Wireless Transmitter

Infared Remote Control Transmitter-Receiver

2009-11-11T06:46:00.000-08:00

Here's schematics for infared remotes. This remote transmits a tone using an infared LED. This tone is decoded by the receiver. Since the receiver only switches when it "hears" the tone, there are no accidental activations.

Infrared Remote Control Schematics
IR Transmitter schematic:

IR Receiver schematic:

Remote Control Setup

To adjust the circuit, hold down S1 while pointing LED1 at the receiver. Adjust R6 until you hear the relay click.
You can increase range by using a high output LED for LED1.
Bright light will stop the receiver from responding to the transmitter.
There is an error in the schematic. There should be a 1 megaohm resistor between pin 3 of IC1 and ground. This provides a 0 volt reference to bias the IC.

Parts List
R1 - 22K 1/4W Resistor
R2 - 1 Meg 1/4W Resistor
R3 - 1K 1/4W Resistor
R4, R5 - 100K 1/4W Resistor
R6 - 50K Pot
C1, C2 - 0.01uF 16V Ceramic Disk Capacitor
C3 -1 100pF 16V Ceramic Disk Capacitor
C4 - 0.047uF 16V Ceramic Disk Capacitor
C5 - 0.1uF 16V Ceramic Disk Capacitor
C6 - 3.3uF 16V Electrolytic Capacitor
C7 - 1.5uF 16V Electrolytic Capacitor
Q1 -1 2N2222/2N3904 NPN Silicon Transistor
Q2 -1 2N2907 PNP Silicon Transistor
Q3 - NPN Phototransistor
D1 - 1N914 Silicon Diode
IC1 - LM308 Op Amp IC
IC2 - LM567 Tone Decoder
LED - 1 Infa-Red LED
RELAY - 6 Volt Relay
S1 - SPST Push Button Switch
B1 - 3 Volt Battery Two 1.5V batteries in series
MISC - Board, Sockets For ICs, Knob For R6, Battery Holder
RELAY - 6 Volt Relay

Price 16.69$ Wireless Remote Control Shutter for Canon EOS Digital Rebel XT, XTi, XSi, & ELAN SLR Cameras

Source: Infra-Red Remote Control

Simple SWR Meter 2.4 GHz

2009-11-11T06:25:00.000-08:00

Here is a simple 2.4 GHz SWR meter which can be easily found, which is based around surplus microwave hardware . The main component is a MECA -20/-20 dB Directional Coupler which has a frequency range of approximately 700 MHz to 2.5 GHz.

This particular directional coupler above has two ports, each coupled by 20 dB. What that means is, a signal is "coupled" to these ports which is identical to the main signal passing through the directional coupler, only it's attenuated by 20 dB. By placing diode detectors on the outputs of these two ports and comparing the resulting voltages on an oscilloscope, you can quickly determine the integrity of your antenna system in reference to 50 ohms.

SWR Meter Diagram

How It Works

Be sure to measure SWR at the antenna! Feedline loss will attenuate the signal and give you a false SWR reading if you measure it directly at the transmitter's RF output. Don't be like those guys at Field Day who brag about their radio's (internal) SWR reading when connected to a homebrew antenna with 200 feet of Radio Shack RG-58 and banana clips.

Also, don't trust "analog" movement reading SWR meters when dealing with digital data transmitters. The needle response time isn't fast enough to provide an accurate reading.

SWR Meter Pictures

Overview of the hardware components used. The directional coupler is the large blue thing. A N-connector female-to-female adapter is needed to connect the Narda 5 Watt, 50 ohm load. The diode detectors are the silver things on top. They both are identical and house 1N23 point-contact diodes. Their voltage output is via integrated BNC connectors.

Close up view of the diode detectors. They are sometimes referred to as "crystal" detectors. 1N23-style diodes can be hard to find. Fair Radio does carry them. You can also find them used as mixer diodes in some 10 GHz gunnplexer-based automatic door openers.

These particular diode detectors generate a negative output voltage. Remember that when looking at the oscilloscope screen shots.

