continues to sound, even when the hand is released from the door knob. The alarm can only be shut off by opening SW2. All is very simple, and the website does a good job at explaining the function of the circuit.

    Here is what I want to do:

    I want to divide the circuit into two: One to detect when the door knob is touched, and the other to trigger the alarm. I want each circuit to be indepently powered (2 different batteries) and I want them to be able to communicate wirelessly, maybe at a distance of no more than 30-40 ft. This way, when a hand touches the doorknob, a signal is sent remotely to the second circuit, which is responsible for triggering the alarm. This way, the alarm can be disarmed at the remote location.

    My questions are these:

    1) By looking at the diagram in the figure from the referenced web page, is it sufficient to simply cut the circuit down a vertical line immediately right of the diode next to SW1? If so, do I have to change the values of any of the capacitors (especially C1 or C7)? If this is as simple as I have suggested I would simply hook up a battery to the left hand side of the circuit, as is done to the right hand side.

    2) What is the best (and cheapest) transmitter and receiver I should use for this project? I have had a hard time researching what the best components would be. I was thinking something along the line of a transmitter-receiver similar to that of a garage door opener. It wouldn't have to be anything fancy, just something that could send a reliable signal across a house (maybe through a couple of walls). The signal would not have to contain any data, other than a digital "high" signaling that the door knob was touched. I would think that there is a cheap transmitter-receiver on the market that would do this, but I have had no luck finding it. (Note: I am aware off FCC complications regarding this application, and have considered that accordingly).

    Thank you for your time....all feedback is greatly appreciated!

    Last edited by a moderator: Dec 6, 2011
  2. jcsd
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