Conex Club Magazine - no.6 - 2004

Conex Club Magazine - no.6 - 2004 - Special alarm with 2 zones made with uc PIC16F84A

Reading time: 3 minute

The main topics presented

1. PC interface with digital analog inputs / outputs

A complex assembly, useful in industrial automation and measurement applications, as well as in laboratory ones. The assembly is controlled by a PC and offers several digital and analog I / O resources.

K8000 card, made by the Belgian company Velleman, allows the communication of the PC with any peripheral. It has 16 digital connections, optically isolated, which can be programmed as inputs or outputs, by 8 analog outputs, plus a precision one, 4 analog inputs and an I2C bus connection.

The card connects to the parallel port of the PC and allows the development of many simple or complex applications in Turbo Pascal, C ++, Qbasic for DOS or Visual Basic for Windows. It comes with a number of libraries and small examples.

2. 13.8V / 4A stabilized source

The source is based on a series linear stabilizer. Its peculiarity consists in the use of specialized integrated circuits, as a series regulating element, instead of power transistors.

The practical scheme presented can supply a voltage of 13.8V, with the possibility of adjustment, at an output current of 4A.

3. Electronic, analog cipher

Although many circuits have been published that control access through the keyboard with code that commands, in case of correct entry of the code (unlocking an electromagnetic lock), the circuit presented has the advantage that it is quite cheap and, based on a monostable circuit (LM555N), does not require software.

The monostable used can discharge a current high enough to avoid the need for an additional amplifier (with a transistor) for the relay that controls the solenoid of the electromagnetic lock.

By using 16 keys of which three (four), entered in the correct order unlock command, and 13 (12) keys return the circuit to its initial state, the combination is not very easy to guess.

Obviously, compared to microprocessor systems, the device is a bit more complicated and less flexible, but for common applications it can be used successfully.

4. Frequency synthesizer - VHF (IV) - Elements of practical realization, commissioning and final adjustment

In this article, the last in the series, are presented elements that are of interest especially for those who want to make a practical synthesis of frequencies that are the subject of this series of 4 articles.

At the beginning the printed wiring of the synthesis part of the actual frequency is presented (PLL block) and the control block for the operation of the entire assembly, equipped with the ATMEGA8 microcontroller.

In the second part some "secrets" are presented in the form of a brief methodology, which allows the commissioning of the frequency synthesis and its calibration-regulation even in the absence of sophisticated equipment.

5. Special alarm with 2 zones made with uC PIC16F84A

It presents a alarm with two inputs for the sensor loop, timed and instantaneous, made with a PIC16F84A microcontroller.

A few issues ago, an alarm control panel with 4 "low cost" zones was published, made with discrete components.

As disadvantages it did not allow the exact setting of system times (the adjustment was made from the trimmer), and the code was "programmed" hardware.

The alarm presented in this issue allows you to change the code on the keyboard, setting system times with good accuracy (with a step of one second), in the programming phase of uC and presents some features that offer a high degree of security to the user (such as inhibiting system disarmament if not entered the reserved area of entrance exit).

6. 4W amplifier - with MOSFET transistors

There is an amplifier for audio signal made entirely with MOS-FET transistors, respectively BS170 and BUZ72, easy to buy.

The diagram does not require comments, the only thing to note is the polarization mode with constant voltage, 6V, of the gates of the driver transistors T1 and T2. The latter, together with the T3, is mounted on a radiator.

The load (speaker) can be between 4 and 8 Ohms. The signal level applied to the input is adjusted from the semi-adjustable P1. The diagram also shows the voltage levels measured relative to the ground, at various points, which can help to put the assembly into operation correctly.

The most relevant technical data of the amplifier are:

  • Power supply at 24Vdc;
  • Absorbed current I = 1.28A;
  • Power measured under load conditions 8 Ohm and THD = 10% at 1kHz: P = 4W "sine";
  • Power measured under load conditions 8 Ohm and THD <20% at 1kHz, P = 5.5W "musical";
  • THD distortions at 1W / 8 Ohm / 1kHz of 4.2%, and at 0.1W of 1.3%;
  • Noise signal ratio S / N = 89dB;
  • Amplification frequency: 83Hz… 155kHz.


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