Wednesday, December 25, 2013

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

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

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

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
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

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
LED On the Air White LED

Tone Selector
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  

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
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.

FM Radio Station Part 5 Index