Low-Cost GPS Clock

Real-time clock (RTC) integrated circuits (ICs) are used for reasonably accurate time displays. The accuracy of RTCs mostly depends on the crystal. In a long run, a small-time deviation will be observed in the RTC circuit due to seasonal and atmospheric temperature changes. In case of a Global Positioning System (GPS) clock, the time/time stamp is received from the satellite(s) and is highly accurate. This project is based on a low-cost GPS clock using AVR ATmega8A microcontroller (MCU).

Circuit and working of Low-Cost GPS Clock

Circuit diagram of the simple and low-cost GPS clock is shown in Fig. 1. It is built around ATmega8A (IC1), NEO 6M GPS receiver module (GPS1) along with an antenna, 16×1 LCD (LCD1) and a few other components.

Fig. 1: Circuit diagram of GPS clock

NEO 6M GPS receiver module receives National Marine Electronics Association (NMEA) data continuously and transfers the same to ATmega8A MCU. The MCU processes the data and picks the date stamp from the received data string. The following data format is searched in the received data, which is used to pick the time stamp:

$GPGGA, 143621, .

The MCU checks for the keyword $GPGGA and reads the next six characters of the received string and displays it on LCD1. This process repeats in a cyclic process, and the latest time is displayed.

The time stamp received is always in Universal Time Coordinated (UTC) format, which matches with Greenwich Mean Time (GMT). To convert it to local time, add proper time constant to UTC time, which will depend on the country and city. In the program, the local time constant for Indian time is set to +5:30, but you can modify it to set a different value.

The local time constant is declared in C program as:

int ADJ[2] = {5,30};

Initial setup
After connecting the circuit, upload GPSclock.hex file to ATmega8A MCU using any AVR programmer through ISP port. Once the program is loaded, blinker LED1 will glow for one second followed by ‘NEO 6M GPS Clock’ message on LCD1. By default, it will display in UTC format.

To change to local time constant, press any switch (S2 or S3) to display the existing value. By pressing hours and minutes switch, the local time constant changes. If no switch is pressed for more than four seconds, the value is saved in EEPROM of the MCU.

The GPS module will initialise and try to connect to the satellite for data reception. This may take some time, say, three to five minutes. LCD1 will display “Connecting..” till the data is received from the satellite. Once the data is received, the time will be displayed on LCD1 in HH:MM:SS format continuously. In case GPS module is not connected, or is giving a connection error, ‘NO GPS MODULE’ message will be displayed on LCD1.

Use shorting jumpers SJ1 for 12/24-hour format and SJ2 for local/UTC time to be displayed. LED1 will indicate that the MCU is processing the received data. Potmeter VR1 is used for adjusting the contrast for LCD1.

Construction and testing
An actual-size PCB layout of low-cost GPS clock circuit is shown in Fig. 2 and its components layout is shown in Fig. 3.

Fig. 2: PCB layout of GPS clock

Fig. 3: Components layout for the PCB

Assemble the circuit on the PCB and connect 5V across CON1 to operate the circuit. Enclose the circuit in a suitable box and connect GPS1 across CON2. Connect the antenna to GPS1. Place the unit at a suitable location along with LCD1. Jumpers J1 and J2 shown in the PCB can be any conductor wires. Connect these before switching on the circuit.

Jumper setting
Connect SJ1 in 12-hour format, otherwise it will be set in 24-hour format by default. Connect SJ2 for UTC time, otherwise it will be set in local time (UTC+5.30).

For example, the time stamp received by the GPS module is 14:15:16, which is in UTC time in 24-hour format by default. Now, LCD1 will display the time as per the settings for SJ1 and SJ2, as shown in the table, assuming the local time constant as +05:30:00.

To change the local time, press minutes and hours switches (S2 and S3, respectively) during initialisation of the system when NEO 6M GPS message appears on LCD1. Change local time constant only once.

