FM Radio Station Microphonic Part 25

FM RADIO STATION MICROPHONIC PART 25

HOW DO YOU TRANSMIT VOICE WITHOUT A MICROPHONE?

This is a two transistor circuit that makes up a wireless FM radio transmitter with a remarkable feature - it transmits voice without a microphone! 

It uses a floppy coil that moves upon receiving the vibrations from speech or other sources.


"This means it will vibrate when bumped and will even pick up sounds such as talking, music and footsteps and transmit audio just like a microphone.

Forget the phone lines as they are not needed for this example. Build the circuit and use 7 turns of thin wire on an 8-10mm pen and see how the coil picks up every sound in the room. Simply connect a 9v supply and the circuit starts broadcasting. The coil should be 6t and 3mm diameter, using 0.5mm enamelled wire."

Source
http://www.talkingelectronics.com/projects/Spy%20Circuits/SpyCircuits-3.html

FM Radio Station Sensitivity Part 21

FM RADIO STATION SENSITIVITY CONTROL PART 21

Sensitivity circuit, adjust values as needed

This project installs a sensitivity control to the FM radio station. R1 is a static preset 1K ohm resistor as seen in the original schematic. Add an optional resistance with a 0 to 10K ohm variable potentiometer as shown. The variable potentiometer becomes the sensitivity control. Just dial in the amount required.

Calibrate the sensitivity control based on the sound source to improve the sensitivity and quality of the signal. The microphone will be the most sensitive when the value is at zero ohms. R1 is actually an optional resistor as shown. It's purpose is to limit the voice sensitivity, for example, a 10K resistor will resist the voice signal coming from microphone more as compared to the 1K resistor.

FM Radio Station Tone Part 16

FM RADIO STATION TONE CIRCUIT PART 16

Here's a modification to make a tone selector switch for the FM radio station's electret microphone output. Changes for actual use require adjusting the 8.2K ohm resistor and the 1nF capacitor based on the voltage supply.

A tone selector switch is used as a high pass filter to provide some tone adjustment. The 47nF setting reduces the low frequency and puts more of the midrange into useful talk power.

For FM, a 1uF setting is used which provides more full range audio suited to FM voice.

The electret mic element inherently has a strong gain. The intensity of voice and the position of person speaking can be adjusted to provide the best gain without distortion. Experiment with distances and component values to find the best results.

http://humanoidolabs.blogspot.tw/2013/12/fm-radio-station-controls.html

FM Radio Station Part 5 Index
http://humanoidolabs.blogspot.tw/2013/12/fm-radio-station-part-5-index.html

FM Radio Station Microphone Part 15

FM RADIO STATION MICROPHONE PART 15

Electret Microphone
http://sound.westhost.com/project93.htm

A typical electret microphone preamp circuit uses an FET in a common source configuration. The two-terminal electret capsule contains a FET which must be externally powered by supply voltage V+. The resistor sets the gain and output impedance. The audio signal appears at the output, after a DC-blocking capacitor.

THE MICROPHONE is an important part of the radio station. In a simplified circuit, it must deliver reasonable sound quality and provide a usable frequency response and audio level.

An electret condenser microphone (ECM) consists of a very light diaphragm (moving plate) and back plate (stationary or static plate) and has a permanent charge implanted in an electret material to provide polarizing voltage.

The principle of operation is that sound waves impinging on the diaphragm cause the capacitance between it and the back plate to change synchronously, this in turn induces an AC voltage on the back plate.

1. Foil Electret Condenser Microphone (also called Middle or Classic) - Type of condenser microphone where the electret material is the diaphragm. (Sometimes referred to as "Front" type.)

2. Back Electret Condenser Microphone -Type of condenser microphone where the electret material is the back plate.

3. Front Electret Condenser Microphone -Type of condenser microphone where the electret material is the inside of the case of the microphone.

"It must be realized that all electret mics (indeed, all mics) have one limitation we cannot readily change, and that is maximum SPL (Sound Pressure Level). Because electret capsules have an integral amplifier, there will always be a level where they will distort.

A capsule having a 10k feed resistor and supplied from a 15V supply will output well over 1V RMS quite easily, simply by having it close enough to your mouth as you speak loudly.

Even professional microphones (including dynamic types) are quite capable of 0dBm in close proximity to a floor tom or a loud singer. As a result, close vocal work, drums and brass instruments (trumpet, sax, etc) are capable of extremely high SPL, and are not really suitable candidates for electret mics. It is possible to get good performance at up to 115dB SPL quite easily - possibly more. 

