tranmistor.htm

FM MUDULATION SYSTEM COMUNICATION

A small signal diode (1N914 or 1N4148) is used as a varactor diode so
that the total capacity in parallel with the inductor varies slightly
at the audio rate thus causing the oscillator frequency to change at
the audio rate (600 Hz). The ramping waveform at pins 2 and 6 of the
timer is applied to the reversed biased diode through a large (1 Meg)
resistor so that the capacitance of the diode changes as the ramping
voltage changes thus altering the frequency of the tank circuit.
Alternately, an audio signal could be applied to the 1 Meg resistor to
modulate the oscillator but it may require an additional pullup resistor
to reverse bias the diode. The N channel JFET transistors used should be
high frequency VHF or UHF types (Radio Shack #276-2062  MPF102) or similar.
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<center><h3>12 Volt Lamp Dimmer</h3></center>
<p>
Here is a 12 volt / 2 amp lamp dimmer that can be used to dim a
standard 25 watt automobile brake or backup bulb by controlling the duty
cycle of a astable 555 timer oscillator. When the wiper of the potentiometer
is at the uppermost position, the capacitor will charge quickly through both
1K resistors and the diode, producing a short positive interval and long
negative interval which dims the lamp to near darkness. When the
potentiometer wiper is at the lowermost position, the capacitor will charge
through both 1K resistors and the 50K potentiometer and discharge through
the lower 1K resistor, producing a long positive interval and short negative
interval which brightens the lamp to near full intensity. The duty cycle of
the 200 Hz square wave can be varied from approximately 5% to 95%.
The two circuits below illustrate connecting the lamp to either the positive
or negative side of the supply.
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<center><h3>Automatic 12 Volt Lamp Fader</h3></center>
<p>
This circuit is similar to the "Fading Red Eyes" circuit (in the LED section)
used to fade a pair of red LEDs. In this version, the lamps are faded by
varying the duty cycle so that higher power incandescent lamps can be used
without much power loss. The switching waveform is generated by comparing
two linear ramps of different frequencies. The higher frequency ramp waveform
(about 75 Hz.) is produced from one section of the LM324 quad op-amp wired
as a Schmitt trigger oscillator. The lower frequency ramp controls the
fading rate and is generated from the upper two op-amps similar to the
"fading eyes" circuit. The two ramp waveforms at pins 9 and 1 are compared
by the 4th op-amp which generates a varying duty cycle rectangular waveform
to drive the output transistor. A second transistor is used to invert
the waveform so that one group of lamps will fade as the other group
brightens. The 2N3053 will handle up to 500 milliamps so you could
connect 12 strings of 4 LEDs each (48 LEDs) with a 220 ohm resistor in
series with each group of 4 LEDs. This would total about 250 milliamps.
Or you can use three 4 volt, 200 mA Xmas tree bulbs in series. For higher
power 12 volt automobile lamps, the transistor will need to be replaced
with a MOSFET that can handle several amps of current. See the drawing
below the schematic for possible hookups.
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</p><center><h3>Other possible hookups </h3></center>
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<center><h3>1.5 Hour Lamp Fader (Sunset Lamp)</h3></center>
<p>
    Similar to the one above, the sunset lamp comes on at full brightness
    and then slowly fades out over 1.5 hours time and stays off until
    power is recycled.

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<center><h3>Sunrise Lamp </h3></center>
<p>
    In this circuit, a 120VAC lamp is slowly illuminated over a approximate
    20 minute period. The bridge rectifier supplies 120 DC to the MOSFET
    and 60 watt lamp. A 6.2K, 5 watt resistor and zener diode is used to
    drop the voltage to 12 volts DC for the circuit power. The bridge
    rectifier should be rated at 200 volts and 5 amps or more. In operation,
    a 700 Hz triangle waveform is generated at pin 1 of the LM324 and a
    slow rising voltage is obtained at pin 8. These two signals are compared
    at pins 12 and 13 to produce a varying duty cycle rectangular waveform
    at pin 14, which controls the MOSFET and brightness of the 60 watt lamp.
    When power is applied, the lamp will start to illuminate within a minute
    or so, and will slowly brighten to full intensity in about 20 minutes.
    You can make that longer or shorter with adjustments to the 270K resistor
    at pin 9. The 2.2 ohm resistor and .015uF cap connected to the lamp serve
    to supress RFI. The diode at pin 9 and 10K resistor on pin 8 are used to
    discharge the 3300uF cap when power is removed. Power should be off
    for a few minutes before re-starting.

    </p><p>
    Caution:

    This circuit is connected directly to the AC line and presents a hazard
    if any part is touched while connected to the line. Use caution and do
    not touch any parts while the circuit is connected to the AC line.
    You may want to use a 9 volt battery connected across the 12 volt
    zener to check the basic operation. The DC voltage at pins 1,2,3,5,6,7
    will all be around 4.3 volts if the circuit is working correctly.
    If the DC voltages are all correct, you can use a variac to slowly
    apply the full line voltage and check for proper operation.

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<center><h3>Simple Op-Amp Radio</h3></center>
<p>
This is basically a crystal radio with an audio amplifier which is fairly
sensitive and receives several strong stations in the Los Angeles area with
a minimal 15 foot antenna. Longer antennas will provide a stronger signal
but the selectivity will be worse and strong stations may be heard in the
background of weaker ones. Using a long wire antenna, the selectivity
can be improved by connecting it to one of the taps on the coil instead of
the junction of the capacitor and coil. Some connection to ground is required
but I found that standing outside on a concrete slab and just allowing the
long headphone leads to lay on the concrete was sufficient to listen to the
local news station (KNX 1070). The inductor was wound with 200 turns of #28
enameled copper wire on a 7/8 diameter, 4 inch length of PVC pipe, which
yields about 220 uH. The inductor was wound with taps every 20 turns so the
diode and antenna connections could be selected for best results which turned
out to be 60 turns from the antenna end for the diode. The diode should be a
germanium (1N34A type) for best results, but silicon diodes will also work if
the signal is strong enough. The carrier frequency is removed from the
rectified signal at the cathode of the diode by the 300 pF cap and the audio
frequency is passed by the 0.1uF capacitor to the non-inverting input of the
first op-amp which functions as a high impedance buffer stage. The second
op-amp stage increases the voltage level about 50 times and is DC coupled to
the first through the 10K resistor. If the pairs of 100K and 1 Meg resistors
are not close in value (1%) you may need to either use closer matched values
or add a capacitor in series with the 10K resistor to keep the DC voltage at
the transistor emitter between 3 and 6 volts. Another approach would be to
reduce the overall gain with a smaller feedback resistor (470K). High
impedance headphones will probably work best, but walkman stereo type
headphones will also work. Circuit draws about 10 mA from a 9 volt source.
Germanium diodes (1N34A) types are available from Radio Shack, #276-1123.
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