Sourced: EFU Author : Fayaz Hassan

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AM transmitter using Integrated Circuit

Many beginners give up playing with RF due to the difficulty of building coils and problems with self oscillations that are common in many electronic circuits of RF. I always point out to beginning the construction of a small FM transmitter, myself have made available a simple circuit and an explanatory tutorial in Article Micro FM Transmitter and Micro Spy FM Transmitter Spy Bug in SMD.

The RF is really a very critical area, but on the other hand, it is the most rewarding, especially when we see one of our creations give the air of grace and drop its waves through the air. I always say that we learn more from mistakes and making mistakes is part of the electronics, I even today burst capacitors and transistors see loose tufts of smoke, the difference is that today I have fun with these errors.

Here I will publish a simple electronic circuit RF, so simple that does not use a single coil. This is a small tone transmitter modulated (AM) for the band of medium waves (AM) using an integrated inverter hex 4049 and a crystal circuit. The circuit is very simple, but effective, as can transmit smoothly.

AM transmitter using Integrated Circuit

The operation of this AM transmitter is simple, the RF oscillator in the transmitter circuit uses an inverter (7-6), whose frequency is determined by the 1 MHz crystal, as we see in the scheme. Two more drives (9-10 / 11-12) amplify the 1MHz oscillator signal.

Have the two inverters (3-2 / 5-4) produce an audio tone, which is modulated with the RF signal by the last inverter (14-15). You can use a piece of wire as an antenna, the signal, a buzz, should be easily capitate until a few meters from the transmitter at 1000 kHz in any AM radio.

With this circuit it is possible to work with the fundamental frequency and its harmonics at all integer multiples of a 1 MHz (or 2 MHz, 3 MHz, ... 10 MHz). This circuit can also be used as a frequency standard to verify the calibration of the display of a short wave radio.

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F-V Converter With Sample And Hold Circuit Diagram

This is the simple Frequency/Voltage Converter With Sample And Hold Circuit Diagram. U1 is a frequency/voltage converter, feeding sample-and-hold circuit using an LF381. An LF351 provides 10-V M-scale output. The circuit produces 1-V/kHz output.

F-V Converter With Sample And Hold Circuit Diagram

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Simple Sensitive RF Voltmeter Circuit Diagram

This is the Simple Sensitive RF Voltmeter Circuit Diagram. This schematic shows a peak-reading diode voltmeter driven by two stages of amplification. A 100-ILF capacitor provides a fairly large time constant, which results in satisfactory meter damping. 

 Simple Sensitive RF Voltmeter Circuit Diagram

The limited differential output voltage coupled with an overdamped meter prevents most needle pinning when you select an incorrect range position, or make other errors. An SPST toggle switch selects additional series resistance. This X2 function gives some more overlap of the sensitivity ranges. The resistance values shown are correct for use with a 100-!LA meter with 1500-0 internal resistance.

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10 Mhz universal counter Circuit Diagram

This is the simple 10 Mhz universal counter Circuit Diagram. The ICM7216A or can be used as a minimum component complete Universal Counter. 

 10 Mhz universal counter Circuit Diagram

 

This circuit can use input frequencies up to 10 MHz at INPUT A and 2 MHz at INPUT B. If the signal at INPUT A has a very low duty cycle it may be necessary to use a 74121 monostable multivibrator or similar circuit, to stretch the input pulse width to be able to guarantee that it is at least 50 ns in duration.

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Low Low Power FM Transmitter Circuit Diagram

This article should satisfy those who might want to build a low power FM transmitter. It is designed to use an input from another sound source (such as a guitar or microphone), and transmits on the commercial FM band - it is actually quite powerful, so make sure that you don't use it to transmit anything sensitive - it could easily be picked up from several hundred meters away. The FM band is 88 to 108MHz, and although it is getting fairly crowded nearly everywhere, you should still be able to find a blank spot on the dial.

NOTE: A few people have had trouble with this circuit. The biggest problem is not knowing if it is even oscillating, since the frequency is outside the range of most simple oscilloscopes. See Project 74 for a simple RF probe that will (or should) tell you that you have a useful signal at the antenna. If so, then you know it oscillates, and just have to find out at what frequency. This may require the use of an RF frequency counter if you just cannot locate the FM band.