Source: Simple 2.4 GHz SWR Meter

MP3 FM Transmitter Circuit Using SMD

2009-11-09T06:58:00.000-08:00

With this simple VHF FM transmitter you could play audio files from an MP3 player or computer on a standard VHF FM radio. The circuit use no coils that have to be wound. This FM transmitter can be used to listen to your own music throughout your home. When this FM transmitter used in the car, there is no need for a separate input to the car stereo to play back the music files from your MP3 player.

This FM transmitter uses a chip made by Maxim Integrated Products, the MAX2606 [1] To keep the circuit simple as well as compact . This IC has been specifically designed for low-noise RF applications with a fixed frequency.

The VCO (Voltage Controlled Oscillator) in this IC uses a Colpitts oscillator circuit. The variable-capacitance (varicap) diode and feedback capacitors for the tuning have also been integrated on this chip, so that you only need an external inductor to fix the central oscillator frequency.

It is possible to fine-tune the frequency by varying the voltage to the varicap. Not much is demanded of the inductor, a type with a relatively low Q factor (35 to 40) is sufficient according to Maxim. The supply voltage to the IC should be between 2.7 and 5.5 V, the current consumption is between 2 and 4 mA. With values like these it seemed a good idea to supply the circuit with power from a USB port.

A common-mode choke is connected in series with the USB connections in order to avoid interference between the circuit and the PC supply. There is not much else to the circuit. The stereo signal connected to K1 is combined via R1 and R2 and is then passed via volume control P1 to the Tune input of IC1, where it causes the carrier wave to befrequency modulated. Filter R6/C7 is used to restrict the bandwidth of the audio signal. The setting of the frequency (across the whole VHF FM broadcast band) is done with P2, which is connected to the 5 V supply voltage.

The PCB designed uses resistors and capacitors with 0805 SMD packaging. The size of the board is only 41.2 x 17.9 mm, which is practically dongle-sized. For the aerial an almost straight copper track has been placed at the edge of the board. In practice we achieved a range of about 6 metres (18 feet) with this. There is also room for a 5-way SIL header on the board. Here we find the inputs to the 3.5 mm jack plug, the input to P1 and the supply voltage. The latter permitsthe circuit to be powered independently from the mains supply, via for example three AA batteries or a Lithium button cell. Inductor L1 in the prototype is a type made by Murata that has a fairly high Q factor: minimum 60 at 100 MHz.

Take care when you solder filter choke L2, since the connections on both sides are very close together. The supply voltage is connected to this, so make sure that you don’t short out the USB supply! Use a resistance meter to check that there is no short between the two supply connectors before connecting the circuit to a USB port on a computer or to the batteries.

P1 has the opposite effect to what you would expect (clockwise reduces the volume), because this made the board layout much easier. The deviation and audio bandwidth varies with the setting of P1. The maximum sensitivity of the audio input is fairly large. With P1 set to its maximum level, a stereo input of 10 mVrms is sufficient for the sound on the radio to remain clear. This also depends on the setting of the VCO. With a higher tuning voltage the input signal may be almost twice as large (see VCO tuning curve in the data sheet). Above that level some audible distortion becomes apparent. If the attenuation can’t be easily set by P1, you can increase the values of R1 and R2 without any problems.

Measurements with an RF analyzer showed that the third harmonic had a strong presence in the transmitted spectrum (about 10 dB below the fundamental frequency). This should really have been much lower. With a low-impedance source connected to both inputs the bandwidth varies from 13.1 kHz (P1 at maximum) to 57 kHz (with the wiper of P1 set to 1/10).