Sensitivity
The sensitivity can be reduced, simply by reducing the value of the feed resistor. Again, there is a limit, as the internal FET amplifier can be driven into distortion regardless of what you do on the outside of the capsule. It is feasible to modify the capsule itself - but this is only possible with some models unless you are willing to make a few sacrifices (you can guarantee that you will ruin a couple in the process)." 

Frequency response
http://www.digikey.tw/Web%20Export/Supplier%20Content/PUI_668/PDF/PUI_ElectretCondenserMicrophone%20Basics.pdf?redirected=1

"Frequency response is the microphone's sensitivity performance in the frequency range of 0 to 20 kHz. Compared with the dynamic types, ECMs tend to have an extended response both at low and high frequencies, which is also smoother.In the case of dynamic types, the response limits are defined where the sensitivity has fallen by 3 dB relative to its value at 1 kHz. It is not stated in such terms for ECMs since they have a much wider frequency response." 

Usage

"To obtain the best results, avoid placing near reflective surfaces. If possible, mount in soft rubber to insulate from vibration. Treat ECMs as stat ic sensitive. Use specified heat sinking when soldering to ECMs. Use as little heat as possible for shortest time consistent with good joint. Avoid subjecting ECMs to high temperature and humidity that can degrade performance. The frequency response of ECMs may be too extended for communications application, in which case, it can be limited by suitable acoustic filtering in the intended housing or handset."

Microphone Setup
The legs on the electret microphone can extend through the cabinet. The mic is supported using a single strand color insulated copper wire. The mic electrical cable has a negative shield side to reduce interference. The microphone has a foam covering fabricated from a discarded headset. The wire support is bendable for best positioning.

Microphone Modifications
One improvement, since moving very close to the mic produces a much better signal and greater clarity, is a funnel that can direct the sound into the mic. This may be a great improvement and needs further testing. It may resemble a miniature old fashioned Thomas Edison "Gramophone." The cone can be fabricated from paper, cardboard or plastic from a bottle.

Microphone Measurement
The microphone resistance is measured at 1.37 ohms.

Microphone Impedance
http://www.edaboard.com/thread217554.html

http://www.gearslutz.com/board/geekslutz-forum/764918-what-should-follow-electret-mic-capsule.html 

Definitions
SPL - Sound Pressure Level

Microphone - Receiver that converts sound energy to electrical energy. Also described as a sound sensor.

Condenser - Another term used for a capacitor: two plates separated by a fixed distance capable of storing an electric charge.

Electret - An electrical property which describes the capability to retain electrical charges.

Pascal (Pa); bar (1Pa = 10 u bars) - Unit of pressure. For microphone applications, this unit refers to 
the pressure of sound on a microphone.

Links
http://humanoidolabs.blogspot.tw/2013/12/fm-radio-station-controls.html

http://en.wikipedia.org/wiki/Electret_microphone

FM Radio Station Part 5 Index
http://humanoidolabs.blogspot.tw/2013/12/fm-radio-station-part-5-index.html

FM Radio Station Mute Function Part 14

Radio Station mute function schematic

FM RADIO STATION MUTE FUNCTION PART 14

This circuit shows details for adding a mute function to the FM radio station. A switch will mute the microphone. When muted, the LED will stay lit.


Microphone Mute Function with LED monitor
When the DPDT switch is in a normalized position, the electret microphone is active, the LED is not connected, and broadcast takes place. When switched, the microphone cuts out, resistor R2 is connected to replace the microphone, and the LED lights. A carrier signal continues to broadcast but there is no sound. The LED is on a separate circuit and does not interfere with the microphone. The microphone resistance is measured at 1.37 ohms (when not in the circuit).

Note: On the schematic, replace the text SPDT with DPDT.

http://humanoidolabs.blogspot.tw/2013/12/fm-radio-station-controls.html

FM Radio Station Part 5 Index
http://humanoidolabs.blogspot.tw/2013/12/fm-radio-station-part-5-index.html 

FM Radio Station Controls Part 9

FM RADIO STATION CONTROLS PART 9
Over time, the simple FM Radio Station will have upgrades for more controls and more functionality. Below are numerous possible combinations.

Spartan "Basic Two" Radio Station 
On/Off Switch
Power On Red LED
http://humanoidolabs.blogspot.tw/2013/12/fm-radio-power-monitor-mod-part-13.html

A very spartan version with rudimentary controls is possible by using a rectangular project box with a hole for the mic protrusion and holes for the on/off battery switch and red LED.

Four Version Radio Station 
On/Off Switch
Power On Red LED
http://humanoidolabs.blogspot.tw/2013/12/fm-radio-power-monitor-mod-part-13.html

The on/off is a toggle switch wired to turn the 9 volt battery supply on or off. The LED wires in with the on/off switch and a dropping resistor to the LED to indicate when power is on.

