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Building a door knob alarm...need help with some simple questions

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mendojazzhead

New Member
Hi. I am building a door knob alarm as depicted in the circuit here http://www.diy-electronic-projects.com/p260-Door-Alarm. 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!

-Chris
 

ash20

New Member
1) sw2 is for stopping the beep signal. so you cant remove this.
2) sw1 is for closing after the proper functioning of the circuit, that is when you tune the circuit, as given in the notes, then you will have to switch sw1 on. that is it will have to be closed. so there is no point in making a wireless system between left and right part of sw1.
3) so you should not think of cutting the circuit.

4) i think you can use a walkie talkie set available. this is just a guess. might be considered. and you can use this before the beeper but it would have to be amplified to be transmitted easily.
 

mendojazzhead

New Member
Thanks for the reply. I realize the importance of SW 1, and I would not cut the circuit until after SW1. I cannot see anything to the right of SW1 that is needed for proper functioning of the left hand side of the circuit (other than the battery), so I don't see any reason why it could not be cut, except for maybe a change in capacitance.

As for a wireless transmission, something like a walkie talkie sounds good. Do you know a website where I can find these transmitters in bulk?

Thanks,
Chris
 

ash20

New Member
but why do you want to cut it? only for the reason tht it becomes bulky??
and for the website part, you might have to google it for the providers in your location.
 

alec_t

Well-Known Member
Most Helpful Member
I would cut the circuit at C4, keeping C4 with the left part and duplicating C4 in the right part.
 

mendojazzhead

New Member
Thanks alec_t The only problem is that I wanted to keep transistors Q2 and Q3 and LED D3 in the left side of the circuit. If I cut immediately to the right of SW1, do you think I will have to duplicate and/or change the values of capacitors C7 and C1? Thanks again!
 
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