PWM Modulator

If you ever thought of
experimenting with pulse-width modulation, this circuit should get you
started nicely. We’ve kept simplicity in mind and used a dual 555 timer,
making the circuit a piece of cake. We have even designed a small PCB
for this, so building it shouldn’t be a problem at all. This certainly
isn’t an original circuit, and is here mainly as an addition to the
‘Dimmer with MOSFET’ article elsewhere in this website. The design has
therefore been tailored to this use. A frequency of 500 Hz was chosen,
splitting each half-period of the dimmer into five (a low frequency
generates less interference).

The finished project

The finished project

The first timer is configured as a standard astable frequency
generator. There is no need to explain its operation here, since this
can easily be found on the Internet in the datasheet and application
notes. All we need to mention is that the frequency equals 1.49 /
((R1+2R2) × C1) [Hz] R2 has been kept small so that the frequency can be
varied easily by adjusting the values of R1 and/or C1. The second timer
works as a monostable multivibrator and is triggered by the
differentiator constructed using R3 and C3.

PCB layout

PCB layout

The trigger input reacts to a rising edge. A low level at the
trigger input forces the output of the timer low. R3 and C3 have
therefore been added, to make the control range as large as possible.
The pulse-width of the monostable timer is given by 1.1xR4xC4 and in
this case equals just over a millisecond. This is roughly half the
period of IC1a. The pulse-width is varied using P1 to change the voltage
on the CNTR input. This changes the voltage to the internal comparators of the timer and hence varies the time required to charge up C4.

Circuit

Circuit diagram

The control range is also affected by the supply voltage; hence
we’ve chosen 15V for this. The voltage range of P1 is limited by R6, R7
and R5. In this design the control voltage varies between 3.32 V and
12.55 V (the supply voltage of the prototype was 14.8 V). Only when the
voltage reaches 3.51 V does the output become active, with a duty-cycle
of 13.5 %. The advantage of this initial ‘quiet’ range is that the lamp
will be off. R8 protects the output against short circuits. With the
opto-coupler of the dimmer as load, the maximum current consumption of
the circuit is about 30 mA.

Power supply

Power supply

COMPONENTS LIST

Resistors:
R1 = 270k
R2,R3 = 10k
R4 = 100k
R5,R8 = 1k
R6,R7 = 220R
P1 = 2k2, linear, mono

Capacitors:
C1,C4 = 10nF
C2,C5,C6 = 100nF
C3 = 1nF
C7 = 2µF2 63V radial
C8 = 100µF 25V radial

Semiconductors:
D1 = 1N4002
IC1 = NE556
IC2 = 78L15

Miscellaneous:
P1 = 3-way pinheader
K1 = 2-way pinheader

Author: Ton Giesberts
Copyright: Elektor Electronics

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Converting a DCM Motor

We recently bought a
train set made by a renowned company and just couldn’t resist looking
inside the locomotive. Although it did have an electronic decoder, the DCM
motor was already available 35 (!) years ago. It is most likely that
this motor is used due to financial constraints, because Märklin (as you
probably guessed) also has a modern 5-pole motor as part of its range.
Incidentally, they have recently introduced a brushless model. The DCM
motor used in our locomotive is still an old-fashioned 3-pole series
motor with an electromagnet to provide motive power. The new 5-pole
motor has a permanent magnet.

We therefore wondered if we couldn’t improve the driving
characteristics if we powered the field winding separately, using a
bridge rectifier and a 27 Ω current limiting resistor. This would
effectively create a permanent magnet. The result was that the driving
characteristics improved at lower speeds, but the initial acceleration
remained the same. But a constant 0.5 A flows through the winding, which
seems wasteful of the (limited) track power. A small circuit can reduce
this current to less than half, making this technique more acceptable.
The field winding has to be disconnected from the rest (3 wires).

A freewheeling diode (D1, Schottky) is then connected across the
whole winding. The centre tap of the winding is no longer used. When FET
T1 turns on, the current through the winding increases from zero until
it reaches about 0.5 A. At this current the voltage drop across R4-R7
becomes greater than the reference voltage across D2 and the opamp will
turn off the FET. The current through the
winding continues flowing via D1, gradually reducing in strength. When
the current has fallen about 10% (due to hysteresis caused by R3), IC1
will turn on T1 again. The current will increase again to 0.5 A and the FET is turned off again. This goes on continuously.

The current through the field winding is fairly constant, creating a
good imitation of a permanent magnet. The nice thing about this circuit
is that the total current consumption is only about 0.2 A, whereas the
current flow through the winding is a continuous 0.5 A. We made this
modification because we wanted to convert the locomotive for use with a DCC
decoder. A new controller is needed in any case, because the polarity
on the rotor winding has to be reversed to change its direction of
rotation. In the original motor this was done by using the other half of
the winding. There is also a good non-electrical alternative: put a
permanent magnet in the motor. But we didn’t have a suitable magnet,
whereas all electronic parts could be picked straight from the spares
box.