Description

The circuit of the transmitter is shown in Figure 1, and as you can see it is quite simple. The first stage is the oscillator, and is tuned with the variable capacitor. Select an unused frequency, and carefully adjust C3 until the background noise stops (you have to disable the FM receiver's mute circuit to hear this).
.

Figure 1 - Low Power FM Transmitter

Because the trimmer cap is very sensitive, make the final frequency adjustment on the receiver. When assembling the circuit, make sure the rotor of C3 is connected to the +9V supply. This ensures that there will be minimal frequency disturbance when the screwdriver touches the adjustment shaft. You can use a small piece of non copper-clad circuit board to make a screwdriver - this will not alter the frequency.

The frequency stability is improved considerably by adding a capacitor from the base of Q1 to ground. This ensures that the transistor operates in true common base at RF. A value of 1nF (ceramic) as shown is suitable, and will also limit the HF response to 15 kHz - this is a benefit for a simple circuit like this, and even commercial FM is usually limited to a 15kHz bandwidth.

Capacitors
All capacitors must be ceramic (with the exception of C1, see below), with C2 and C6 preferably being N750 (Negative temperature coefficient, 750 parts per million per degree Celsius). The others should be NPO types, since temperature correction is not needed (nor is it desirable). If you cannot get N750 caps, don't worry too much, the frequency stability of the circuit is not that good anyway (as with all simple transmitters).

How It Works
Q1 is the oscillator, and is a conventional Colpitts design. L1 and C3 (in parallel with C2) tunes the circuit to the desired frequency, and the output (from the emitter of Q1) is fed to the buffer and amplifier Q2. This isolates the antenna from the oscillator giving much better frequency stability, as well as providing considerable extra gain. L2 and C6 form a tuned collector load, and C7 helps to further isolate the circuit from the antenna, as well as preventing any possibility of short circuits should the antenna contact the grounded metal case that would normally be used for the complete transmitter.

The audio signal applied to the base of Q1 causes the frequency to change, as the transistor's collector current is modulated by the audio. This provides the frequency modulation (FM) that can be received on any standard FM band receiver. The audio input must be kept to a maximum of about 100mV, although this will vary somewhat from one unit to the next. Higher levels will cause the deviation (the maximum frequency shift) to exceed the limits in the receiver - usually ±75kHz.

With the value shown for C1, this limits the lower frequency response to about 50Hz (based only on R1, which is somewhat pessimistic) - if you need to go lower than this, then use a 1uF cap instead, which will allow a response down to at least 15Hz. C1 may be polyester or mylar, or a 1uF electrolytic may be used, either bipolar or polarised. If polarised, the positive terminal must connect to the 10k resistor.

Inductors
The inductors are nominally 10 turns (actually 9.5) of 1mm diameter enamelled copper wire. They are close wound on a 3mm diameter former, which is removed after the coils are wound. Carefully scrape away the enamel where the coil ends will go through the board - all the enamel must be removed to ensure good contact. Figure 2 shows a detail drawing of a coil. The coils should be mounted about 2mm above the board.

For those still stuck in the dark ages with imperial measurements (grin), 1mm is about 0.04" (0.0394") or 5/127 inch (chuckle) - you will have to work out what gauge that is, depending on which wire gauge system you use (there are several). You can see the benefits of metric already, can't you? To work out the other measurements, 1" = 25.4mm

NOTE: The inductors are critical, and must be wound exactly as described, or the frequency will be wrong.

Figure 2 - Detail Of L1 And L2

The nominal (and very approximate) inductance for the coils is about 130nH.This is calculated according to the formula ...

L = N² * r² / (228r + 254l)

... where L = inductance in microhenries (uH), N = number of turns, r = average coil radius (2.0mm for the coil as shown), and l = coil length. All dimensions are in millimetres.

Pre-Emphasis

It is normal with FM transmission that "pre-emphasis" is used, and there is a corresponding amount of de-emphasis at the receiver. There are two standards (of course) - most of the world uses a 50us time constant, and the US uses 75us. These time constants represent a frequency of 3183Hz and 2122Hz respectively. This is the 3dB point of a simple filter that boosts the high frequencies on transmission and cuts the same highs again on reception, restoring the frequency response to normal, and reducing noise.

