2 km FM transmitter

Description

With a matching antenna, the FM transmitter circuit shown here can transmit signals up to a range of 2 kilo metres. The transistor Q1 and Q2 forms a classic high sensitive preamplifier stage. The audio signal to be transmitted is coupled to the base of Q1 through capacitor C2. R1, R3, R4, R6, R5 and R9 are the biasing resistors for the preamplifier stage comprising of Q1 and Q2. Transistor Q3 performs the collective job of oscillator, mixer and final power amplifier.C9 and L1 forms the tank circuit which is essential for creating oscillations. Inductor L2 couples the FM signal to the antenna.

Circuit diagram

Notes.

• Assemble the circuit on a good quality PCB.
• The circuit can be powered from anything between 9 to 24V DC.
• Inductor L3 can be a VK220J type RFC.
• For L1 make 3 turns of 1mm enamelled copper wire on a 10mm diameter plastic former. On the same core make 2 turns of 1 mm enamelled copper wire close to L3 and that will be L2.
• Frequency can be adjusted by varying C9.
• R9 can be used to adjust the gain.
• For optimum performance, value of C8 must be also adjusted.
• Using a battery for powering the circuit will reduce noise.

Using the 74xx47 BCD to Seven-segment display

The 74xx47 chip is used to drive 7 segment display. You must use the 74xx47 with a common anode 7-segment display (e.g. Kingbright part number SA03). The input to the 74xx47 is a binary number DCBA where D is 8s, C is 4s, B is 2s and A is 1s. The inputs DCBA often come from a binary counter.The display is only sensible if the binary number is between DCBA=0000 (0) and DCBA=1001 (9); this is called Binary Coded Decimal or BCD for short. If the number is larger than 9 you get a strange output on the display. Try this out by moving your mouse over the truth table.
The inputs /,  and  are usually connected to 5v

Simple use

1. Connect V_cc [pin 16],  [pin 3], / [pin 4] and  [pin 5] to 5v.
2. Connect Gnd [pin 8] to 0v.
3. connect DCBA [pins 1, 2, 6 and 7] to DCBA on your counter.
4. Connect        [pins 9-15] to abcdefg on the common anode 7-segment display.

Inputs							Outputs
D	C	B	A	/										Display
0	0	0	0	1	1	1	0	0	0	0	0	0	1
0	0	0	1	1	1	1	1	0	0	1	1	1	1
0	0	1	0	1	1	1	0	0	1	0	0	1	0
0	0	1	1	1	1	1	0	0	0	0	1	1	0
0	1	0	0	1	1	1	1	0	0	1	1	0	0
0	1	0	1	1	1	1	0	1	0	0	1	0	0
0	1	1	0	1	1	1	1	1	0	0	0	0	0
0	1	1	1	1	1	1	0	0	0	1	1	1	1
1	0	0	0	1	1	1	0	0	0	0	0	0	0
1	0	0	1	1	1	1	0	0	0	1	1	0	0
1	0	1	0	1	1	1	1	1	1	0	0	1	0
1	0	1	1	1	1	1	1	1	0	0	1	1	0
1	1	0	0	1	1	1	1	0	1	1	1	0	0
1	1	0	1	1	1	1	0	1	1	0	1	0	0
1	1	1	0	1	1	1	1	1	1	0	0	0	0
1	1	1	1	1	1	1	1	1	1	1	1	1	1

Advanced use

 stands for Lamp Test. When  is low all the segments on the 7-seg display are lit regardless of DCBA.  stands for Blanking Input. When  is low the display is blank so all the segments on the 7seg display are off regardless of DCBA.
 stands for Ripple Blanking Input. When  is low and DCBA=0000 the display is blank otherwise the number is displayed on the display. This is used to remove leading zeroes from a number (e.g. diplay 89 instead of 089). To use with more than one display connect  (Ripple Blanking Output) from most significant 74xx47 to the  of the next 74xx47.
Connect  of the least significant 74xx47 to 5v unless you want the diplay to turn off when the number is 0.

Inputs							Outputs
D	C	B	A	/										Display
X	X	X	X	0	1	1	1	1	1	1	1	1	1
X	X	X	X	1	1	0	0	0	0	0	0	0	0
0	0	0	0	1	0	1	1	1	1	1	1	1	1
0	0	0	1	1	0	1	1	0	0	1	1	1	1
0	0	1	0	1	0	1	0	0	1	0	0	1	0
0	0	1	1	1	0	1	0	0	0	0	1	1	0
0	1	0	0	1	0	1	1	0	0	1	1	0	0
0	1	0	1	1	0	1	0	1	0	0	1	0	0
0	1	1	0	1	0	1	1	1	0	0	0	0	0
0	1	1	1	1	0	1	0	0	0	1	1	1	1
1	0	0	0	1	0	1	0	0	0	0	0	0	0
1	0	0	1	1	0	1	0	0	0	1	1	0	0
1	0	1	0	1	0	1	1	1	1	0	0	1	0
1	0	1	1	1	0	1	1	1	0	0	1	1	0
1	1	0	0	1	0	1	1	0	1	1	1	0	0
1	1	0	1	1	0	1	0	1	1	0	1	0	0
1	1	1	0	1	0	1	1	1	1	0	0	0	0
1	1	1	1	1	0	1	1	1	1	1	1	1	1