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Twin-T RC Notch Filter

The two T-notch filter
can be used to block unwanted frequencies or if placed in an op amp as a
bandpass filter. the notch frequency occurs where the capacitive
reactance equals the resistance (Xc = R) and if the values are close,
the attenuation can be very high and the notch frequency virtually
eliminated,Insertion of the filter depends on the load, the output is
connected to the subject so that the resistors must be of much lower
value than the load for minimal loss.

Circuit

Circuit diagram

For audio frequencies, the filter may act as a bass and treble boost
circuit by attenuating the midrange. With 1.5K resistors and capacitors
0.1 uF, the band is in stop-10dB 500 Hz to 2 kHz. The depth and breadth
of the reaction can be slightly adjusted to a value of 0.5 R and adding
some resistance at the C-values,If the circuit is an operational
amplifier is used as a bandpass filter, the response should be reduced
to avoid oscillation.

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1kH Synthetic Inductor

Inductors can be
mimicked quite easily using operational amplifiers. The circuit shown
here was developed to have an inductance of 1000 H (say, one thousand
Henry) with good damping. Using this design you can build a resonant
circuit with a center frequency of less than 1 Hz. The slow behavior
allows you to use conventional measuring instruments to investigate the
circuit in real time. The circuit can also be used as part of a filter
design. Opamp1 operates as an Integrator, Opamp2 as a difference
amplifier.

The output voltage of Opamp2 is equal to the voltage drop across R1
and P1, which is proportional to the output current. This voltage is
differentiated by Opamp1, C1 and R2. The net effect is that the circuit
behaves as an inductor. P1 allows adjustment of the inductance value. P2
allows adjustment of the Q factor of the coil by altering the symmetry
of the difference amplifier and with it the stability of the circuit.

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Color Organ

I have had many requests
for this circuit. It was a very popular unit years ago. The basic idea
of the project is to make different colored bulbs light at different
frequencies of music. The circuit connects to the speaker outputs of
your stereo or to the back of your speaker. The music passes through the
transformer and the volume level is adjusted by the 5k ohm pot. Each
light bulb is turn on by a different frequency of sound based on the
resistor & capacitor combination in the gate circuit of the SCR. If the resistors R1, R2, or R3 are changed, the frequency of sound that will trigger the SCR will change. The isolation transformer is for protection.

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65W Notebook Laptop Power Adapter

Using TOP269EG off-line
switcher IC, (U1), in a flyback configuration can be designed a very
simple high efficiency notebok laptop power adapter.TOP269EG IC has an
integrated 725 V MOSFET and a multi-mode controller. It regulates the output by adjusting the MOSFET duty cycle, based on the current fed into its CONTROL
pin.This laptop power adapter circuit will provide a fixed 19 volts
output voltage at a maximum current of 3.5A. input voltage range is
between 90 to 265VAC.

Common-mode inductors L3 and L4 provide filtering on the AC input. X
class capacitor C1 provides differential filtering, and resistors R1
and R2 provide safety from shock if the AC is removed, by ensuring a
path for C1 to discharge. This is required by safety agencies when the
capacitor value exceeds 100 nF. Bridge rectifier D1 rectifies the AC
input, and bulk capacitor C2 filters the DC.

Y capacitor C11, connected between the primary and secondary side
provides common mode filtering.Capacitor C7 provides the auto-restart
timing for U1. At startup this capacitor is charged through the DRAIN
(D) pin. Once it is charged U1 begins to switch. Capacitor C7 stores
enough energy to ensure the power supply starts up. After start-up the
bias winding powers the controller via the CONTROL pin. Bypass capacitor C6 is placed as physically close as possible to U1.

U1 utilizes an output overvoltage shutdown function. An increase in
output voltage causes an increase in the bias winding on the primary
side, sensed by VR1. A sufficient rise in the bias voltage causes VR1 to
conduct and bias Q1 to inject current into the Voltage Monitor (V) pin
of U1. When the current exceeds 336 uA, U1 enters the overvoltage
shutdown mode and latches off.

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Using LM317 with Overvoltage Protection

The circuit diagram
shows the over voltage protection as a feature of LM317 linear voltage
regulator.

Over voltages come from several source or factors which are usually
in the form of transients. Transients are represented in spikes which
are short and fast disturbance or change in the voltage and current
component.

It only occurs in circuits containing conductance and capacitance.
The causes are typically from power outages, short circuits, tripped
circuit breakers, lightning spikes, inductive spikes and other
malfunctions from power company.

