A Highly Efficient DC Lamp Dimmer

The simplest lamp dimmer circuit consists of a rheostat, in series with the lamp, which one
may adjust to obtain the required brightness. Such linear regulators are quite inefficient since
a lot of power is wasted in them. Moreover, in the rheostat the moving contacts are likely to
get damaged in the long run, as its value is frequently adjusted by moving the slider. Such
linear control circuits provide an overall efficiency of no more than 50 per cent. This wastage
of power can be avoided if one uses pulse width modulation (PWM) which can be made to
control an electronic rheostat. The circuit shown here is based on PWM principle. Gate N1
and its associated components constitute an oscillator producing oscillations of approximately
200 Hz with a pulse width of 0.1 ms. This output is fed to transistor T1 for level shifting. At the
output of this transistor is a potentiometer VR2, using which a DC component can be added
to the pulses emerging from transistor T1. By adjusting this potentiometer/trimmer, one can
have a good linear control of the lamp brightness from completely off state to 100 per cent on
state. The signal is inverted by gate N2 and fed to MOSFET 12N10. IC CD40106 provides six
inverting buffers with Schmitt trigger action. The buffers are capable of transforming slowly
changing input signals into sharply defined jitter-free output signals. They are usually used as
wave and pulse shapers. IC CD40106 possesses high immunity and low power consumption
of standard CMOS ICs along with the ability to drive 10 LS-TTL loads. In this circuit loads up
to 24W can be connected between MOSFET drain and 12V supply without using a heatsink.
The loads can even be DC motors, miniature heating elements, etc. If one uses a low RDS
(on) MOSFET, a higher efficiency can be achieved. By using the components as shown in the
circuit, an efficiency of approximately 95 per cent can be achieved. The flexibility of the design
makes it possible to change the MOSFET with a similar one, in case of non-availability of
12N10. The circuit by itself does not draw much current when the load is disconnected.
Ensure proper ESD protection while handling the MOSFET to prevent damage. Lab note: The
circuit was tested using MOSFET IRF640 with RDS (on)=0.18 ohm.

Intelligent Automotive Ignition System

INTRODUCTION
Electronic ignition is nothing new. Many “electronic”
ignition systems still rely on mechanical properties of
the distributor for RPM sensitive modifications
(advance/retard) and for actual spark “distribution”.
The proposed system uses a PIC12C508 for total
spark control on a 4-cylinder engine. This system could
be adapted to 6 and 8-cylinder engines by using a
“double-fire” (firing on power and exhaust strokes).
In this system, each cylinder has its own high voltage
coil, allowing a “hotter” spark than is supplied through
the arcing and inaccuracies of a mechanical distributor.
The PICmicro could use either a single or dual sensor
(IR) reading from a “code-wheel”. The dual system
would indicate top dead center of cylinder #1 (or some
other relevant timing mark) and single marks for each
cylinder. The single sensor system would only require
the TDC detector.
The PICmicro would time TDC detections, thereby
determining engine RPM. This RPM would be used in
a lookup table to determine the spark timing (single
sensor) or cylinder detect (dual). Each cylinder would
fire at the appropriate time.
System Benefits:
(over a mechanical design)
Inexpensive processing power means system can be
easily tuned for performance/emissions or other criteria.
Stronger, more accurate spark can be delivered.
No parts to wear, arc, or corrode.

Free Flight Model Aircraft Dethermalizing Timer

INTRODUCTION
This application is for a Free Flight Dethermalizing
Timer for Model Aircraft. This is usually a mechanical or
fuse timer which spoils the lift of a model aircraft after 5
minutes. This is to prevent losing the aircraft in a strong
atmospheric thermal.
Using a bicolor LED and pushbuttons, a delay time of
between two and seven minutes in 10 second increments
is indicated B1 places the device in program mode and the LED
shows green. B2 then enters the number of 10 second
intervals with the LED flashing red for each interval in
groups of 5 for ease of reading. After the interval is
loaded, the timer is armed by pressing B2 again. When
armed the LED goes red. To start the timer, B2 is
pressed and held. The timer starts on release of B2.
When the unit times out, the 2N2222 is turned on, the
wire heats and contacts, and the control surface is
actuated and the 2N2222 is deactivated.
Nictol wire is a shape memory nickel titanium alloy
which contracts 3.5% of its length when heated. Nictol
wire also has a high resistance similar to nichrome
wire. Using 6-10 mil wire, 200 mA will heat the wire to
its activation temperature.

