Conex Club Magazine - no.5 - 2004

Conex Club Magazine - no.5 - 2004 - Protective coatings for implanted plates (conformal coatings)

Reading time: 4 minute

The main topics presented

1. Introduction to digital potentiometers (I)

The potentiometer is a variable resistor at which the user can continuously or gradually change the electrical resistance in a certain interval [Rm, RM], interval determined in the manufacturing process.

Considering how the resistance variation is achieved, potentiometers fall into two categories:

  • mechanical potentiometers, in which the variation of the resistance is obtained by the mechanical displacement of a mobile contact, called cursor, on the resistive element;
  • digital (electronic) potentiometers, in which the variation of the resistance of a resistive network is modified by means of electronic switches as a result of some digital commands.

Depending on the role they play in electronic circuits, potentiometers are classified into:

  • control potentiometers, used to change and control the circuit parameters during operation of the device (eg change of amplification (volume), tone, balance of a radio receiver, change of brightness or contrast in the case of a TV, change of voltage and current of a power supply, etc. );
  • adjustment potentiometers (adjustment), used for adjusting the parameters of electrical / electronic circuits to the nominal value; these potentiometers are generally used to compensate for the deviation of some electronic components from the theoretical values, values ​​with which the respective circuits were dimensioned in the design phase; in practice they are also called "semi-adjustable".

2. AD7376 +30 V / +/- 15 V Operation - 128-Position Digital Potentiometer

The AD7376 is a 128-bit digital potentiometer produced by Analog Devices as a replacement for the unreliable classic counterpart components.

The circuit must be regarded as a fixed resistor, traversed by a cursor with 128 possible contact points. The cursor position is determined by a 7-bit number, transmitted on a 3-line serial interface.

AD7376 is produced in four variants, with the resistance between terminals A and B of the integrated circuit having the following values: 10 KOhm, 50 KOhm, 100 KOhm, 1 MOhm.

The temperature coefficient is -300ppm / ° C, and the resistance between the cursor and each terminal varies linearly, depending on the numerical value present in the latch (figure 2 in the article).

3. Protective coatings for implanted plates (conformal coatings)

The materials known as "conformal coatings" have been known and used for more than 30 years in the world, but in Romanian there was no way to define them except through an imperfect and incorrect translation - compliant coatings (with what? ) - or by a phrase such as "coatings adapted to the shapes of the components", difficult to use, especially speaking.

Going over the difficulties of definition, let's talk about the materials themselves and how they are used.

"Compliant" coatings are resin systems, usually dissolved in volatile solvents with a precisely determined evaporation rate. The resins are selected according to their electrical and thermal properties and their ability to form, after polymerization, a resistant film to protect the surface of the component plate, at the demands of the environment.

In general, synthetic resins such as polyurethanes, acrylics and epoxy are suitable for these requirements. If there are special demands at high temperatures, special resins are used, such as those based on silicones or polyimides, and for special dielectric strength, polystyrene resins are used.

4. Automatic lighting (with IR barrier)

A simple idea, such as an infrared (IR) barrier, has applications in various fields (especially domestic ones) with great practical utility.

The lighting machine uses the IR barrier principle, and in the presented application, it offers a saving of electricity, the destination being the storage rooms in the households.

The device can also function in electronic security systems, as well as a control element for the automatic opening of an access door in a building.

The oscillator made with the G2 gate provides pulses with a frequency of 36kHz with which the infrared LED (IR LED) LED2 is controlled. This is the working frequency (to which it is sensitive) of the S1 sensor, type SFH5110, optically coupled with LED2.

In the situation where the optical "connection" (barrier) works, the capacitor C1 is charged, the gate G3 has a logic 0 at the output, and the relay Re1 is at rest.

When the optical beam is closed, the transistor Q1 opens, discharging through the diode D1 the capacitor C1, which leads to the tripping of the relay Re1.

5. TV modulator - for surveillance video cameras

If you own a newly purchased surveillance video camera and do not have a monitor, the solution is to watch directly on a TV that also has video input (RCA type), as are most devices currently sold.

But what is to be done, if it is of an older generation, possibly "dust was deposited on it", being unused even without such an entrance? The solution: a simple video modulator!

The video modulator presented provides significant savings in a home video surveillance system, unpretentious, with a single video camera (or more, if an electronic switch is used).

An ordinary A / N TV can be used for viewing, may be taken out of use, which usually does not have a special input for the video signal (the signal provided by the video camera).

The assembly converts this signal into one similar to the one coming from the antenna, the TV set being used without modifications. The mounting output connects to the TV antenna connector.

The mounting input connects to the video output of the used camera. Very simple, the assembly is essentially an oscillator with a transistor, the oscillating circuit being in the collector (RC group).

6. IC-Prog serial programmer - for uC PIC and 24Cxx memories

What is noteworthy about the serial programmer scheme for PC PIC in the article is the use of a specialized circuit, MAX232, for PC interface (RS232) - microcontroller.

The latter, for those who do not know, works with TTL signals. MAX232 contains buffers and RS232-TTL voltage converters.

Also with the role of buffers and signal formers, with the precise purpose of eliminating the noises that can falsify the data circulating in both directions (PC - uC and vice versa), the NAND gates (YES-NO) contained in the integrated circuit capsule are used. CD4093.

7. Up-grade to three-relay card (II)

The article presents a graphical interface made with the help of Visual Basic 6 program, through which three switching elements can be controlled directly through the computer's serial port.

As a hardware element is used the card with three relays (three triacs) presented in issue 02/2004 of the magazine. The order can be made manually or by calling time functions related to the clock and date of the computer system.

8. Voice activated electronic switch

The circuit shown is a electronic relay switch, activated by an audio signal applied to the input, higher than 5mV.

The activation of the relay is performed only for signals located in the frequency band 50Hz… 3kHz, corresponding to the human voice. The operational amplifier offset is adjusted from RV1. C4 provides a delay in actuating the relay. For more demanding applications it is recommended to use a filter for 1 kHz.

There are many applications in the field of automation or security systems that can be imagined (turning on a standby light, by voice, automatically or activating an equipment from the noise captured by the microphone of a surveillance video camera or as a sensor in an alarm system , there are only three applications as an example).


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