In this circuit the pre-emphasis of the input is missing. Radios in Europe have a built-in de-emphasis network of 50 μs (75 μs in the US). The sound from the radio will therefore sound noticeably muffled. To correct this, and also to stop a stereo receiver from mistakenly reacting to a 19 kHz component in the audio signal, an enhancement circuit Is published elsewhere in this issue (Pre-emphasis for FM Transmitter, also with a PCB). Author: Mathieu Coustans, Elektor Magazine, 2009

MP3 FM Transmitter Parts List
Resistors (all SMD 0805)
R1,R2 = 22kΩ
R3 = 4kΩ7
R4,R5 = 1kΩ
R6 = 270Ω
P1 = 10kΩ preset, SMD (TS53YJ103MR10 Vishay Sfernice, Farnell # 1557933)
P2 = 100kΩ preset, SMD(TS53YJ104MR10 Vishay Sfernice, Farnell # 1557934)
Capacitors (all SMD 0805)
C1,C2,C5 = 4μF7 10V
C3,C8 = 100nF
C4,C7 = 2nF2
C6 = 470nF
Inductors
L1 = 390nF, SMD 1206 (LQH31HNR39K03L Murata, Farnell # 1515418)
L2 = 2200Ω @ 100MHz, SMD, common-mode choke, 1206 type(DLW31SN222SQ2L Murata, Farnell #1515599)
Semiconductors
IC1 = MAX2606EUT+, SMD SOT23-6 (Maxim Integrated Products)
Miscellaneous
K1 = 3.5mm stereo audio jack SMD (SJ1-3513-SMT
CUI Inc, DIGI-Key # CP1-3513SJCT-ND)
K2 = 5-pin header (only required in combination with 090305-I pre-emphasis circuit)
K3 = USB connector type A, SMD (2410 07 Lumberg, Farnell # 1308875)

Notice. The use of a VHF FM transmitter, even a low power device like the one described here, is subject to radio regulations and may not be legal in all countries.

Source: MP3 FM Transmitter Circuit

Simple FM Transmitter-Single Transistor

2009-11-09T06:40:00.000-08:00

This FM transmitter is simple using a single transistor. It provide very clear wireless sound transmission through an ordinary FM radio over a remarkable distance. I've seen lots of designs through the years, some of them were so simple, some of them were powerful, some of them were hard to build etc.

Here is the last step of this evolution, the most stable, smallest, problemless, and energy saving champion of this race. Circuit given below will serve as a durable and versatile FM transmitter till you break or crush it's PCB. Frequency is determined by a parallel L-C resonance circuit and shifts very slow as battery drains out.

Main advantage of this circuit is that power supply is a 1.5Volts cell (any size) which makes it possible to fix PCB and the battery into very tight places. Transmitter even runs with standard NiCd rechargeable cells, for example a 750mAh AA size battery runs it about 500 hours (while it drags 1.4mA at 1.24V) which equals to 20 days. This way circuit especially valuable in amateur spy operations.

Transistor is not a critical part of the circuit, but selecting a high frequency/ low noise one contributes the sound quality and range of the transmitter. PN2222A, 2N2222A, BFxxx series, BC109B, C, and even well known BC238 runs perfect. Key to a well functioning, low consumption circuit is to use a high hFE / low Ceb (internal junction capacity) transistor.

Not all of the condenser microphones are the same in electrical characteristics, so after operating the circuit, use a 10K variable resistance instead of the 5.6K, which supplies current to the internal amplifier of microphone, and adjust it to an optimum point where sound is best in amplitude and quality. Then note the value of the variable resistor and replace it with a fixed one.

The critical part is the inductance L which should be handmade. Get an enameled copper wire of 0.5mm (AWG24) and round two loose loops having a diameter of 4-5mm. Wire size may vary as well. Rest of the work is much dependent on your level of knowledge and experience on inductances: Have an FM radio near the circuit and set frequency where is no reception. Apply power to the circuit and put a iron rod into the inductance loops to chance it's value. When you find the right point, adjust inductance's looseness and, if required, number of turns.

Once it's OK, you may use trimmer capacitor to make further frequency adjustments. You may get help of a experienced person on this point. Do not forget to fix inductance by pouring some glue onto it against external forces. If the reception on the radio lost in a few meters range, than it's probably caused by a wrong coil adjustment and you are in fact listening to a harmonic of the transmitter instead of the center frequency. Place radio far away from the circuit and re-adjust.

