Conex Club Magazine - no.6 - 2002

Conex Club Magazine - no.6 - 2002 - TDA 7381 - Car deck power amplifier (4 x 25W)

Reading time: 3 minute

The main topics presented

1. Professional manufacture of printed circuits in our own laboratory

The readers were able to follow in the technology section various articles oriented towards the issue of electrical design and evaluation of printed circuits.

They wanted to be a necessary introduction of the Romanian hobbyists regarding the particularly subtle phenomena that arise within the apparently common banal printed circuit boards and will be followed by other materials on this topic.

Starting with the article entitled "Rapid manufacture of printed circuits by engraving technologies", the authors wanted to approach the issue of PCB structures from another angle, namely from their manufacture.

Any electronics enthusiast has asked his question at least once "How can you make a wiring at a level close to the factory with the modest laboratory resources at home?".

The answer will be given starting with this article, an article that highlights in a brief but clear manner the PCB manufacturing process in a laboratory developed in your own home.

From the beginning it must be said that there are some critical points in the production process (exposure / development / corrosion times, concentrations of substances, etc.) and that there are also more expensive equipment (quality laser printer) or difficult to obtain / build (exposure to ultraviolet - UV radiation).

2. Frequency meter

In order to be able to make a frequency meter that works up to frequencies of at least 60MHz, integrated circuits made in HC technology, more precisely of the type, were used for the basic counter 74HC490.

This integrated circuit is a double decadent divider. Some specimens have "read" frequently up to 70MHz. No specimens were found that did not work until 66… 68MHz.

Therefore, it is recommended that the first counter (CI-8, figure 1 of the article) be "selected" from the 6 required copies.

Figure 1 of the article shows the wiring diagram containing the basic counter, the decoders for the LED displays as well as the input preamplifier-TTL signal former compatible with the input of the first counter.

The transistor was used as a preamplifier BF215 but, with the same performance, a transistor of the type can be used BF173.

3. GPS - Introduction

It has become widely used worldwide to accurately determine the geographical location of a particular receiver. The system, initiated and built by the US Department of Defense is known to the general public through its initials GPS meaning (Global Position System).

Using the physical properties of electromagnetic waves, a system of 4 artificial satellites of the earth permanently transmits to the user receiver information regarding the place where it is on the Earth's surface or in space.

4. TDA 7381 - Car deck power amplifier (4 x 25W)

Technical data:

  • output power: 4 x 18W / 4 Ohm at 14.4V / f = 1kHz, THD = 10%;
  • restriction input detector;
  • minimal distortions;
  • low output noise;
  • depending on STAND-BY;
  • MUTE function;
  • MUTE function when the supply voltage drops below normal parameters;
  • internal gain 26Db;
  • 180mA quiescent current;
  • 25 connection pins;

Diagnostic function for:

  • entry into limitation;
  • short output and ground;
  • short output and Vcc;
  • overtemperature blocking;

Protection at:

  • short circuit output-Vcc and GND output;
  • temperature rise;
  • high inductive load;
  • overtemperature;
  • reversing the supply voltage.

5. TV micro-transmitter

The transmission of the video signal from the camera to the receiver can be done by cable or by electromagnetic waves depending on the place where the camera was installed and the location of the receiver.

Currently, the installations that give the possibility to mount the camera in various places to correspond to a desired surveillance are most often used.

The presented montage is modulated in frequency and transmits only video - it was made by E. Paulitsch - OE6ACG if it refers to a transmitter working in 23cm band (1278… 1296MHz) which can develop a power of about 70mW.

6. Position indicator

The assembly is mainly intended to indicate the position of an antenna or group of antennas based on an optoelectronic digital transducer mounted on a pillar (outdoor), and a display circuit with circularly arranged LEDs, which indicate the orientation directions (indoor).

It may have other applications that require angular displacement transducers.

In the application presented, an optoelectronic angle-number transducer generates the numerical code corresponding to a certain angle. This transducer is made with the help of an encoder disc with optical slots, which rotates with the antenna axis in the space between a group of infrared LEDs and a group of phototransistors that receive the emitted radiation.

7. Timed voltage measurement relay

The voltage measuring relay presented in this article performs both the function of a minimum voltage relay and that of a maximum voltage relay.

The operation of the electromechanical relays can be delayed, in the interval 0… 25,5 seconds. The measuring relay is thus included in the class of time relays.

The measured voltage is displayed on a three-digit LED display with a common cathode.

We opted for a circuit MAX150, with the following features: conversion time 1.34 microseconds, supply voltage + 5Vdc, input voltage 0… + 5Vdc, 2.5Vdc built-in voltage reference, does not require connection interfaces with the microcontroller.

The floor is protected against long-lasting overvoltage by diode D1 and ultra-fast fuse SIG1.

The control block, made with the 89C51 microcontroller, which manages the measured voltage, the signals for lighting the displays and warning lights, the signals for controlling the electromechanical relays, the commands from push-button group K1 - K5, and the circuit for the serial interface made with RS232 for the case of connecting the relay to a process computer.


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