This is a low cost circuit which is  simple and easy to build. The main components of this  circuit is based on dual op-amp MC1458 and LM 380. The two op amps inside the MC 1458 are used to produce square and triangular waves. LM 380 is used to amplify the output. The first op amp IC1a is wired as an astable multi vibrator and second op amp IC1b is wired as an integrator, to make the square wave triangle.

This is door knob touch alarm for your home security purpose. The alarm will be activated when someone touch the metal door knod. This circuit won’t work on full metal door.

Download the schematic drawing

A Many security systems use a closed loop of wires and switches arranged so that whenever a door or window is opened, the loop will be broken and the alarm will sound. An obvious problem is that someone can tamper with the system, short out the loop, and later on, come back and burglarize the premises without sounding the alarm. Hiding a known resistance in the loop, as you propose, is a very good idea. That way, the alarm can distinguish a short circuit from a correctly functioning closed loop.

FIG.l – ONE OF THREE OUTPUTS goes low depending on whether loop resistance is too high, too low, or just right.

Figure 1 shows a circuit that does the job. It’s a somewhat unusual application of a National Semiconductor LM3915 IC, normally used to drive LED’ bargraph displays. That chip happens to contain the right combination of comparators and logic circuits to do what you need.