The main topics presented
1. Linear voltage regulators
In overwhelming proportion, the electronic assemblies that require power supply, continuous and stabilized, use linear regulators, positive or negative, from series 78_ and 79_ respectively.
The 78L (79L) variants are for low load currents, below 100mA, simple variants, for example 7805, for currents of up to 1A.
There are also variants of regulators such as 78M_, 78T_, etc., which operate at much higher currents. Useful for troubleshooting electronic assemblies equipped with these regulators is the synthesis presented (in the article) in drawings (meaning of pins) and table.
Careful! The regulators, depending on the polarity of the stabilized voltage and the type of capsule, have a different arrangement, from one model to another. Many electronics are wrong, mounting a 79L as well as a 78L.
2. "Low cost" fast charger for NiCd and NiMH elements
A cheap charger for charging batteries with Ni-Cd or NiMH elements, it is easy to build using timer 555 and some transistors. Charging is done quickly, and the scheme ensures the elimination of the unwanted effect of memory in NiCd cells.
Generally, NiCd batteries are charged protected (trickle charge), at a current of 1 / 10C for 14h. However, following the charging curves in the article, the elements can be charged quickly, following the dV / dt slope or monitoring the battery limit temperature (38 ° C for NiMH or 35 ° C for NiCd).
The presented scheme is designed for 4 elements, model "AA", with effective charging time 1h. For charging a rectangular signal with 3s charging and 10ms discharge is used.
Charging is done at a current of 70mA. SW1 allows the start of the fast loading process of approx. 1… 1.2h. LED1 signals fast charging in progress, LED2 signals normal charging (at 1 / 10C) when the power supply is connected.
Elements Q7-ZD1 - thermistor TH1 and R14 are optional; they monitor the temperature of the batteries. R14 is chosen for approx. 37 ° C.
Q5 is a transistor that supports at least a current of 250mA. It can be replaced with BC639 or BD135 for example, in which case it is recommended to mount a resistor in series with the base (4k7… 10k).
Instead of the transistor ZVN42066 can be used BS170, BS250 or BS108and ZTX107, can be replaced with BC547 or 2N2222. For TIP141, the equivalent is BDW83C.
3. Keyboard with conversion from decimal to BCD
With the help of simple diodes 1N4148 or 1N4007 you can make a decimal keyboard with BCD code output.
Most digital circuits work with BCD or hexadecimal code. It is very difficult to procure digital circuits that perform the conversion from decimal to BCD, and the solution presented is very cheap to implement.
Very easily, the scheme can be implemented for a 16-key keyboard, resulting in a value-to-value converter from HEX to BCD.
Careful! The keyboard used does not have to be a matrix type! The idea is taken from the website of electronic circuits made by Ron J., where the printed circuit from the article is presented, which can be implemented with push-buttons on a printed circuit for tests, with a trace of holes at 2.54mm.
When a key is pressed, the code generated in the BCD will appear at outputs A, B, C and D. A reset output also appears on the printed circuit (rst - to bring a controlled circuit, for example, to the initial state, by pressing the 0 key).
4. Vu-meter with transistors
A specialized circuit for designing a professional vu-meter, with linear or logarithmic display, made with specialized circuits (see for example LM3914 respectively, LM3915) costs a lot. In comparison, a transistor BC547 has an average price of approx. 10… 20 bani; a LM3914 it costs around 20 lei!
Ori the application presented will cost you, making it, even less than 5 lei, (in terms of design - making the printed circuit and purchasing components)!
The signal level applied at the input is adjusted within acceptable limits, from the 100 KOhm potentiometer. If you want a calibration of the display in dB and not a simple qualitative visualization of the audio level, the most convenient is the adjustment by comparison with an industrial product.
5. Musical ringtone
The circuit presented in the article offers two advantages: low consumption (battery power - 4 1.5V elements) and the possibility to change the ringtone according to the user's preferences.
The oscillation frequency, which is heard in the speaker 0.5W (impedance 8 Ohms), is comprised around a 2 and 5kHz. Jongland with capacitor values of 470uF and 1uF (from the gate oscillation network NAND 7413) other ringtones are obtained.
The sound is heard in the speaker, timed, for a period of a few seconds, by pressing the "Ring Button" button.