Source: Simple FM transmitter with a single transistor

MP3 FM Transmitter Circuit Powered by USB

2009-11-09T06:13:00.000-08:00

Here's a small FM transmitter ciruit for your laptop. This FM transmitter, which is powered by USB, recovers output on your computer or your MP3 player to the relay on the tape FM (frequency 108 MHz). For Assemblying this FM transmitter kit, an electronics hobbyist will have built in about 30 minutes.

FM Transmitter Construction
It is not necessary to drill the transmitter PCB. All components will be soldered to the plate with their legs folded.

The two transistors and the LEDs are polarized:
The transistor has a flat side, the LED a foot longer than the other is the anode (A), the other is the cathode (K). The audio cable (minijack) must be transformed from a stereo cable into a cable.

Mono Sound:
Soldering together the white and red cables, leaving aside the yellow cable (mass). The frequency setting will be turning the variable capacitor gently with a screwdriver or thin cardboard but rigid.

FM Transmitter Parts List
* 1 Ohm resistor 510 (green - brown - brown)
* 100 resistor 1 kOhm (brown - black - yellow)
* 1 MOhm resistors (brown - black - green)
* 1 capacitor 0.1 uF (0.1)
* 1 nF capacitor 47 (0.047)
* 1 capacitor 4.7 pF (479)
* 2 pF capacitors 22 (22)
* 1 variable capacitor 1.5 pF ... 15
* 2 transistor BF 246 (F246A)
* 1 red LED
* 1 audio cable (minijack)

Source: USB Powered FM Transmitter Circuit

Broadcast FM Transmitter 88-108 MHz

2009-11-07T08:08:00.000-08:00

The circuit is identical to the Wideband FM Transmitter but with a few small additions. TR1 (BC547) is an inverted Hartley oscillator which based upon an inductor fabricated on the PCB. This makes it megga-stable, and setable anywhere in the VHF FM band (76MHz to 119MHz) and the BB105 varicap makes it voltage tuneable over about 8MHz of that band. The inductor has one tapping for feedback and a second to feed an optional prescaler. TR2 is a buffer/amplifier and TR3 it the PA stage.

Mainly, the PCB; it has all the optional bits on it to make it into a FM pirate radio broadcast transmitter. It is NOT limited to just pirate radio, you can still use it for other purposes and frequencies. The noise floor is low enough to make a good NBFM transmitter. But let me assume you want to build a braodcast bands TX and it is legal in your country.

The Prototype
Now that is a bit of a laugh. I have already built loads of these on the same PCB so the use of the term "prototype" is a bit misleading. All functioned well and very close to each-other in specification. Output power of the last six units is about 25% higher than that observed from the first V7, probably because the first one had so much "hacking" around the PCB. Anyway, here is a picture of the final unit.

In these two pictures you can see the completed unit, plus a view of the PA stage. At the time of the photograph it was loaded with a 6v 0.1A bicycle lamp and fed with +15v DC supply. The lamp is 600mW lamp at 60 Ohms when fully lit. Here it is lit to well over half brilliance (lowers impedance). This particular unit is delivering +27dBm, according to my Hewlet Packard analyser. I think I am happy with that!

Download PCB
Continue reading: WBFM TX V7b

Wideband FM Transmitter 88-108 MHz

2009-11-07T07:39:00.000-08:00

This FM transmitter is for the 88MHz to 108 MHz band. This particular TX is of special interest to those wishing to build low power Power Amplifiers for the VHF bands since it used impedance matching, power amplifier and antenna filtering, all of which should be used by radio constructors, whether it be for amateur radio or any other form of radio.

FM Transmitter Circuit
The circuit itself is fairly conventional, with a couple of small refinements. It all begins with TR1 (BC547) in an inverted Hartley oscillator configuration. The feedback to the Base of TR1 is via a small 4.7pf capacitor to help keep the oscillations as weak as possible whilst allowing the oscillator to be a reliable starter. The frequency of the oscillator is determined by L1 and the 22pf trimmer capacitor and functions in the range of about 76MHz to 119MHz using the PCB I have made.

