Build-very-low-cost-FM-transmitter

1. No. of stage: 4

2. Frequency of operation: About 100MHz

3. Antenna type: Folded 300 ohms dipole.

4. Range obtained in free space: Up to 4km with dipole antenna 30 feet above ground level. More   range with yagi antenna.

Brief Description:

The transmitter is built on a Printed Circuit Board. This board uses track inductor for L1, L2 and part of L3. The section built around Q1 is the oscillator section. Oscillation frequency is determined by L1, C4 & C5 which forms the tank. Actually C5 is the feedback capacitor. This is required to sustain oscillation. This also influence the operation of tank formed by L1 & C4. Modulation is directly applied to the base of Q1 via C2. A microphone is connected here to serve this purpose. You can alternately feed direct audio here after disconnecting the microphone biasing resistor R1. Q2, Q3 & Q4 gradually raises the output power up to the desired level.

As most of the inductors are PCB etched, there is practically very little frequency drift provided you use a highly regulated and ripple free power supply.

RF output from the transmitter is taken from the junction of C11 & C12. This is unbalanced output of around 75 ohms impedance. But a folded dipole is a balanced type antenna of around 300 ohms impedance. So we need to use a 'BALanced to UNbalanced transformer' or 'BALUN'. A 1:4 type BALUN is employed here for this purpose. Antenna connection is taken from this BALUN via a 300 ohms flat parallel feeder cable commonly used in television to receive terrestrial broadcast. No coaxia is used to feed antenna. This saves cost. Also a parallel feeder cable provides much less signal loss compared to a coaxial.

Design of BALUN
The BALUN is made using a two-hole binocular ferrite bead as shown above. You need to use parallel insulated twin wire to construct this. This wire is commonly used to wind TV BALUN transformer. If you want to get rid of this, then buy a ready-made TV BALUN that is generally used at the back of your television set for interfacing with feeder wire.  

 
If you prefer to build this yourself, the circuit diagram is given above. You need to carefully construct it keeping in mind about the 'sense' & 'direction' of turns. See there are four coils. Two coils in the upper section, which are red and blue, required to be wound on left side of the BALUN and the remaining two (blue & red) in the lower half to be wound on right side. Connection marked 'A' and 'B' at the left side of the circuit is reqired to be connected to the PCB at the shown point. As dipole antenna is balanced type, so you need not to worry about its connection.

 

PCB design details
The transmitter is built on a single sided PCB. As mentioned earlier, this PCB has a number of etched inductors. For this reason, you need to very carefully construct the PCB as mentioned below.

 

In the copper side view, you can see that there are three track etched inductors that resembles 'RCL' Every corner and track width/length are calculated and then they are drawn so that each 'RCL' section becomes an inductor of required value. Never play with this; otherwise, optimum result could not be achieved.

You need to use a laser printer or a high quality printer to get a printout of the drawings. First, save the picture to disk. Now try to print it from such a software which permits you to control print size. 'Paint Shop Pro' is such a software. Of course you can use any other software. Print the drawing so that copper side drawing is exactly 59mm X 59mm. Few trial will give you the perfect print. Now construct the PCB using 'Photo-etching' method so that all the tracks becomes exactly same as you are now seeing. Now drill the PCB carefully. The PCB is now ready to populate.

Start population according to the component mounting plan. You can also get a true size copy of this plan printed and glued to the PCB. This will help you work fast.Part of L3 is required to be constructed. This is described in parts list.

Please note that in the picture of the transmitter kit, capacitor C1 & C10 are not mounted by mistake and the kit is filmed. Please add these two capacitors. Try to keep all component leads as short as possible.

Now you need to design the dipole antenna to use with the kit.

N.B: Believe it or not, a 2N2369 from Philips, used in the final power amplifier section, can give this much of range. 

Detailed Parts List:

RESISTORS

R1 - 22K

R2 - 100K

R3, R7, R9 - 1K

R4, R8 - 100E

R5 - 390E

R6 - 330E

R10 - 15E

R11 - 10K

CAPACITORS

C1, C3, C10 - 1n

C2 - 100n

C4,C8,C9 - 47pF

C5, C11 - 10pF

C6 - 100uF/25V Electrolytic

C7 - 100pF

C12 - 3pF

TRANSISTORS

Q1, Q2, Q3 - BC548

Q4 - PN2369 (Plastic casing) or 2N2369 (Metal casing)

MISC.

L3 - 7 turns, 22SWG wire, 3mm ID, Close wound, Air core.

Two hole binocular BALUN core, BALUN wire, 300 ohms TV feeder wire,

JP1 to JP5 - All jumper wires.

This completes the Project. Please mail me with your feedback. It will really encourage me to give you more & more project like this.