6. Switch-detector for the minimum load supplied in alternating current
The scheme detects a minimum current 250mA which feeds a load in alternating current. The detection is made without electrical contact between the circuit and the monitored load; basically, a U-shaped sheet coil is used, which surrounds the phase conductor, and which supplies the load (an incandescent bulb, an electric motor, the mayor of a transformer that supplies an electronic module, etc.).
The circuit can also be used as a short-circuit or overload protection, if for example the capture coil is mounted around the conductor that supplies the primary of a transformer from an electronic assembly, a relay will switch off the power supply of the supervised assembly.
L1 must have 800 turns. This will produce approx. 4mV peak to peak for a current of approximately 250mA (corresponding to a load of 30W).
Basically, the sensitivity depends on the number of coils wound for L1. Any general purpose operational amplifier can be used instead LM1458.
7. FM transmitter microphone - for the band 88-108 MHz with a single transistor
The assembly is done around an oscillator COLPITTS. The signal picked up by the microphone is modulated in frequency with the help of this oscillator and emitted in the antenna ether from the collector of the transistor T1 (model BC550 or BC557 / 549), made directly on the wiring (strip).
Making the antenna directly on the wiring eliminates the disturbances with the oscillator. The transmission frequency is adjusted from trimmer C4. The L1 coil is made of air, has a diameter of 4mm and is made of CuEm conductor with a diameter of 0.7mm. The consumption of the assembly is approx. 6.5mA.
8. Constant current source for charging the batteries
The application is a switching source that, powered by 20V (+/- 4V), offers in pregnancy 1.7A, in which case, on the battery, the voltage can take any values in the range 5… 17V and this if charges from 4 to 12 Ni-Cd elements.
The efficiency is better than 85% if series transistors with very low saturation Vce are used, such as the model FZT949 or FZT789 from ZETEX.
They can be replaced, with poorer results, with BC910 or BC912 (or TIP42C). A specialized integrated circuit is used to generate the switching signal, respectively TL494.
The series transistor has as driver a circuit with main elements - the 2N3904 transistor (or BC547) and the 68uH inductor.
9. Selective capture of colored light pulses
For those who want an economical method of selection (capture) of light pulses of a certain color, a method presented in Electronic Design is recommended.
This consists in using a LED (with colored lens) as a photodiode. A reverse polarized LED behaves like a general purpose photodiode. In white light, any strongly excited LED, regardless of color, provides voltage at the output.
Depending on the light with which the LED is excited, it provides this voltage at the output. If, for example, a green LED is excited by the green light it will respond accordingly; if it is excited only with red light it will be insensitive.
An LED will not tell the difference in color if the white spectrum is also the usual white color (daylight). The LED only differentiates the color it emits.
Thus, you can make a selection of the emitted colors and more a remote control system with 3-4 channels, in which the transmitters are simple LEDs of different colors.
Obviously, the installation will work well in rooms where ambient light does not reach. The optical signal captured by the LED and converted into voltage is compared in level with a voltage reference set from the 25k resistor and coming from + 5V.
The signal from the comparator output is reversed with a gate 74HC04 and further applied as the trigger-has signal for a timer with 555 (or 1/2 of 556, working as a monostable).
An LED will signal that the light of the selected color has been received. The warning time is a few seconds.
10. Using an LED as a photodiode (light detector)
The photovoltaic effect of a red LED junction is around 2V. It is best to use AO TL071 because it has inputs with JFET transistors, so a high input impedance.
The resistor above 10 MOhm ensures the corresponding polarization of the reversing input (0V) in the dark. Its value can be increased even more, taking into account that the AO has an offset voltage of 10… 15mV, and an LED offers as photodiode approx. 50mV.
3 resistors of 10 MOhm can be mounted in series, parallel on the diode. The AO output provides zero volts (0V) with the LED in the dark. Adjust the semi-dimmer so that the corresponding gain and voltage level desired at the output are obtained when the LED is exposed to light.
11. Universal plate with 5 triacs
A universal plate with 5 triacs is a unique proposal. It represents the reduced version of the Conex Electronic CNX143 kit (universal board with 8 triacs).
The assembly is used for the isolated control (optically) of some alternating current loads (bulbs, motors, etc.). The controls, in direct current, are given on the internal LEDs in the optocouplers LTV817 and LTV847 (which have in series, on each input, a resistor of 1k… 3k3) from a separate source of + 5V… 12Vdc, by means of buttons.