The 15pf capacitor couples the top of L1 to the varicap diode which serves to add more capacitance to the tuned circuit to alter the frequency. R1 adds the supply voltage to the varicap, with a little noise decoupling (the 0.1uf capacitor). If you are to use synthesiser control then it is important to remove R1 from the circuit, then connect the synthesiser loop filter output to the terminal marked "Ctrl". Audio is coupled to the BB105 via a 47K resistor. There is only 47pf of decoupling in order not to restrict the AF bandwidth of the complete transmitter. The AF bandwidth is flat from 3Hz to about 72KHz, but if we look beyond these limits, there is an increase of +6dB at DC. This is because the two 47K resistors divide the AF input voltage by 2, but at DC the 0.1uf capacitor has time to charge, the two 47K resistors do not therefore divide.

TR2 (BC547) is both biassed and directly connected to the Emitter of TR1, which is a little unconventional in a VHF circuit. I needed to get a good input to TR2 and cut down on components. There are already far too many coils as it is in this circuit. Remember that the BC547 is an audio transistor but works well at VHF. The inductor in the Emitter of TR2 helps to extend the response a little to give a bit more signal to drive the final power amplifier transistor (TR3). TR2 gives no voltage gain; it is current we need to drive TR3. We already have enough volts from the oscillator.

22pf and L3 couple TR2 output into the Base of TR3. These components match the impedances so we get the maximum power possible into TR3 Base. The signal level, however is still quite low, so some DC biasing has been added to turn TR3 ON a bit. The transistor should draw about 5mA with no signal. This is not enough to make it become linear, but it is operating around class "B". This would make a very poor frequency multiplier, so harmonics are also reduced a little by the DC bias. Note that NO emitter resistor has been used. The prototype units all worked well without one and the drive level is not enough to cause the transistor to conduct very much. The small standing DC bias of 5mA doesn´t even "tickle" TR3. In operation the DC voltage on the Base of TR1 will be negative due to the drive level, conduction of TR3 Base/Emitter junction and the 22pf capacitor. TR3 does NOT need a heatsink.

If you want to use a different transistor in place of TR3 then I suggest you remove L3 and substitute a current meter in place of L4. Apply volts to the transmitter. The current should be about 5mA. Select the value of the 47K resistor if required. Any current reading between about 2mA to 8mA "will do nicely sir" (even without your American Express card!)

The collector of TR3 (2N4427) has a big (by QRP standards) choke to pass the supply DC, but presents a high impedance to RF. The RF signal is then matched to 50-Ohms with the 15p, L5 and 56p. The 1nf cap simply blocks the supply voltage that would otherwise pass to the antenna. L5 and 56pf form a low-pass filter that helps to block harmonics present in the output signal. L6 and 47pf are added to further reduce the harmonic levels. This filter is an absolute MUST for all transmitters if one does not wish to offend every other user of the radio spectrum. L5 and L6 have also been positioned on the PCB so that there is a little coupling between them. This coupling serves to cancel out any residual signals, not within the passband of the filter, that may be present at the input to L5. It is this effect that was responsible for the unexpected cleanliness of the first prototype, and a little layout experimenting has now reduced the 2nd and 3rd harmonics to -60dBc at all supply voltages. With 150mW output, this corresponds to 3rd harmonic of 150 nano-watts and a 2nd harmonic level of just 50 nano-watts.

I have "played around" with the values and taken a few liberties. If you want to try adjusting the coils then then will be able to get another 2 to 3dB out of the TX. I have deliberately mis-tuned a couple of times in order that impedances and resonances will improve at the edges of the band. The result is that the performance of the transmitter does NOT vary (much) no-matter which end of the band you are operating at.

Download PCB
Continue reading WBFM TX V7

Crystal Controlled FM Transmitter

2009-11-07T07:12:00.000-08:00

The FM transmitter is relatively simple to build and with only one adjustment it is ideal for the absolute beginner. But before I continue, let me just make one thing clear. With Wide-Band deviation, the VCO is never locked in phase, only in frequency. It uses a Varicap diode to modulate an oscillator, then lock the oscillator in a Phase Locked Loop, and compare it to a crystal oscillator.