The simple transmitter above does not have this built in, so it can be added to the microphone preamp or line stage buffer circuit. These are both shown in Figure 3, and are of much higher quality than the standard offerings in most other designs.

Figure 3 - Mic And Line Preamps

Rather than a simple single transistor amp, using a TL061 opamp gives much better distortion figures, and a more predictable output impedance to the transmitter. If you want to use a dynamic microphone, leave out R1 (5.6k) since this is only needed to power an electret mic insert. The gain control (for either circuit) can be an internal preset, or a normal pot to allow adjustment to the maximum level without distortion with different signal sources. The 100nF bypass capacitors must be ceramic types, because of the frequency. Note that although a TL072 might work, they are not designed to operate at the low supply voltage used. The TL061 is specifically designed for low power operation.

The mic preamp has a maximum gain of 22, giving a microphone sensitivity of around 5mV. The line preamp has a gain of unity, so maximum input sensitivity is 100mV. Select the appropriate capacitor value for pre-emphasis as shown in Figure 3 depending on where you live. The pre-emphasis is not especially accurate, but will be quite good enough for the sorts of uses that a low power FM transmitter will be put to. Needless to say, this does not include "bugging" of rooms, as this is illegal almost everywhere.

I would advise that the preamp be in its own small sub-enclosure to prevent RF from entering the opamp input. This does not need to be anything fancy, and you could even just wrap some insulation around the preamp then just wrap the entire preamp unit in aluminium foil. Remember to make a good earth connection to the foil, or the shielding will serve no purpose.

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SMD FM Transmitter

Let’s construct a low-power FM transmitter using surface-mount devices (SMD) that will be received with a standard FM radio. Soldering surface mounted devices is not so hard and actually is quite easy. There are many designs for small FM transmitters but they have some problems. First, you need an audio amplifier to get enough modulation. Second, the antenna is attached directly to the collector.

Third, the coil L must be wound by hand and adjusted by stretching. It all ads with a weak signal that tends to drift in frequency. In contrastm the transmitter schematic we present here eliminates some of those problems, using varactor diode for tuning and modulation, givind great sensitivity without an audio amplifier.

FM Transmitter – How it works
The figure below shows the schematic of the transmitter which consists of two stages: an oscillator and an output amplifier. Modulation is from an electret microphone but you can use a low power audio source.

Oscillator stage
Transistor Q1 is a Colpitts oscillator where the frequency is determined by the parallel resonant circuit formed by inductor L, varactor V1 and capacitors C7 and C8. Q1 is a common-collector amplifier where the power gain counts. V1 is actually a dual varactor that eliminate the possibility of forward conduction at the sinewave peaks.

The frequency of oscillation is set by adjusting the DC voltage on V1 with potentiometer R2. R4 and C3 form a low-pass filter to prevent RF from feeding back onto the DC.
Capacitors C7 and C8 form an AC voltage divider to provide feedback at the emitter of Q1 to sustain oscillation. A necessary condition for oscillation to start is for the radio (C7+C8)/C7 to be sufficiently bigger than 1.

SMD transmitter circuit schematic

Frequency Modulation
Modulation is done by superimposing an audio signal from the electret mic onto the DC bias applied to V1. R3 and C1 form a low-pass filter to prevent RF from feeding back to the mic. R3, R4 and R2 form a votage divider for the audio.

Transmitter output stage
The output of the oscillator is fed through C9 to the Q2 emitter-follower. The output of Q2 drives the antenna through C11. The Q2 emitter-follower it ensures that the oscillator is not loaded down by the impedance of the antenna and it provides power gain to drive the antenna.

