A detector of magnetic fields

Figure 1: Diagram of the detector of magnetic material. If you approach an object's magnetic core of Ferroxcube choke L1, the LED lights DL1 and the buzzer sound a note.

Pinouts integrated circuits used landmarks as seen from above and U-notches facing left.

After gaining some experience in electronics, through articles and lessons published in your journal, I can now have fun experimenting with new circuits like the one I'll show you, hoping you can give him a little place in your columns.
The circuit that I propose is able to detect the passage of any object on the magnetic coil L1.
For example, if you pass on the blade of a L1 previously magnetized screwdriver, or one of those little magnetic buttons that are used for holding sheets of paper to the wall, or the magnet of a speaker, LED DL1 lights immediately and the buzzer CP1 is immediately hear the note (note that both are connected to pin 3 of NE555). But what use such an arrangement? That may be the question you're asking right now ...
Well, I can assure you that with this device you can easily open a door, gate, etc.. By simply replacing the buzzer by a circuit polarizing the base of a transistor that will excite a relay!
With a magnet, recovered from an old speaker out of order, I managed to turn the LED to 90 centimeters from the core of Ferroxcube choke L1. If you take a look at the wiring diagram in Figure 1, you will see one side of the coil L1 is connected to ground and the other through a capacitor C1 1 uF polyester, the inverting input pin 2 of an operational amplifier μA741 or LS141. Potentiometer R4 100 k, mounted between the output pin 6 and ground, used to adjust the sensitivity, as we shall see in a moment. The voltage taken from the cursor to the trimmer will drive the pin 2 of the NE555 which, beginning to oscillate, lights and sounds DL1 CP1.
As a core for the inductor L1, you should use a short piece (4 or 5 centimeters) in ferrite bar (salvaged from an internal AM antenna in an old transistor radio), or maybe an old plate transformer (among the two elements E and I, I choose).
Do not, cons, a piece of ordinary iron because it is magnetized fast enough and the circuit would no longer work. You can turn electrical coil on a mandrel (see Figure 2) or directly on the core of Ferroxcube. It will take turns rolling a thousand (!) Enamelled copper wire of 0.18 mm in diameter (18 cents). But you can wound more: the more you roll, the more the device will be sensitive.
To adjust, slowly turn the cursor of R4 (or trimmer potentiometer) until DL1 off, then try to go to the core of Ferroxcube any magnetic object and the LED should light up at the same time as the buzzer should sound.

Figure 2: On the core of the inductor L1 must be wound at least 1000 (one thousand!) turns of wire enamelled copper of 0.18 mm in diameter.

Component List
R1 .... 100 kΩ
R2 .... 100 kΩ
R3 .... 10 MW
R4 .... 100 kΩ potentiometer linen.
R5 .... 1 kΩ
R6 .... 100 kΩ
R7 .... 1 kΩ
C1 .... 1 uF polyester
C2 .... 470 nF polyester
C3 .... 10 nF polyester
C4 .... 1 uF polyester
C5 .... 10 uF electrolytic
C6 .... 10 nF polyester
C7 .... 47 uF electrolytic
L1 .... see text
DL1 ... LED
IC1 ... LS141 or μA741
IC2 ... NE555
CP1 ... piezo buzzer


Editor's Note
If you do not find an enamelled copper wire of 18 hundredths of a millimeter, you can take instead of 15 or even 10 / 100, but if you take a larger diameter than a thousand turns and occupy a non negligible.