Circuit Basics
The circuit follows that of a simple textbook synthesiser comprising a Voltage Controlled Oscillator (VCO), a damped Loop Filter, a Reference Oscillator (Crystal) and a Frequency Comparator.

The only difference is that I have added a frequency divider chip to divide the VCO frequency by 64. This means that if the VCO operates at 100MHz, the output from the divider will be 1.5625MHz. If the crystal oscillator is also 1.5625MHz then the loop will be "in-lock." The control voltage from the filter to the VCO steers the frequency of the VCO so that the output of the divider is ALWAYS 1.5625MHz. Any deviation from this will result in a change of the loop voltage to move the VCO back to 100MHz.

Audio frequencies are then added to the loop voltage that control the VCO frequency. It is in this way the synthesised transmitter is modulated.

Circuit Specific
I will not delve too deeply into the in's and out's of synthesisers. I have already written several pages of information about them and the stages that are needed to make a working synthesiser.

The specific circuit is shown above. The "Prescaler" (divider) chip needs to have a supply voltage of only 5v (+/- 0.25v) so the LM317 has been included. I used the LM317T due to it's larger can size (and I have got a lot of them) so it will tolerate a supply voltage of greater than 13.8vDC without burning. All has been somewhat over-engineered.

IC1 (CD4001) is the crystal oscillator with an extra gate used as nothing more than a buffer stage. This feeds the frequency comparator of a CD4046 (IC2). The comparator output is filtered with a "slack- handfull" of resistors and caps to feed the VCO; a BC547. A second BC547 has been used to isolate the VCO from the antenna. Without this device the loop would have the tendency to jump out of lock if you touched the antenna. The VCO is also coupled to the divider, IC3, which can be any one of a selection of chips. MB501, SA701, SP8704 and CA12022 are all the same device.

The filter time-constant is a couple of second or so. This makes it take about one second for the loop to stabilise. If this were not the case then the modulating frequency would be seen as a frequency error and the loop would correct the error (remove the modulation). The modulation input is via a 100K resistor and 3n3 capacitor which provides the little pre-emphasis needed for an FM broadcast transmitter. If your AF input comes from a stereo encoder then remove the 3n3 since the pre-emphasis must occur BEFORE encoding. I hope to post a stereo encoder soon, but I give no promisses.

If you are one of those who likes to dissmantle circuits, then you may notice that I have used the CD4046 Signal and Reference the wrong way around - I have used the signal input for the Reference frequency and the Reference input for the signal frequency. This is because the Reference input is designed to be fed from an external source and so it will respond to small small signals (ca: 200mV) whereas the Signal input is designed to be fed from the CD4046s own CMOS logic level oscillator. IC3 is an ECL device with only 1v output signal and is therefore NOT CMOS compatible. The result of this is that the output sense of the frequency comparator is reversed! That is why the Varicap Diode (BB105) is reference to +5v and Not to Ground.

Download PCB
Continue Reading Xtal-Controlled FM TX (V6)

Long Range FM Wireless Microphone

2009-11-07T03:00:00.000-08:00

Here's a long range FM Wireless Microphone, which also has a better frequency stability, over 1 Km range (under ideal conditions) and is good on microphone sensitivity. This has been achieved by adding an RF amplifier buffer (with 10dB gain) and an AF preamplifier to boost the modulation a little.

