We are constantly surrounded by electromagnetic radiation in the range of tens of kilohertz to several gigahertz. It is created by mobile phones, computer systems WiFi and Bluetooth, poorly shielded microwave ovens and other household and industrial electronic devices. The professional and amateur circles are lively debates about whether physiologically harmless effects of radiation on the human body. There is no consensus on this matter and no.
But, in addition to effects on the body, non-compliance with the EMC electronic devices may cause interference for other equipment. Finding the source of interference is often a difficult task. The article proposes an indicator of the electromagnetic field that can be used to detect radio sources at a short distance (about 3 meters). This device can perform another function — to detect illegally installed listening devices («radiozhuchki ,,) operating in the VHF and UHF bands.
Pre-amplifier transistor T1 is loaded by a rectifier D1, made under the scheme doubling the voltage on the assembly of high-speed Schottky diodes. Through the resistor R5 to the diodes supplied the initial bias current (about 2 uA). The voltage at the anode of the second diode assembly (right under the scheme) is 0.2 … 0.4 V. The transistor is operated at a collector current of about 2 mA at which its noise is minimal. The gain of the cascade in a broad frequency range is about 10 dB (2 … 3 times).
At the conclusion of the collector load resistor R3 T1 dressed ferrite «bead» (ring) Ft, preventing self-oscillation and overload amplifier in switching or in the case of «hanging» (unconnected) inputs. For this fit a ring type «Cambion SA515-3225» or similar which can be found, for example, in the old transistor TV tuner.
Due to the initial displacement of the rectifier begins to operate at the level of the input signal 50 … 100 mV. However, the high sensitivity is to the detriment of stability, so it uses «chopper » — stabilization, ie, voltage cascade is interrupted at a frequency of about 700 Hz. For this is a square wave generator on a chip timer 555, the frequency of which is determined by the nominal values of timing elements R7, R8, C7. Oscillator output enters the amplifier via the filter-C4-R4 R3.
Due to the pulsed power preamplifier output thereof has a square wave with an amplitude proportional to the radio signal received by an antenna. Capacitor C6 removes the DC component of the signal which is fed to an amplifier on a chip IC2b through sensitivity control potentiometer R. The gain of this stage is determined by the resistors R11 and R12.
Since the amplifier has single supply (+ UT) at its non-inverting input with the signal supplied bias voltage equal to half the supply voltage (UT + / 2). Shapers of this voltage serves as a repeater on the chip IC1 and, the source for which is given by the level divider R9-R10.
The amplified signal from the output 1S1 enters the controlled analog switches that make up the chip IC3. Keys are switched on IC1 signal generator and peak detector are synchronized. To reduce the equivalent resistance of the keys are connected in parallel. Phase shift pulse supplying preamplifier offset R15-C11 chain.
In this embodiment, «Hunter bugs» is applied to the LED 10 LEDs and an additional acoustic signal. Using mostly surface mounting, the unit managed to place in the body is much smaller than previous.
LED indicator operates specialized chip LM3915 (IC4) with a logarithmic scale. Microchips family LM3914, LM3915, LM3916 («Dot / Bar Display Driver») differ from each other only in the values of the components of internal reference circuit resistances which are input to a comparator. The device I found most suitable for this purpose LM3915, nevertheless, it is possible without any modification to replace circuit LM3914 linear scale.
LM3915 implements logarithmic scale with steps of 3 dB. In this circuit are connected 6th and 7th findings therefore require full swing input voltage of 1.25 V. For the LEDs connected to respective terminals of IC, thresholds given in the table. LED current can be «programmed» via the resistor R18, the resistance is 22 ohms delivers an output current of about 1.3 mA. The device uses high-brightness blue LEDs, the glow of which is already noticeable at the current 0.5 … 2 mA.
Acoustic (sound) indicator consists of the classic stages integrator comparator key (IC5a, IC5b, T2). The quiescent current at a voltage of 9 V (without headphones) is 0.3 mA, 2×20 Ohm impedance headphones, headphones, both of which are connected in series by connecting the output jack of unconventional Cs — 0,5 mA. Changing the frequency of the sound signal in the range 0 … 1.25 is 0 … 1300 Hz. At rest (no signal) in the phone or there is no sound or hear rare flicks at a very low frequency.