Mute Switch
On the Air LED 
http://humanoidolabs.blogspot.tw/2013/12/fm-radio-station-mute-function-part-14.html

Another handy feature is an audible mute switch for the microphone. This cuts out the microphone and switches to a resistor with the same resistance as the microphone. The result is a transmitted broadcast carrier frequency without any sound from the microphone.

Deluxe Version Radio Station 
This deluxe radio station version has many more advanced controls bringing it closer to a professional radio station.

Switch On/Off
http://humanoidolabs.blogspot.tw/2013/12/fm-radio-power-monitor-mod-part-13.html
 
Switch External Antenna A or B
The external antenna switch requires two antenna banana jacks to plug in two antennas at the same time. The switch can enable antenna A or B.

Switch Power from 9V Battery to External Power Jack
A switch that powers off the 9 volt battery supply. Then, the banana power input jack can accept a lower voltage such 6 volts or 3 volts DC and/or simply run off an external power supply instead of internal batteries. This will affect the transmitters power and range. In some cases, it may be useful and necessary to reduce power and range to comply with local regulations.

Switch Mic Mute
http://humanoidolabs.blogspot.tw/2013/12/fm-radio-station-mute-function-part-14.html

Audible mute switch for the microphone. This cuts out the microphone and switches to a resistor with the same resistance as the microphone. The result is a transmitted broadcast carrier frequency without any sound from the microphone.

LED Power On/Off Red LED
http://humanoidolabs.blogspot.tw/2013/12/fm-radio-power-monitor-mod-part-13.html
 
LED On the Air White LED
http://humanoidolabs.blogspot.tw/2013/12/fm-radio-station-mute-function-part-14.html

Tone Selector
http://humanoidolabs.blogspot.tw/2013/12/fm-radio-station-tone-part-16.html
A tone selector switch is used as a high pass filter to provide some tone adjustment. The first setting at 47nF reduces the low frequency and puts more of the mid-range into useful talk power. For FM, the second setting uses a 1uF capacitor which provides more full range audio suited to FM voice.

Sensitivity Control
http://humanoidolabs.blogspot.tw/2013/12/fm-radio-sensitivity-part-21.html  
This modification replaces 1K resistor R1 with a 10K potentiometer to adjust the sensitivity of the microphone and the transmitter signal.

FM Radio Station Shielding
http://humanoidolabs.blogspot.tw/2013/12/fm-radio-station-shielding-part-22.html
The install of aluminum conductive ground shielding, inside the radio station housing, will minimize frequency shift and interference from outside sources such as a human hand.

Banana Jack External Power Supply Black (- ground)
Banana Jack External Power Supply Red (+ 9 volts max)
These are two banana jacks to plug in an external power supply
 
Banana Jack Ground (plug in for antenna load)
Banana Jack Antenna 
These are two banana jacks for two purposes. One, the plug in antenna and ground allows plugging in a light bulb load to determine power output and run tests. Two, an external antenna can plug in.

Banana Jack External Sound Input
This jack is for plugging in an audio source like an iPod, iPhone, or iPad. It can input a source of music or prerecorded programming for FM radio station broadcast.
 
Control Variable Volume
The variable volume control sets the volume of the input microphone. 

Amplifier Monitor Headset
This is an amplified sound monitor that feeds the mono broadcast sound to a headset for monitoring purposes. The amplifier is an LM386 chip. The circuit requires audio conversion. 

2 Sound Source Mixer
This is a basic two source sound mixer. It can mix one sound source with the microphone. It uses the External Sound Input and a control "mixing" knob.

Band - With different switchable or plug style "band" coils, it would be possible to make large changes to the transmitting frequency. It may be more feasible to interchange values of capacitors. With proper orientation, either the band coils or the capacitors could plug into the cabinet to enable particular transmit frequencies.

Trim - With a variable 0 to 20 pF capacitor connected in parallel with the coil, it would be possible to fine tune the transmitting frequency.

Clock - with added processor, a clock is an added feature useful for keeping programming on time

Compass - with added processor, a compass can verify the same equipment and antenna positioning, since position greatly effects transmission and signal characteristics

Program Timer - with added processor, a timer is useful for keeping programs on schedule

Program Timer Switch - with added processor, a timed switch can automate the start of radio programs

Signal Strength - the idea is to determine the strength of the signal being transmitted, using a common DVM and/or pickup coil to calibrate the transmitter to under 100 mW to comply with FCC Part 15 regulations in the USA. Also refer to the installment blog about determining power.
http://humanoidolabs.blogspot.tw/2013/12/radion.html

FM Radio Station Part 5 Index
http://humanoidolabs.blogspot.tw/2013/12/fm-radio-station-part-5-index.html