Of course, the commands can come from execution relay outputs from other electronic assemblies (or from open collector outputs). You can use any type of general purpose npn transistors (BC547, BC171, BC107, etc.).
12. Universal relay with undervoltage indicator
Using some cheap electronic components (a Zener diode, an LED and a low power resistor - 0.25W) a universal relay can be made with the possibility of supply at higher voltages than the nominal one inscribed on the relay body.
It can also be a compromise solution if a relay with a lower voltage than the one indicated in the electronic assembly is available.
The Zener diode is chosen with a stabilization threshold (voltage) close to the value of the supply voltage. For example for 12V you can choose a diode with a threshold of 10, 9.1 or 8.2V.
In this case, if the supply voltage of the assembly drops below one of the indicated values, the relay is no longer actuated and the LED will indicate the undervoltage condition.
13. Display element - 7 segments with 3 LEDs per segment
Those who want to build an LED display system can use the example printed circuit board presented, to adapt it to various sizes of a display element.
3 LEDs (3mm or 5mm) are used for each segment of a digit and one LED for the decimal point.
The interface elements are two pin-head connectors arranged parallel to the two small sides. The output is standard, being compatible with the integrated display elements, from the SA_ KingBright series.
Thus, the pin marked TP8 is pin 1 from a series display SA11-_, respectively the common anode pin, denoted AC. The type of display is with a common anode, but by reverse mounting the LEDs, the common cathode variant can be achieved!
14. AVR Pony Prog interface - for Atmega8515 uC programming
The application is a standard I / O interface for the COM port used to program uC AVR using the Pony Prog PC program (available for free at www.lancos.com).
The assembly is dedicated to the model Atmega8515 or compatible, packed in TQFP capsule.
If you troubleshoot or build an application with this microcontroller and there is no possibility of programming in the circuit, after assembly (soldering with SMT station), a solution is to use the programmer presented.
With the help of a hot air SMT station, the microcontroller is glued in the indicated place and programmed (see programming instructions using Pony Prog in Conex Club no. 4/2006).
After the programming operation (successful), the component is detached with the same tool from the programmer's plate and glued in the assembly in which it has to work.
15. 1W audio amplifier with TDA7052
An audio amplifier with TDA7052 is ideal for battery power, being simple to turn it into a portable travel kit.
The specifications are as follows:
- Power supply: 3… 15V at max. 300mA;
- Standby current: <8mA;
- Output power> 1W @ 8 Ohms or> 0.25W RMS;
- Band> 20Hz… 20kHz (-10… 50kHz, -3dB);
- THD <1% @ 100mW, typical 0.2%;
- S / N> 70dBA;
- Gain: -30dB maximum;
- Input impedance: ~ 10 KOhm.
16. Remote video signal transmission via UTP / telephone cable
The video signal obtained from a (mini) video camera can be transmitted to a video monitor and by two-wire cable, of a kind used for computer interconnection (PC) and not necessarily by coaxial cable with impedance of 75 Ohm.
The distance at which the signal can be transmitted is up to 100m. The price difference between the two types of cables is of the order of 1: 2… 1: 3. So, for long lengths, advantages worthy of consideration are obtained.
Because there are attenuations of the video signal on such a long cable length, it is necessary to use broadband (video) amplifiers using the specialized integrated circuit of the type uA733 (equivalent to ROB733).
The video signal for the monitor is picked up from the amplifier - receiver on one of the outputs: OUT1 or OUT2. On one of the outputs the signal is positive and on the other negative. Choose the signal you need!
If a shorter cable is used (20… 40m), and the signal losses on the route are smaller, then the value of the 1 KOhm resistors is adjusted in the sense of increasing them, so that at the output we have a 1Vvv signal.
17. Verification of IGBT transistor modules
The most used electronic switching components in the field of power electronics today have become transistors IGBT (Insulated Gate Bipolar Transistor - Insulated Gate Bipolar Transistor).
Due to the different applications in which these transistors appear, the manufacturing companies offer a wide range of IGBTs with different designs. Among the most common are capsules with 2 or 6 IGBT transistors (figure 1 in the article).
How the IGBT transistors in a capsule are connected to their terminals is shown in Figure 2 (from the article).
He will introduce himself how to check a capsule with 2 or 6 IGBT transistors, if it is still functional or if one of the 2 or 6 IGBTs is defective.
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