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 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


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

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

D1 = 1N4002
IC1 = NE556
IC2 = 78L15

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

Author: Ton Giesberts
Copyright: Elektor Electronics


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


Random Number Generator Based Game

This electronic game is
simulation of one-arm bandit game. Electronics hobbyists will find it
very interesting. When toggle switch S1 is in ‘run’ position, all
segments of 7-segment displays (DIS1 through DIS3) will light up. On
turning toggle switch S1 from ‘run’ to ‘stop’ position, displayed digits
will continue advancing and the final display is unpredictable. Thus
the final number displayed in DIS1 through DIS3 is of random nature. The
speed with which the number in 7-segment display keeps changing on
flipping switch S1 from ‘run’ to ‘stop’ condition slowly decays before
stopping with a random number display. To play this game, one has to
obtain three identical numbers in displays DIS1 through DIS3.

The contestant would score 1 (one) point if he manages to get a
final display of ‘000’, 2 points for getting ‘111’ display, 3 points for
‘222’,… and so on—up to ten points for ‘999’. He should try to score
maximum possible points in fixed numbers of attempts (say, 20 to 25
attempts). Apart from using this circuit as a game for entertainment,
one can use it as random number generator for any other application as
well. The decay time with the given component values is around 15
seconds before the display could stop at a final random number.


Circuit diagram

The circuit comprises clock oscillator built around NE555 timer IC4,
three-stage clock pulse counter built using three CD4033 ICs (IC1 to
IC3), and three 7-segment LED displays (DIS1
to DIS3). In clock oscillator circuit, NE555 timer IC4 is used in a
similar way as a free-running astable multivibrator, the only difference
being the additional capacitor C1 introduced between pin No. 7 of IC4
and junction of resistors R22 and R24. When toggle switch S1 is in ‘run’
position, both terminals of capacitor C1 are shorted by switch S1 and
timer IC4 works as a free-running astable multivibrator. The operating
frequency is in the vicinity of 35 kHz, determined by the value of
timing components.

When toggle switch S1 is flipped from ‘run’ to ‘stop’ position,
capacitor C1 is introduced in the discharge path of pin No. 7 of IC4 and
junction of resistors R22 and R24. At the same time, capacitor C4 comes
in parallel with timing capacitor C3 to change the operating frequency
of the astable from around 35 kHz to around 65 Hz. Now capacitor C1
slowly starts charging as it is connected in the discharge path of the
timing capacitors C3 and C4. The clock frequency of IC4 gradually
reduces and after 15 seconds, when capacitor C1 is sufficiently charged,
the oscillating frequency gradually drops and finally it stops
oscillating. Thus, pin 3 of IC4 becomes low.

Second part of the circuit comprises three cascaded ICs, IC1 through
IC3 (CD4033 decade upcounter cum 7-segment decoder). In conjunction
with three 7-segment displays (DIS1 to DIS3), these form a 3-digit clock
counter. The clock counting speed is dependant upon the clock pulse
frequency of IC4. It is connected to clock input pin 1 of IC1 while chip
enable pin 2 of IC1 to IC3 are held low. Thus all clock counter ICs
advance by 1 for every positive clock transition. Reset pin 15 of all
counter ICs is held low through resistor R25. Thus reset facility is not
used in this circuit. Due to persistence of vision, one can not
distinguish 0-9 counting in DIS1 to DIS3 when the clock frequency is

All 7-segment displays appear to show digit 8, while the red LED1
remains lit continuously, indicating clock counter is in running
condition. On sliding toggle switch S1 from ‘run’ to ‘stop’ position,
the counting speed of individual digits falls immediately due to the
clock frequency changing to around 65 Hz. Now, the counting speed will
be 65 Hz for DIS3, 6.5 Hz for DIS2, and 0.6 Hz for DIS1. This speed of
individual digit counting slowly decays, until the counter stops and
LED1 stops blinking, and the final count (random numbers) are displayed
in DIS1, DIS2, and DIS3.


Cupboard Lights circuit

This Cupboard lights circuit is an automatic White LED lamp used to illuminate the interior of cupboard to search things. The lamp automatically turns on when the door of the cupboard opens and stays on for three minutes then turns off.

The circuit uses the popular timer IC NE 555 for the time delay. Components R1 and C2 gives three minutes time delay during which the lamp remains on. Power to the circuit is obtained from a 9 volt PP3 battery. A magnetic reed switch is used for the automatic operation of the circuit. The Normally closed (NC) contacts of the reed switch break when the magnet is close to it if the door is closed. When the door opens, magnet move away and the contacts of reed switch closes and the circuit gets power. Since the trigger pin 2 of timer is grounded, timer triggers and its output become high to light the white LED lamp. Fix the reed switch and circuit in the frame of the cupboard and magnet in the door so as to keep them close when the door is closed.

