Conex Club Magazine - NR.12 - 2003

Conex Club Magazine - no.12 - 2003 - RF detector - For "spy" transmitters

Reading time: 4 minute

The main topics presented

1. Universal USB interface - Velleman K8055

Velleman offers electronics a kit that allows familiarization in the field of applications where the supervision of operation and control of complex installations is ensured through the computer.

Because analog or digital signals, received from various sensors, transducers or other electrical equipment, cannot be recognized and interpreted by the computer, it is necessary to insert an interface that makes communication possible.

To cover a wider class of experiments, the module was provided with 7 inputs (5 digital + 2 analog) and 10 outputs (8 digital + 2 analog).

The number of inputs / outputs can be extended by connecting additional interfaces to the computer's USB port. All communication routines are contained in DLL (Dynamic Link Library).

There is also the facility to write Windows applications in Delphi, Visual Basic, C ++ or any other 32-bit development application that can communicate through the DLL.

2. RF Detector - For "spy" transmitters

There are specialized services in the search for transmitters, in discovering the most ingenious possible hiding places, but this can also be done individually using high-performance equipment or having somewhat more modest qualities.

The presented article aims to make a general description, as well as a practical presentation of an easy to make model.

First, some essential things for the RF detector must be established:

  1. To cover as high a frequency band as possible;
  2. Its sensitivity should not be too high to be able to accurately locate the object that hides the transmitter;
  3. During the search to emit a sound that can activate an transmitter with automatic operation;
  4. To indicate optically and acoustically the moment of approach;
  5. Consume as little current as possible from a portable voltage source.

The printed wiring has the size of 75 / 55mm and the drawing shown in figure 2 (in the article).

3. LM3876 - Audio Power Amplifier SeriesHigh-Performance 56W Audio Power Amplifier w / Mute

The LM3876 circuit is a high performance audio amplifier capable of delivering 56 continuously on a 8 Ohm load with THD + N = 0.1% in the 20Hz… 20kHz band. The integrated circuit is designed and made by NATIONAL SEMICONDUCTOR.


  • continuously delivered power 56VV / 8 Ohm;
  • 100W instantaneous output power;
  • signal to noise ratio> 95dB;
  • "mute" input;
  • protection of the output in case of short circuit to ground or to the supply bar, by limiting the current;
  • protection of the output to transient overvoltages due to inductive loads;
  • undervoltage protection | Vee | + | Vcc | <12V to avoid transient start and stop regimes;
  • encapsulated in TO 220 with 11 pins;
  • wide supply voltage range 20V… 94V.


  • stereo component;
  • CD cassette;
  • active speakers;
  • surround-sound amplifiers;
  • televisions.

4. Electronic thermostat

In automation systems, automatic regulation loops are frequently used, among which are temperature stabilizers, also called thermostats. The field of application is wide, in industry, but also at home.

The thermostat shown is intended to work in the home, to stabilize the temperature in a room.

Can order an apartment boiler or an electric heater. Compared to mechanical thermostats, it is superior in the accuracy of the actuation thresholds, the low hysteresis and the maintenance of the characteristics unchanged over time. For convenient use it contains a digital thermometer.

The temperature sensitive element consists of two integrated circuits LM 335 (D5 and D6) connected in series. LM 335, connected in the circuit as a Zener diode, has at the terminals a linear voltage proportional to its absolute temperature, the proportionality factor being 10mV / K.

Binding two circuits in series increases the sensitivity of the transducer to 20mV / K, respectively a value that can be further processed without special precautions.

5. Access control keyboard for electronic lock

The installation described is a "low cost" variant of the controller with keyboard dedicated to "stand-alone" (independent) access control systems.

The access (code) combination of most current controllers is chosen (programmed) directly from the keyboard, they are based on the operation of a microcontroller.

That's why they are a bit more expensive. The peculiarity of the presented assembly comes from the fact that that code combination is changed in "hardware" mode, with the help of some straps.

Admittedly, this method can sometimes become inconvenient, accessible only to specialists, but the inconvenience is offset by the low realization price.

The electrical diagram of the application is relatively simple (figure 4 of the article), based on a logic integrated circuit from the series CD - 4022, Johnson counter, divisor with 8, a BS170 type VMOS transistor and some passive components.

The code combination consists of 4 digits, but can be extended up to 7.

6. Liquid crystal displays - LCD (I)

It presents a series that presents the physics of liquid crystal displays (LCD), testing and identifying the terminals for them and various applications.

Liquid crystal physics

In order to better understand the composition and operation of liquid crystal displays, LCDs, it is necessary to briefly go through some notions of liquid crystal physics.

Liquid crystals are organic chemical compounds, which have two distinct states in the liquid phase: the anisotropic state and the isotropic state.

The anisotropic phase (also called mesophase) is present immediately after the melting point, in the sense of increasing the temperature. The liquid has some special properties, the most important being double refraction, which occurs (normally) only in solid crystals.

If the temperature continues to rise, the liquid crystal enters the isotropic phase, after exceeding the critical point. In this phase it no longer has special properties.

Several categories of liquid crystals are known - in chemistry. Nematic liquid crystals are used for the production of LCDs.

7. ICL7660 and MAX1044 - CMOS voltage converters with switched capacities

The two circuits are used to make low power voltage converters using switched capacitors (capacitors).

The integrated circuits ICL7660 (Intersil) and MAX1044 (Maxim) are voltage converters with switched capacities, made in CMOS technology, which can reverse (change the polarity of the input voltage), double, divide or multiply a positive input voltage.

There are many practical applications in which it is necessary to have several values ​​of low power voltages, some of which are even negative and not just one.

The transformation of this unique positive voltage that we have into other voltage values ​​(some even negative, as we mentioned before) is possible and can be solved practically in several ways, simpler or more complicated.


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