Posted on Sunday, October 23, 2011 • Category: FM Transmitters
A high quality stereo FM transmitter circuit is shown here. The circuit is based on the IC BA1404 from ROHM Semiconductors. BA1404 is a monolithic FM stereo modulator that has built in stereo modulator, FM modulator and RF amplifier. The FM modulator can be operated from 76 to 108MHz and power supply for the circuit can be anything between 1.25 to 3 volts.
In the circuit R7, C16, C14 and R6, C15, C13 forms the pre-emphasis network for the right and left channels respectively. This is done for matching the frequency response of the FM transmitter with the FM receiver. Inductor L1 and capacitor C5 is used to set the oscillator frequency. Network C9,C10, R4,R5 improves the channel separation. 38kHz crystal X1 is connected between pins 5 and 6 of the IC. Composite stereo signal is created by the stereo modulator circuit using the 38kHz quartz controlled frequency.
Posted on Sunday, October 23, 2011 • Category: Power Supplies
This is an add-on over voltage protection circuit for LM317 voltage regulator. It is a voltage regulator that allows a 6V portable supply to be derived from the 12V car battery. You can add a 6.2V zener diode and a LED to warn you when the input supply is overvoltage. If you could find a relay that would operate from 6.2V right up to 12v that you could connect in such a way that if over voltage occurred, then the relay would automatically switch off the output preventing damage to any connected equipment.
Such a relay would be quite difficult to find, so I designed this, it is a simple two transistor circuit which will switch off the output should the voltage raise above 6.2v this can be changed by selecting a different value of zener diode.
Posted on Tuesday, October 18, 2011 • Category: FM Transmitters
This simple FM Transmitter takes audio input through a 1/4" phono jack and, constructed as shown, without the optional antenna connections, will broadcast an FM radio signal about 30 feet. This is the standard model of simplest FM transmitters includes a trim capacitor to adjust the transmitting frequency. It can be powered by a 9V battery and uses a hand-turned copper coil. The circuit is extraordinarily simple and could be built on perfboard or on a panel almost as easily.
Posted on Wednesday, October 12, 2011 • Category: FM Transmitters
This is a simple design of a small FM Transmitter Bug that's perfect for transmitting and eavesdropping purposes. Due to the high sensitivity, even the ticking of the clock to hear. The range is estimated at anything from 50 meters. With a small piece of wire as an antenna to get at least the whole house. L1 and L2 are two equal air pools. They each consist of 5 turns at a diameter of about 4 mm. The thickness of the wire does not matter, 0.5 mm works perfectly. C4 is the frequency adjustment. Tune an FM radio in an empty area of the FM band and C4 to turn your silence or hear a whistle. From what you can precisely adjust the radio and the transmitter installed in a room somewhere to intercept. Note: Because these transmitter bugs inherently unstable, you better read the short legs of the components keep the circuit mechanically tightly together up. Also placing a 1 nF capacitor (C6) will benefit stability.
R1, R3, R4: 4K7
R6: 270 Ohms
C1, C2: 10 uF
C3, C6: 1 nF
C4: 2-18 pF trimmer
C5: 5.6 pF
L1, L2: air puddle windings on May 4 mm in diameter (see text)
T1, T2: 547 BC
Ook het plaatsen van een 1 nF condensatortje (C6) over de voedingsaanluitingen komt de werking ten goede. [Origineel TinyCAD ontwerp]
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Posted on Saturday, October 8, 2011 • Category: Power Supplies
Presented here is a circuit for 30V 10A variable bench power supply that offers variable voltage and current adjustment. Power supply is based around a LM723 voltage regulator chip and has current limiting. I often end up with the power clips shorting out on the bench and with no problems. I have had this circuit in use now for over 20 years and has never let me down and is one of the most handiest gadgets i have built. The 2N3055 transistors are a well proven high current transistor. More 2N3055 transistors can be connected together for more output current. The transistors will need to be mounted on a good size heatsink.
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