Battery charge controller

A charge controller circuit for battery makes you hassle-free by not bothering about switching the charger with different battery levels, the main advantages being improved life and low current consumption as it switches off the charger when battery full charged. So far we have published a simple battery charge controller circuit using 555 IC where many of our readers commented that it was difficult for them to set the threshold levels and also it seemed like it was harder to analyze working of the same.
For the above reasons, I have decided to introduce another one where you could find it easier to set the threshold levels. Furthermore, this charge controller design makes it easier for debugging. The main component of this circuit is a LM324 comparator IC which has four inbuilt comparators inside; here we are making use of only one. You can use any other comparator ICs instead of this one, like LM358, LM317, LM339 etc. I have used lm324 here owing to its high availability. For basic electronics learners see what is a comparator.
Read: Battery charging circuit with battery level indicator
As the threshold levels to turn ON and turn OFF the charger are different (hysteresis), it helps to avoid oscillation problems of the relay due to leakage current of battery. The threshold levels can be set by varying the potentiometer. As we are switching the charger with the help of relays, you can connect any capacity battery. High current transformers are recommended when using higher capacity batteries to decrease the charging time. For small capacity batteries you should use a series resistance to limit the charging current and that’s not necessary for higher capacity batteries.
This charging controller circuit can be applied with any system that makes use of rechargeable batteries like emergency, UPS, inverters, telephone receivers etc. Here we have used LEDs to indicate the charging status. Also at the bottom I have uploaded a video demonstration to guarantee you that this charge controller schematic is 100% working.

Battery charge controller circuit diagram

Components Required:

1. LM324 IC
2. Zener diode 5.1V
3. Bridge IC/1N4007X4
4. Transistor BC548X2
5. Led Green
6. Capacitor 1000uF
7. Resistor 1KX4, 100K
8. Pot 50K/47K
9. Relay 12V/10A
10. Transformer 0-15V/2A

Working:

• Step down transformer steps down the line voltage to 15 volts and bridge rectifier does the AC to DC conversion process (rectification), it’s followed by the capacitor filter which removes AC ripples.

• The power supply for the working of charger circuit is given from the battery (as it should always be monitoring the battery voltage level even when the mains supply is disconnected).

• The non-inverting terminal of the comparator is connected to a zener diode and 1k resistor which are used to make a reference voltage of 5.1 volt, and its inverting terminal is used to monitor the battery level.

• Usage of zener diode instead of resistors makes sure that the reference value is independent of battery level and temperature variances.

• In fact the comparison is not between reference voltage and the actual battery voltage, but between a proportional value of battery level and reference value. This proportional value is achieved by using a potentiometer.

• If the battery level is below the Lower Threshold Point (say LTP), then the reference voltage becomes higher than the proportional battery value. Thus output of comparator becomes positive.

• Then the transistor is switched ON (as output of comparator is connected to the base of transistor BC548 through a 1k resistor) followed by the relay. I have used a freewheeling diode along with the relay to de-energize the inductor.

See:Working of relay with animation

• And when the comparison goes reverse, output of comparator is low and transistors are OFF resulting in the disconnection of supply.

• Output of comparator is also connected to another BC548 through a 1k resistor which make hysteresis. That means when the charger is ON, transistor is ON and it makes a 100k resistor parallel with the respective voltage divider, then the drop across the divider will again decrease increasing the charging level.

• Upper Threshold Point (voltage level when charger is switched OFF) can be set by varying the potentiometer.

• The difference between UTP and LTP (hysteresis) can be varied by changing the 100k resistor, decreasing the resistance increases the hysteresis and vice versa.

• A 8.2ohm 10 watt resistor connected in series with the circuit helps to limit the charging current. It’s not required for higher capacity batteries as the charging time will increase.

 

IR Remote Switch Circuit

This circuit lets you control any line powered electrical device (a lamp, television, fan, etc.) using any infra-red remote control. Almost everyone these days has a pile of old IR remotes left over from appliances they have long ago disposed of them. With this circuit, you can put them back into use. The circuit looks for any modulated IR source and uses it to control a TRIAC, which then switches any appliance connected to it's socket. For example, you can use it to control the room lighting in your home theater setup using any of the remotes you already have. The circuit is powered using a simple transformer less power supply from the line itself, making it compact and easily built into a light switch, wall box, power bar or even the appliance you wish to control. Schematic

Part	Total Qty.	Description
R1	1	3 Meg 1/4W Resistor
R2	1	1.2 Meg 1/4W Resistor
R3	1	680 Ohm 1/4W Resistor
R4	1	2K 1/4W Resistor
R5	1	4.7K 1/4W Resistor
R6	1	150 Ohm 1/4W Resistor
C1	1	0.001uF Ceramic Disc Capacitor
C2, C5	2	1uF 50V Tantalum Electrolytic Capacitor
C3	1	47uF 50V Tantalum Electrolytic Capacitor
C4	1	10uF 50V Tantalum Electrolytic Capacitor
C5	1	150 Ohm 1/4W Resistor
D1	1	1N4733 5V Zener Diode
D2	1	1N4003 Rectifier Diode
Q1	1	2N6071A TRIAC
U1	1	GP1U52X IR Module
U2	1	MC74HC74 D-Type Flip Flop
U3	1	MOC3011 Opto Isolator
MISC	1	Board, Sockets For ICs, Mains Socket, Mains Plug and Cord, Wire

 

1. Under normal circumstances, Q1 should not need a heatsink.
2. The circuit is designed for a supply voltage of 120V.
3. The printed circuit pattern is reproduced here larger then real life for clarity. It will need to be resized to the scale at the bottom of the image if you intend to transfer it to a board.

The circuit functions as an on/off flip flop. Illuminate it with your remote once to turn it on, then again to turn it off.