Tehnium Magazine no. 3 - 2004

Tehnium Magazine no.3 - 2004 - Automatic position maintenance system

Reading time: 4 minute

The main topics discussed

1. Schmitt tipping circuit

Tilting circuit (triggers) Schmitt is very widespread in electronic assemblies - including those designed by amateurs - due to its simplicity, as well as safety and precision in operation.

Viewed as a circuit for transforming electrical signals, it fulfills the function of transforming an input signal Ui variable in time according to a certain law, into an output signal Uo of "crenellated" shape, whose levels (upper level, respectively the lower bearing), as well as the transition durations between them, depend exclusively on the internal construction of the trigger.

2. Design a Hi-Fi audio amplifier

In the field of audio frequency amplifiers, a multitude of technical solutions, more or less sophisticated, have been approached and practically realized.

There is practically three groups of audio amplifiers, namely:

  • amplifiers with transistors;
  • hybrid amplifiers (integrated circuit + transistors);
  • amplifiers with specialized integrated circuits.

Obviously, it is also realized audio amplifiers with electronic tubes, expensive and high-performance solutions for some audio parameters.

Unfortunately, purchasing electronic tubes has become a major difficulty for the amateur HI-FI hearing aid manufacturer. For this reason, the approach was proposed for the design of a power frequency amplifier with bipolar transistors, which any amateur manufacturer who wants to learn electronics, to improve, to become at least the professional technician of tomorrow, to understand, address and complete successfully.

The article is addressed mainly to electronic technicians who want to present a graduation project of an industrial high school in terms of the professional competence exam, but can be successfully approached by any amateur electronics technician.

It is also good for professional engineers in the field, who do not hesitate to update a series of knowledge "left in the cellars of memory".

3. Calibration of RLC measuring bridges

The bridge for measuring resistances, capacitances and inductors is a very good measuring device for the amateur builder. Its use is simple and the accuracy of the measurements is more than satisfactory.

Also, its construction is relatively easy, with minimal expenses. The only problem is finding the values ​​for the parts used as a standard, which must be as accurate as possible, so selected.

But the most delicate operation remains calibration. One possibility is to use a box with decadent resistors, but it means using the goodwill of a school or enterprise laboratory, which is often impossible for some amateur builders.

For them, the article presents a more "old-fashioned" method, but more accessible and precise enough. It's about "resistive wire" calibration method.

4. Automatic position holding system

The application possibilities are numerous, generally where the control of a certain actuating or signaling organ can be obtained by varying the illumination of a photosensitive element.

The system fulfills the function of keeping constant the value of one or more parameters of a technical process.

The appearance or disappearance of the light beam falling on the photosensitive element is sufficient to control a mechanical or other action. The assembly was used at a packing machine, where the movement of the foil must be maintained between two limit positions.

5. Time relay with coupling delay

The time delay relay for coupling finds its many applications in practice.

Of these, only one is given some examples where the presence of such a relay with coupling delay is absolutely necessary:

  • to high power electronic tubes, which are still used today in broadcast equipment or HI-FI audio amplifiers, it is necessary to first heat the filaments and only then to be supplied with anodic voltage;
  • in industry, medium and high power asynchronous motors they initially start in the star. After a few seconds, when they have reached nominal speed, they switch to a triangle;
  • asynchronous motors with coiled rotor they also require the gradual short circuiting (in three or four stages) and timing of the rheostat resistors in the rotor circuit.

This operation can also be performed manually, but it is indicated to use timing relays with delay in coupling;

6. Bread oven automation equipment

Most private bakeries and pastry shops use electric ovens for baking bread and other pastries.

Due to the constructive and functional complications, the very high cost price, gas heated bread ovens are usually found only at large bread factories.

In most cases, the temperature control of the electric oven is performed with bimetallic thermal contacts. They are inaccurate, have a short lifespan and fail frequently. The defect itself and their replacement do not matter much in the last resort. What matters is the high cost of the batch of bread or compromised pastries.

The equipment presented in the article is cheap, and can be easily made. Once put into operation, it works for years without any maintenance or adjustment.

The operation is precise, the deviation from the prescribed temperature not exceeding 2-3C. This deviation is insignificant under the given conditions.

7. How do we build a 50g mule?

With a little patience and skill, you can build a scale for small weights (up to about 50g), simple and quite accurate. It will be very useful in different situations.

The base material is a piece of cardboard, thin but strong, or other material, such as rigid plastic or thin sheet.

8. Digital capacitance meter

It shows a capacitor that adapts (connects) with a digital multimeter.

The measuring range is 1 nF - 500 nF, a fairly wide beach. It consists of two parts: an oscillator with NE555 and a capacitance - voltage converter, namely with LM 2907N-8.

A frequency meter, a digital multimeter and several 1 nF 1%, 500 nF 1% standard capacitors will be used for calibration.




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