Conex Club Magazine - no.2 - 2004

Conex Club Magazine - no.2 - 2004 - Switching voltage stabilizers - LM2575, LM2576, LM2577

Reading time: 4 minute

The main topics presented

1. Two-wire intercom - Full duplex

An interesting communication system, which is recommended to be used as an intercom for the office, at the counters of banks or public institutions or for domestic utilities such as the supervision of babies in an adjoining room.

Conex Electronic designed and made a intercom that works in duplex system. This means that you can simultaneously talk and listen to your partner.

For the interconnection of two intercoms (CNX207 kits) a line similar to those used in telephony can be used, using a two-wire cable with a section of 0.5mm2.

With relatively low line impedance, on the order of 1 KOhm, there is no need to use shielded cable. The device is powered from the 220V / 50Hz network and consumes about 2W.

The audio power is 0.5W, enough to make a light sound connection. The drawing of the printed wiring is presented in figure 3 (from the article) on a 1: 1 scale.

Description of the wiring diagram

The microphone is connected to terminals K1-K2 after which, by means of the semi-adjustable P1, the resistor R2 and the capacitor C3, the captured audio signal is applied to the reversing input of the amplifier IC1A (LM358) where the necessary amplification is performed.

2. Alternative method of ultra-fast manufacturing of printed circuits (Toner Transfer System)

The article explains in detail a new method of making, in a few tens of minutes, the wiring printed in prototype mode.

In previous articles, published during 2002, methods and techniques for manufacturing printed circuit boards in their own laboratory or in specialized laboratories were presented.

The author, together with his collaborators, provided details of a technical nature both for the method of making the wiring by mechanical etching of the conductive foil, and for the professional photo, which uses laminates covered with photoresist foils and exposure equipment using radiation. ultraViolet

Disadvantages of the professional photo method (in addition to the many benefits well known to specialists and enthusiasts) are given by the use of special equipment, the use of dark rooms or premises, the use of several chemicals, some toxic and extremely dangerous or sometimes the use of photographic negatives.

In this article the author wants to present a new technique (figure 1 of the article), included in the category "direct etch" and useful both for the extremely low processing time (15 minutes, for going through all the phases), and for the very good resolution that the final wiring can have - 6mil (0.15mm).

3. Switching voltage stabilizers - LM2575, LM2576, LM2577

The integrated fixed / adjustable stabilizers (ADJ) operating in switching, from this family, are manufactured by Nationai Semiconductor and are of the type "step-down" (voltage reducers), LM2575 / LM2576 and respectively "step-up" (voltage lifters): LM2577.

They have a modest selling price, depending on the type, and are distinguished by an extremely simple use (requiring very few external components) and a very wide range of working voltages.

This series of stabilizers is also distinguished by very high yields (over 80%) and by a decrease in dissipated power, which is strong enough arguments for their use instead of the classic 3-pin stabilizers.

Usually, these stabilizers do not require a cooling radiator, and if it is still needed, it is much smaller than in the case of integrated 3-pin stabilizers.

These integrated circuits are delivered in either 5-pin TO220 capsule (LM257XT) - version with adjustable voltage (ADJ) or in 4-pin TO-3 capsule (LM257XK) - having a fixed output voltage.

The LM2575 series of integrated switches represent voltage stabilizers (step-down or buck) capable of delivering a maximum load current 1A, with a very good stabilization coefficient.

The circuits are available in a fixed version, with output voltages of 3.3V, 5V, 12V and 15V and the adjustable version (ADJ).

4. TDA8571J - 4 x 40W audio amplifier

TDA8571J is a monolithic integrated circuit, designed and produced by PHILIPS, a class B audio power amplifier assembled in a 23-pin SIL type capsule. It contains 4 amplifiers configured in the bridge, each having the gain (fixed internally) of 34dB.

Among the circuit performances we can list the following:

  • requires very few external components;
  • high output power;
  • low offset voltage on outputs;
  • internally fixed voltage gain;
  • diagnostic facilities (distortions, short circuit on outputs, overtemperature);
  • good rejection of the supply voltage ripple;
  • mode selection input
  • operation (operational, "mute", "standby");
  • short-circuit protected on outputs (to ground, to power supply, during pregnancy);
  • low thermal resistance.

5. 3 triac card (3 relays) with RS232 interface for PC

There are two applications for ordering and supervising computer-assisted domestic or industrial processes.

Industrial processes or PC-driven or household utilities are no longer a novelty.

Imagine that you are leaving home and programming your computer to turn on the twilight light a few minutes before arrival, in a darker hallway.

Or to turn on / off the boiler at certain hours, automatically. Or imagine how useful it would be to use the PC to simulate your presence in the house, when in fact you are on vacation for a few days (turning on / off the lights automatically of the TV, radio, etc.).

Not to mention the usefulness of the application for farmers (at a poultry house the simulation of a solar cycle).

The described assemblies allow the control via the PC (via RS232) of three power loads, respectively through an interface with output on power triacs or on 10A / 220V relays. The command is made optically isolated from the serial port of the PC.

The applications are based on the information taken from the French specialized magazine Interfaces PC, no. 9, under the title "Port serie - 3 sorties sur triacs", signed by David REY.

It presents in the mentioned publication a serial interface with output on 3 power triacs and the appropriate software written in Delphi 4, in two variants: on / off switch type application for each channel, and the other, a bistable switch with programmable timer (in hh: mm: ss, start - stop format) that uses the PC system time.

6. Automatic for cyclists

The machine was presented in the French magazine Electronique et Loisir Magazine in August 2003 and can prove to be a device particularly useful for amateurs of cycling or sports.

As the headlight bulbs are powered directly from the bike's dynamo (direct current generator, pulsating), it is obvious that if the bike's speed is slowed or braked, they light up very little or not at all.

There is thus the danger of an accident at night. The assembly ensures a relatively constant voltage supply of the bulbs, by automatic switching, depending on the situation, from the power supply from the dynamo, to the one on batteries (4 elements of 1,2V type NiCd) and vice versa.


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