SMD Transmitter layout
The figure below shows the layout of the PCB and it uses surface-mounted devices like resistors and capacitors (non-polar devices). All the caps are size 0805 and all resistors are size 1206. use through-hole components for Q1, Q2, IC1 and V1. You can use an SOT-89 device for IC1 and an SOT-23 device for V1. Use MPSH10 or a transistor equivalent. Here you can learn how to solder smd chips

The inductor
A coil would consist of two or three turns of wire but for this schematic we will use an inductor with loops of copper on the PCB. Such flat spiral inductor are common at these frequencies.
One formula for flat spiral inductors is:
flat spiral inductors formula where
L = inductance in uH
r = radius of coil (outer radius + inner radius divided by 2 ) inches
N = number of turns
d = depth of coil (outer radius minus inner radius) inches

Tuning range
While commercial FM band goes from about 88 MHz to 108 MHz, the L and C values used in this design allow tuning up to 100 MHz.

Transmitter testing
You will need a portable FM radio and an assistant. First, find an empty spot on the FM dial and set your radio about 30 feet away (9 meters). The radio’s volume control should not be set too high to prevend feedback. Next, power-up your transmitter and talk to yourself as you adjust the frequency with the trim-pot. When your assintant hears you, your transmitter is tuned. You might have to adjust the radio’s tuner slightly for best reception.

Have fun with it but remember that using the transmitter as a bugging device may not be legal in your country. To use the circuit as a wireless microphone, increase the value of R3. The transmitter range is about 100 feet (30 meters) inside a building.
Parts list

 

Source by :www.circuitsproject.com

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FM Stereo Transmitter

You'll find that this is a very easy project to build. It will transmit good quality sound in the FM band ( 88 - 108 mhz ). One inportant item is that the IC chip operates on 3 volts DC. The chip will get destroyed if it is operated on any voltage higher than 3.5 volts. The antenna can be a standard telescopic antenna or a 2 foot length of wire. The input is in the millivolt range and you may need to add additional pots for the inputs. I was able to use this circuit for a walkman and a portable CD player in my car. I used the headphone jack on both and varied the signal with the volume control.

Circuit Diagram

To adjust the circuit tune your FM radio to a quite spot then adjust the trimmer capacitor C8 until you hear the signal that you are transmiting. When you have a strong signal adjust the resistor R4 until the stereo signal indicator lights. If the input is to high of a signal you may over drive the IC chip. Use two 15 turn pots on the input signals to bring the level down. You can balance the signal by using headphones. The inductor L1 is 3 turns of .5 mm wire on a 5 mm ferrite core.

Sourced: Circuitsstream

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Four Stage FM Transmitter

This FM transmitter circuit uses four radio frequency stages: a VHF oscillator built around transistor BF494 (T1), a preamplifier built around transistor BF200 (T2), a driver built around transistor 2N2219 (T3) and a power amplifier built around transistor 2N3866 (T4). A condenser microphone is connected at the input of the oscillator.

Working of the circuit is simple. When you speak near the microphone, frequency-modulated signals are obtained at the collector of oscillator transistor T1. The FM signals are amplified by the VHF preamplifier and the pre-driver stage. You can also use transistor 2N5109 in place of 2N2219. The preamplifier is a tuned class-A RF amplifier and the driver is a class-C amplifier. Signals are finally fed to the class-C RF power amplifier, which delivers RF power to a 50-ohm horizontal dipole or ground plane antenna. Use a heat-sink with transistor 2N3866 for heat dissipation. Carefully adjust trimmer VC1 connected across L1 to generate frequency within 88-108 MHz. Also adjust trimmers VC2 through VC7 to get maximum output at maximum range.


Regulator IC 78C09 provides stable 9V supply to the oscillator, so variation in the supply voltage will not affect the frequency generated. You can also use a 12V battery to power the circuit. Assemble the circuit on a general-purpose PCB. Install the antenna properly for maximum range. Coils L1 through L5 are made with 20 SWG copper-enamelled wire wound over air-cores having 8mm diameter. They have 4, 6, 6, 5 and 7 turns of wire, respectively.

EFY note. This transmitter is meant only for educational purposes. use of this transmitter with outdoor antenna is illegal in most parts of the world. The author and EFY will not be responsible for any misuse of this transmitter.

Copyright: EFY Mag

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