Circuit

Circuit Diagram

As an example using a 12V battery source, the LM317 voltage
regulator can be used to obtain 6 Volts. To protect any device from over
voltage, there are ways such as adding relays or a zener diode. A relay
switch functions by opening or closing under the command of another
circuit. But finding a relay that would limit the output from 6 V to 12 V
is not easy.

Fortunately, zener diodes are more abundant. A 6.2V zener diode
rating can be used to surpass any excessive voltage set by the voltage
regulator, to prevent more damage to the circuit. The components will be
as follows:

ZD1 – 6.2 Volts
R1 – 1K ohms
R2 – 1K ohms
T1 – NPN Transistor (low power)
T2 – NPN Transistor (acts as a switch)

Every circuit design needs to be tested carefully as to avoid
further damage to the equipment that will be connected. The trial can be
done by using a multimeter, and gradually increasing the circuit
voltage. Once the circuit turns off the supplied voltage, take note of
the reading which will signify the threshold voltage of the zener diode.

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One Transistor Radio

Here is a simple circuit
for a one transistor Audion type radio powered by a 1.5 V battery. It
employs a set of standard low-impedance headphones with the headphone
socket wired so that the two sides are connected in series thus giving
an impedance of 64 Ω. The supply to the circuit also passes through the
headphones so that unplugging the headphones turns off the supply. Using
an Audion configuration means that the single transistor performs both
demodulation and amplification of the signal.

The sensitivity of this receiver is such that a 2 m length of wire
is all that is needed as an antenna. The tap on the antenna coil is at
1/5th of the total winding on the ferrite rod. For details of the
antenna coil see the article Diode Radio for Low Impedance Headphones.
This circuit is suitable for reception of all AM transmissions from
long-wave through to shortwave.

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Auto Power Off

We are surrounded by
battery operated equipment of all kinds, and this array is growing
still. Manufacturers and designers lean over backwards to make sure that
their equipment draws a small current and can thus be operated by a
battery. This has its flip side, too. because even if the equipment in
question draws only a small current, when it is not switched off, the
battery is flat after a few days or weeks. The circuit presented here
can prevent this happening. It may be added to all kinds of equipment
operating from a 9 V battery and switches this off automatically one
minute after a preset time has elapsed. The peak switching current is 20
mA, which is more than enough for most applications.

Circuit diagram:

Automatic Power Off Circuit

Automatic Power Off Circuit Diagram

The switch is formed by a p-n-p darlington, T1, which is actuated by
push-button switch S1. The very high amplification of the darlington
enables it to be kept on fairly long with the aid of a relatively
small-value capacitor, C1 (= 100 µF). Resistor R3 limits the charging
current of C1 to ensure a long life of S1. Resistors R1 and R2, in
conjunction with C1, determine the switch-on time. When this time has
elapsed, R1 ensures that T1 is switched off. Since the darlington can
handle a UBE of –10 V, a polarity protection diode is not needed.

Author:H. Bonekamp
Copyright: Elektor ELectronics

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Push Off Push On

The ubiquitous 555 has
yet another airing with this bistable using a simple push-button to
provide a push-on, push-off action. It uses the same principle of the
stored charge in a capacitor taking a Schmitt trigger through its
dead-band. Whereas the Schmitt trigger in that reference was made from
discrete components, the in-built dead-band arising from the two
comparators, resistor chain and bistable within the 555 is used instead.
The circuit demonstrates a stand-by switch, the state of which is
indicated by illumination of either an orange or red LED,
exclusively driven by the bipolar output of pin 3. Open-collector
output (pin 7) pulls-in a 100-mA relay to drive the application circuit;
obviously if an ON status LED is provided elsewhere, then the relay, two LEDs and two resistors can be omitted, with pin 3 being used to drive the application circuit, either directly or via a transistor.

Circuit diagram:

Push Off Push On Circuit

Push Off Push On Circuit Diagram

The original NE555 (non-CMOS) can source
or sink 200 mA from / into pin 3. Component values are not critical; the
‘dead-band’ at input pins 2 and 6 is between 1/3 and 2/3 of the supply
voltage. When the pushbutton is open-circuit, the input is clamped
within this zone (at half the supply voltage) by two equal-value
resistors, Rb. To prevent the circuit powering-up into an unknown
condition, a power-up reset may be applied with a resistor from supply
to pin 4 and capacitor to ground. A capacitor and high-value resistor
(Rt) provide a memory of the output state just prior to pushing the
button and creates a dead time, during which button contact bounce will
not cause any further change. When the button is pressed, the stored
charge is sufficient to flip the output to the opposite state before the
charge is dissipated and clamped back into the neutral zone by
resistors Rb. A minimum of 0.1 µF will work, but it is safer to allow
for button contact-bounce or hand tremble; 10 µF with 220 k gives
approximately a 2-second response.

Author: Trevor Skeggs – Copyright: Elektor July-August 2004

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