This inductive measuring device is designed to detect high-voltage wires, electrical appliances, and other charged devices through sound signals without direct contact. It offers fast testing, safe operation, and reliability, making it an essential tool for both electronics enthusiasts and electricians.
The working principle of the device is based on a circuit divided into two main parts: a high-voltage induction control circuit and an acoustic generation circuit. The key component, VT1, is a junction field-effect transistor (JFET) that takes advantage of its high input impedance and voltage gain to sense and amplify the signal. VT2 and VT3 are complementary transistors with opposite polarities, forming a self-oscillating multivibrator when connected with feedback from capacitor C1 and resistor R3.
When the push button switch SB is pressed, the circuit powers on. If no high-voltage source is detected, VT1 remains in a zero-bias state, allowing a low resistance between its drain and source. This causes the base of VT2 to be at a low level, preventing oscillation. However, if a charged body is nearby, the induced voltage triggers the circuit, resulting in an audible alarm.
For component selection, VT1 can be either a 3DJ6 or 3DJ7 JFET with a saturated drain current of IDSS ≥ 50 mA. VT3 can use a 9012 or 3CG23 PNP transistor with a β value greater than 30. The trimmer potentiometer RP is a WH7-A type, while resistors R1 to R3 are RTX-1/8W carbon film resistors. Capacitors C1 and C2 are ceramic and electrolytic types, respectively. The speaker B is a small, lightweight 8Ω unit for compact design.
The metal sensor plate is a thin piece of iron or copper, approximately 15x15 mm. The power switch SB is a 14x14 mm tactile switch, and the battery GB consists of two AA batteries in series for 3V. The printed circuit board measures about 40x30 mm, and during soldering, it's crucial to ensure the soldering iron is properly grounded.
The device is housed in an insulating case measuring around 110x32x20 mm. The sensor is mounted inside the front panel, and the speaker has a dedicated opening for sound output. During commissioning, adjust the trimmer potentiometer RP until the circuit starts oscillating, ensuring the resistance of R3 is not too high to avoid excessive power consumption.
In practical use, the device can detect AC and DC voltages from 36V up to 10kV, with detection ranges varying from 0.5cm to over 80cm depending on the voltage. The power consumption is very low, less than 1mA when inactive and up to 100mA when active, making it energy-efficient.
Beyond basic detection, this tool can help locate hidden wires in walls or identify breaks in cables or electric blankets. Simply move the device along the surface, and when a sound is heard, you've found the location of the wire or fault.
Additionally, the alarm frequency changes depending on the type of signal detected. A vibrato sound indicates AC power, while a steady tone suggests an electrostatic field. This feature allows users to easily distinguish between different types of electrical sources.
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