Conex Club Magazine - no.3 - 2006

Conex Club Magazine - no.3 - 2006 - Tricks and applications with LM3914 (LM3915)

Reading time: 4 minute

The main topics presented

1. Mini-timer with bargraph

Probably this application idea will delight the hunting electronics of useful, simple and cheap electronic mini-projects. A non-specific application with LM3915 (_4) (voltage voltmeter with linear / logarithmic LED display) is the timer.

As LM3914 (_5) contains a current source (available at pin 7), the idea is based on charging an electrolytic capacitor, C3, at constant current, through R3 (by pressing the PB1 button). After releasing the button, C3 is discharged through the resistive group R4 - semi-adjustable R7; the voltage on this group decreases exponentially, with a time constant equal to C3 * (R4 + R7).

This voltage variation also applies to the measuring pin of the LM3914 (_5), a variation that is shown in descending order by the LED bargraph (from D10 to D1).

The time elapsed from the opening of the PB1 button to the complete download of the C3, is displayed in a selected mode (pin 9, MODE, plus - bargraph, offline - a single LED).

From here, the application can be developed according to its own design requirements, by using optocouplers instead of LEDs D1 and D10, and through an interface with logic gates to generate various commands.

2. Night light with white LEDs and variable brightness

The current global trend is to replace incandescent bulbs (be they krypton) in portable, high-power (white) LED lamps.

The diagram denotes a way to make a lamp with 10 super bright LEDs (white or blue, of several thousand gcf), arranged on the area of ​​a circle for example, each controlled by a constant current generator, so that the emitted light is uniform.

This calls for an integrated circuit whose outputs are drivers for LEDs, constant current, respectively LM3914.

To change the brightness of the LED lamp, the number of lit LEDs can be changed from a potentiometer!

3. Capacity indicator for Li-Ion batteries

With the help of the attached diagram you can build a tester for Li-Ion batteries, with nominal voltages of 3.6V (such as those in phones), 7.2V or 12V.

Depending on its rated voltage, the battery to be tested is connected to the GND and A, B or C terminals, respectively.

On a bargraph with 10 LEDs it is displayed, in percentages, if the battery under test is fully charged (100%, all LEDs on) or discharged (with 10% of the nominal capacity, one LED on).

To calibrate the mounting, on the input pin (5) and the reference pin RHI (6) of the LM3914, it must be measured, with a charged battery, 1.25V, and on the reference pin RLO (4), 1.135V. The current through the LEDs is a few mA.

4. Tester for operational amplifiers

Its own electronics workshop will be equipped, by building this electronic assembly, with a high-performance tester for operational amplifiers and comparators. (in DIP8 or DIP14 capsule).

Due to the complexity and diversity of the functions implemented by the linear integrated circuits, the realization of a universal tester (like the catheters of the past for testing electronic tubes), is practically almost impossible.

But if we limit ourselves to operational amplifiers (AO), the situation is greatly simplified, because, despite their diversity of coding, the basic function retains common elements, and in addition, the arrangement of the terminals is compatible pin-by-pin, a few integrated circuit groups, thus making it possible to include a very large number of types of operational amplifiers in a unitary and simple test system.

It should be noted that this tester does not aim to measure the parameters of the amplifiers, but only to test their operating condition, to quickly remove defective components.


  • functionally tests AO and / or single, double (DIP8 capsule) or quad (DIP14 capsule) comparators;
  • pin-to-pin compatibility with all general purpose AOs;
  • the test result is displayed optically, with LEDs;
  • test frequency: 1 Hz.

5. Telephone alarm

The device described in the article is a useful accessory where two bad phones are used, connected on the same line. Powered by its own source (two R6 batteries) the device indicates, by lighting a green LED, the use of a telephone.

A second red LED indicates the presence on the line of a branch phone while using the main phone.

The device is especially suitable when one of the phones is electronic. The impedance differences of the two devices are substantial from the point of view of the line and the detection of an electronic bypass phone is much more difficult.

The usual combinations of series / parallel LEDs are not enough for good signaling.

6. Parallel connected telephones - restrictions and confidentiality of calls

With the help of the electronic assembly presented, you can ensure the confidentiality of calls on phones connected in parallel, on a line. There are also some restrictions regarding the calls made.

The application eliminates the inconvenience created by phones connected in parallel (someone picks up the handset over an ongoing call!). The installation is very suitable for small business offices, apartments or large houses, equipped with a single telephone line (wired).


  • modular design (configurable for the desired number of phones connected in parallel);
  • the desired number of phones with (slave) or without (master) restrictions can be programmed;
  • from the internal lines defined as master you can make and receive calls confidentially and you can listen to a defined slave phone;
  • internal lines defined as slaves cannot make calls (do not receive a tone), but can take an external call directly (when the phone rings). From this line you cannot listen to the call of another phone connected in parallel (through the module).

7. PIC16F84 microcontroller (I) - use and programming

A new series in Conex Club magazine, dedicated to PIC microcontrollers, especially for PIC16F84. This time, in a different approach.

The author, Neboja Matic, MikroElektronika, is an official consultant to the UK manufacturer, Microchip, and a well-known contributor to Conex Club magazine. Excerpts from his book are presented "PIC microcontrollers", translated into Romanian by Cristian Secrieru.

The paper is structured as follows:

In the first part a reference will be made to the construction of uC PIC16F84 (and in general of uC PIC), the basic external functional elements, the structure of a program and the program instructions.

The second part will present work programs with uC PIC, and in the third part various simple applications will be studied, corresponding to some ways of interfacing the CPU with various external components (LEDs, buzzers, relays, LCD display, etc.)

8. How to perform serial to USB conversion?

The most commonly used way to convert USB to serial communication is to use a specialized integrated circuit, FT232, produced by the company FTDI (Future Technology Devices International).

Made in small SMD capsule, LQFP, with 32 pins, FT232 includes a standard serial interface, available at 9 pins (pins 16, 18… 25) and a USB interface (pins 8 and 9).

The role of the circuit is to convert the signals from one format to another, and if a similarity is allowed, such as a MAX232 (or MAX3232) circuit performs an RS232 / TTL conversion.

It is presented in an example of a standard application, for educational purposes, how to use the FT232 circuit, to understand how to convert signals from USB to serial and to eliminate confusion, as a USB adapter - RS232, is a " simple adapter cable, only with connectors at the end ".


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