Inductive measuring circuit diagram

Crystal oscillator

Industrial Router Crystal 3.2*2.5mm 3225 26M (26.000MHZ) 12PF 10PPM 20PPM 30PPM

Photocoupler

This inductive measuring device is a safe and efficient tool that can detect high-voltage wires, electrical appliances, and other devices without direct contact. It uses sound signals to alert the user, making it ideal for both professionals and hobbyists working with electricity. The device is known for its fast testing speed, reliability, and ease of use.

**Working Principle** The circuit of the inductive measuring device is divided into two main parts: the high-voltage induction control circuit and the acoustic generation circuit. The key component is VT1, a junction field-effect transistor (JFET), which takes advantage of its high input impedance and voltage amplification capabilities to sense the presence of a charged body. Once the signal is picked up, it is amplified and sent to the next stage. VT2 and VT3 are complementary transistors—VT2 is an NPN type, while VT3 is a PNP type. They work together with positive feedback from capacitor C1 and resistor R3, forming a self-excited multivibrator circuit. This setup allows the circuit to generate oscillations when the device detects a voltage source.

When the push-button switch SB is pressed, the circuit powers on. If no high-voltage source is detected, the JFET VT1 remains in a zero-bias state, keeping the drain-source resistance low. This means the base of VT2 is at a low level, preventing the circuit from oscillating.

**Component Selection** VT1 should be a 3DJ6 or 3DJ7 JFET with a saturated drain current of at least 50 mA. For VT3, a 9012 or 3CX200 transistor is suitable, as long as its β value is above 30. The trimmer potentiometer RP is a WH7-A type, and resistors R1–R3 are RTX-1/8W carbon film types. Capacitors C1 and C2 are CT1 ceramic and CD11-10V electrolytic, respectively. The speaker B is an 8Ω ultra-thin micro-coiled unit for compactness. The metal sensor is a thin iron or copper sheet measuring about 15x15 mm. The power switch SB is a 14x14 mm tactile switch, and the battery GB consists of two No. 5 dry cells in series, providing 3V.

**Installation and Commissioning** The printed circuit board (PCB) measures approximately 40x30 mm. During soldering, make sure the soldering iron is properly grounded to avoid damage. The PCB is housed in an insulating case of about 110x32x20 mm. The metal sensor is glued to the inside of the front panel, and the speaker is placed with a pre-drilled hole. The power switch is mounted on the side panel. The debugging process is straightforward. In a non-charged environment, press the switch SB and adjust the trimmer RP using a small screwdriver. Ensure that R3’s resistance is not too high, as this could increase power consumption and cause unwanted oscillation.

**Performance Indicators** Under normal conditions, the device can detect AC and DC voltages between 36–100V at distances of 0.5–4 cm. For 220V AC, the detection range is at least 5 cm. It can detect 10kV transmission lines from 80 cm away, and TV picture tube voltages from 20–50 cm. When not detecting any voltage, the power consumption is less than 1 mA, and during alarm, it stays under 100 mA—making it very energy-efficient.

**Usage Tips** Beyond basic voltage detection, this device has several unique applications. To locate hidden wires in walls, move the device close to the wall while holding the power button. When you hear a sound, you’ve found the wire. To find a broken wire in a cable or electric blanket, connect one end to the live wire and move the device along the other end. A sound will indicate the break point. The device’s sensitivity increases with the voltage level, allowing users to estimate voltage based on detection distance. Additionally, the oscillation frequency changes depending on whether it detects AC or static fields. AC signals produce a vibrato sound, while static fields result in a steady tone. This helps users distinguish between different types of electrical sources.