Infra Red Cordless Mouse

INTRODUCTION
Can anybody imagine that this little wonder,
PIC12C509, be used to control a cordless mouse?
Incredible! Just a handful of components, that's all! In
fact the circuit is small enough and perfectly suitable to
be fitted in the mouse housing with batteries. Current
consumption is minimized by the power reducing
SLEEP mode of the chip.
The circuit consists of two parts. A transmitter, which is
enclosed in the mouse, and the receiver, connected to
the PC via RS 232 link.
APPLICATION OPERATION
Transmitter
The PIC12C509 forms the heart of the circuit. Thanks
to the PIC12C509, it's use greatly simplifies hardware
design and the software. It senses the mouse movements,
mouse buttons and transmits the information to
the PC through infra red light emitting diodes (IR
LED's). The internal oscillator of the PIC12C509
enables one to use all of the I/O pins. The power-on
reset feature of the PIC12C509 rules out any need for
external reset circuitry, thereby saving one precious I/O
pin. Out of six I/O pins, one pin is configured to be output,
while the rest of the five pins are used as inputs.
The output pin drives two IR LED's through a MOSFET
BS170. Note that the MOSFET and one IR LED can be
saved and current consumption reduced by driving the
IR LED directly through the PIC12C509 pin at the
expense of limiting the range.
Three input pins out of the five are interfaced to the
three mouse buttons. Of course, two mouse buttons
can be used if desired. Flexibility of the design is evident.
Thanks to the PIC12C509 again! The remaining
two input pins are movement sensing inputs. Optical
sensing is used, which consists of an opto coupler with
a toothed wheel in between the LED and the phototransistor.
There are two such wheels, one for horizontal
movement and another for the vertical movement.
The wheels are mechanically coupled to the mouse ball
so that they rotate and electrical pulses are generated
with mouse movement. PIC12C509 senses the pulses
and converts the information into the appropriate format,
to be transmitted to the receiver via IR LED's. The
information, in the form of pulses, is then fed to the IR
LED through the driving MOSFET BS170. Thus the
information gets transformed into infra red light which is
transmitted to the receiver. When the microcontroller
transmits the motion information it produces exactly the
same pulses as would be produced by a regular
mouse.
Receiver
This is also a very simple circuit consisting of an IR
receiver, SFH505A, for instance and an op-amp
CA3140. The IR receiver receives the IR pulses and
transfers them into equivalent electrical pulses. The opamp
acts as an amplifier cum lavel shifter so as to make
these pulses compatible to RS 232 voltage levels. Note
that no extra power supply is needed for the receiver
circuit as it derives the power from the serial port itself.
Since this arrangement appears as a regular mouse to
the PC, there is no need to write device driver, and the
mouse can be used with the existing driver. Just plug
and play!

Infra Red Monitor

APPLICATION OPERATION
The application of IR monitor is to check an IR
emitting device such as a TV Video remote controller.
This IR monitor requires only 7 resistors, 4 LED s, 1
miniature switch, 1 IR photo transistor and 1
PIC12C671 uC.
This circuit uses an analog converter from PIC12C671

to measure the infra-red intensity from the IR photo transistor.

The intensity is displayed on the bargraph
LEDs.
OPERATION
1. Put an emitting device in front of the IR monitor
(from 1 inch to 2 feet).
2. Press the switch on the IR monitor once to
wake-up the micro-controller.
3. Press one key on the TV video controller and
watch the 4 monitor LEDs. The LEDs blink if
data is received, if all 4 LEDs stay off, no IR is
sensing. If high IR power is received, more
LEDs will be on. If IR monitor didn't receive anything
for 17 seconds, it will turn off (sleep mode).
Note: The OPTIC IR photo-transistor must be protected
from daylight source to avoid false bargraph level.