Simple metal detector with 4011
Posted on Jul 30, 2012 7943
This simple metal detector requires only a handful of components and an evening's work. Built around a cmos4011 IC, is very robust and versatile. The 250 kHz reference oscillator is built with two gates (U1/1 and U1/2), C1, R1 and P1. The search oscillator uses only one gate (U1/3), two capacitors and the search coil. The outputs of the two oscillators are fed to the fourth gate acting as a mixer and filtered with C4.
After assembly, connect the headphones and slowly turn P1. The pitch will get lower until it disappears. Continuing to rotate P1 in the same direction will cause the pitch to rise again. The point at witch the pitch is the lowest and disappears is called "zero beat". If you can not get this zero beat frequency for the entire turn of P1 you may have to select different values for C1. Turn P1 close to the zero beat position, then move the search coil near a metallic object. The tone should change, depending on the size and distance of the metal. Note that this simple detector's performance is not comparable to more advanced commercial products. It will only detect relative large metallic objects at a short distance. Coins and other small objects will be much harder to find! Parts list: U1: CD4011 (Quad 2-input NAND Gate) U2: LM78L05 (5V Regulator IC) R1: 2.2k 5% resistor R3: 330k 5% resistor R4: 270k 5% resistor R5: 1k 5% resistor C1: 390pF NPO capacitor C2, C3: 10nF C4: 100nF C5: 100uF/16V electrolytic C6: 220uF/16V electrolytic C7: 100nF ceramic P1: 4.7k lin. potentiometer L1: 22cm diameter, 14 turns, AWG 26 K1: SPDT toggle switch J1: Headphone jack 1/4 or 1/8 inch