Gun Detector (ART471E)

Passing a sensor on an object, such as a case, if it contains metallic objects in its previous one, the change in the magnetic field will be enough to allow its detection by the circuit that we present. Thereby, the circuit triggers a monostable 555 which triggers an audio oscillator for a short time.

We then issue an alarm beep. Figure 1 illustrates how to use the detector by passing the sensor over a suitcase.

Figure 1 - Using the sensor

The sensor consists of a coil around a ferrite rod and to create the magnetic sensing field, a small magnet is attached to the assembly as shown in Figure 2.

Figure 2 - The sensor

The more coils the coil has, the more sensitive the circuit will be. We recommend rolling 1 000 to 5 000 turns of fine wire or take advantage of a device reel that already has this coil, such as a residential cicada.

The circuit operates with a high gain operational amplifier at its input. The signal from this amplifier is applied to an amplifier transistor that triggers a 555 in the monostable version.

The alarm time is given by this circuit, set by R8, which can be changed over a wide range of values. The 555 output controls a small audio oscillator with two transistors whose frequency depends on R7 and capacitor C3.

These components can also be changed over a wide range of values.

Assembly

In figure 3 we have the diagram of the weapons detector.

Figure 3 - Weapon detector

For assembly, we can use a printed circuit board with the pattern shown in figure 4.

Figure 4 - Printed circuit board for mounting

Observe the positions of the integrated circuits and transistors in the assembly. The resistors are 1/8 W with any tolerance and the electrolytic capacitors to 12 V or more.

The speaker can be 5 cm with 4 or 8 ohm and small batteries (AA) can be used for power. In figure 5 we have details of the coil using the spool of the primary winding of a small transformer from which the core has been withdrawn.

Figure 5 - The sensor

The magnet may be of the type found in objects to attach to refrigerators or of the type found in small engines. The final assembly of the apparatus in a plastic box is shown in figure 6.

Figure 6 - Final assembly

For connection of the sensor to the circuit, a shielded cable must be used, otherwise noise from the network of nearby devices can trip the circuit erratically. To test and use simply pass the sensor quickly near a metal object.

There should be the trip of the circuit with the emission of a sound through the speaker. To change the duration of the sound, change the value of resistor R5 and to change the tone, change R7.