Construction is quite simple. L1 is 3.25 turns in spiral form and is an integral part of the PCB foil pattern. The two BC547 transistors can be replaced with (almost) any small-signal NPN transistor, such as the 2N2222. The final stage is a BC557 PNP general purpose device. If you use different devices then you should select the 1M0 resistor for 5-volts DC at the collector of the the first transistor. Select the 47K resistor for 3 - 4 volts on the collector of the third transistor. Here is the V5 component overlay drawing. Note that there is a modification:

Download PCB

There used to be a 1n0 5mm cap for supply decoupling, but after a cange of component supplier (manufacturer?) there developed some form of RF instability when the gain of the PA transistor was a little above normal. Replacing the 1n0 to an electrolytic capacitor of 22uf cured this problem totally. Any "radial" (the leads both come out of the same end) type electrolytic capacitor from 0.47uf upwards cures the problem. The finished unit draws about 30mA which should vary as you touch the tuned circuit, a good test that the unit is oscillating. You should remove the 4K7 resistor if you use a dynamic microphone.

The PCB is 50mm x 25mm, a little larger than the first version but there are three stages instead of just the one. The first prototype is shown above, beside the battery powering it. The output power is about +10dBm which is about 10dB more than the first FM Wireless Microphone. This would theoretically give it 3.12 times the range (1.6Km) but I have only tested it using a handheld receiver with the TX laying on the bench indoors. But I got a comfortable 700 meters (and a few funny looks from our neighbours).

Above you can see the addition of a "gimmick" capacitor added across the 12p tuning capacitor to lower the frequency of the transmitter. Make the capacitor by twisting two lengths of single core insulated hook-up wire, about 2cm long. This will reduce the frequency to the bottom end of the band. Cut short the capacitor to increase the frequency to the desired final frequency. If you cut it a few KHz too high then just twist the gimmick a little tighter.

Source: High Power FM Mic

Simple FM Wireless Microphone

2009-11-07T02:45:00.000-08:00

Here's a miniature FM Wireless Microphone transmitter. The transmitter circuit is very simple and needs no explanation for construction although the kids needed some guidance when soldering.

The Tuning Coil
The coil version uses a 1/4" (4mm) diameter coil wound on a drill bit, although the PCB version has the coil fabricated on the PCB itself. A simple piece of insulated wire about 60 cm (2 feet) was fine for the antenna, and is connected to a 1-turn tapping of the coil. Use tinned copper wire for the tuning coil and not the enamelled wire for kids to build. It is much easier for them to solder the antenna, without "mashing-up" the coil, whilst trying to remove the enamel. The PCB version is ideal for the kits as there is no coil to wind, see the photographs on this page.

Since the antenna is coupled directly to the tuned circuit coil, the final frequency of the oscillator will vary if the antenna or the battery is touched. This could make this little circuit seem unstable. This is a normal feature of this type of circuit. To avoid this you will need to add an antenna buffer/amplifer but this only adds to the complexity - not exactly ideal for beginners. See this FM wireless microphone if you need a more stable circuit.

Finished Mic
For those who would like to see what a finished microphone lookes like, here is one ready built. Note that there is no pad for the microphone, the wires are to be fitted accross the 1n0 capacitor.Download PCB

Microphones
The 4K7 resistor feeds the Electret condenser microphone with DC and the internal FET amplifer will develop an AF signal accross this resistor. I originally used a 47K resistor here. It worked fine but many electret microphones require up to 1mA to operate. Reducing the value to 4K7 increases the microphone sensitivity quite a lot. If you want to use a dynamic microphone then you may omit this resistor completely although it will do no harm to leave it in circuit. Please note that the audio sensitivity of the microphone is not fantastic - i.e. you cannot use it to bug a room. It is intended for you to speak directly into the microphone as you would with any other normal microphone. Do NOT expect to hear a whisper at 5 meters (15 feet) because you won't.

The microphone sensitivity is quite low. This is normal for this type of circuit not incorporating a microphone amplifer. A high-output type microphone is therefore required. If you need a microphone amplifer then see this FM wireless microphone for a suitable circuit.

Source: Simple FM Mic

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2009-05-18T01:32:00.001-07:00

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2009-05-18T01:31:00.000-07:00

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If you have any questions regarding the content in this weblog, about the products that are mentioned, or just any questions at all don’t hesitate to contact me at the following email address: wirelesstx@telkom.net.

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2009-05-18T01:30:00.003-07:00

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I'm just a blogger to spend my spare times.