Cupboard Light Circuit diagram

Cupboard Lights circuit

555 datasheet

IC NE 555 and Reed Switch

Cupboard Lights circuit #2
Cupboard Lights circuit #3

Pin connections of IC 555

Cupboard Lights circuit #4

LED technology 555 datasheet

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High Low Voltage Cutout Without Timer

This inexpensive circuit
can be connected to an air-conditioner/fridge or to any other
sophisticated electrical appliance for its protection. Generally, costly
voltage stabilizers are used with such appliances for maintaining
constant AC voltage. However, due to fluctuations in AC mains supply, a
regular ‘click’ sound in the relays is heard. The frequent
energisation/de-energisation of the relays leads to electrical noise and
shortening of the life of electrical appliances and the
relay/stabilizer itself. The costly yet fault-prone stabiliser may be
replaced by this inexpensive high-low cutout circuit with timer.

The circuit is so designed that relay RL1 gets energised when the
mains voltage is above 270V. This causes resistor R8 to be inserted in
series with the load and thereby dropping most of the voltage across it
and limiting the current through the appliance to a very low value. If
the input AC mains is less than 180 volts or so, the low-voltage cut-off
circuit interrupts the supply to the electrical appliance due to
energisation of relay RL2. After a preset time delay of one minute
(adjustable), it automatically tries again. If the input AC mains supply
is still low, the power to the appliance is again interrupted for
another one minute, and so on, until the mains supply comes within
limits (>180V AC).


Circuit diagram

The AC mains supply is resumed to appliance only when it is above
the lower limit. When the input AC mains increases beyond 270 volts,
preset VR1 is adjusted such that transistor T1 conducts and relay RL1
energises and resistance R8 gets connected in series with the electrical
appliance. This 10-kilo-ohm, 20W resistor produces a voltage drop of
approximately 200V, with the fridge as load. The value and wattage of
resistor R8 may be suitably chosen according to the electrical appliance
to be used. It is practically observed that after continuous use, the
value of resistor R8 changes with time, due to heating. So adjustment of
preset VR1 is needed two to three times in the beginning.

But once it attains a constant value, no further adjustment is
required. This is the only adjustment required in the beginning, which
is done using a variac. Further, the base voltage of transistor T2 is
adjusted with the help of preset VR2 so that it conducts up to the lower
limit of the input supply and cuts off when the input supply is less
than this limit (say, 180V). As a result, transistor T3 remains cut off
(with its collector remaining high) until the mains supply falls below
the lower limit, causing its collector voltage to fall. The collector of
transistor T3 is connected to the trigger point (pin 2) of IC1. When
the input is more than the lower limit, pin 2 of IC1 is nearly at +Vcc.

In this condition the output of IC1 is low, relay RL2 is
de-energised and power is supplied to the appliance through the N/C
terminals of relay RL2. If the mains supply is less than the lower
limit, pin 2 of IC1 becomes momentarily low (nearly ground potential)
and thus the output of IC1 changes state from ‘low’ to ‘high’, resulting
in energisation of relay RL2. As a result, power to the load/appliance
is cut off. Now, capacitor C2 starts charging through resistor R6 and
preset VR3. When the capacitor charges to (2/3)Vcc, IC1 changes state
from ‘high’ to ‘low’. The value of preset VR3 may be so adjusted that it
takes about one minute (or as desired) to charge capacitor C1 to

Relay is now de-energised and the power is supplied to the appliance
if the mains supply voltage has risen above the lower cut-off limit,
otherwise the next cycle repeats automatically. One additional advantage
of this circuit is that both relays are de-energised when the input AC
mains voltage lies within the specified limit and the normal supply is
extended to the appliance via the N/C contacts of both relays.

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Wire Tracer (Transmitter)

The circuit depicted
here forms one half of a device that will prove extremely handy when
tracing the path of electrical wiring in a building or to locate a break
in a wire. The system is based on similar equipment that is used by
technicians in telephone exchanges. The operation is straightforward.
You require a generator that delivers an easily recognizable signal
which, using a short antenna, is inductively coupled to a simple, but
high gain, receiver. To create a useful transmitter it would suffice to
build a simple generator based on a 555. But as the adjacent diagram
shows, a 556 was selected instead. The second timer (IC1a) is used to
modulate the tone produced by IC1b.


Circuit diagram

The output frequency alternates between about 2100 Hz and 2200 Hz.
This is a very distinctive test signal that is easily distinguished from
any other signals that may be present. Resistor R6 is connected to a
piece of wire, about ten centimeters long, that functions as the
antenna. The ground connection (junction C2-C3) is connected to ground.
When the antenna is connected directly to a cable, it is possible to
determine at the other end of the cable, with the aid of the receiver,
which conductor is which (don’t do this with live conductors!). The
schematic for the matching receiver may be found elsewhere in this


SDR Soundcard Tester

The key to using a
soundcard successfully in digital signal processing or digital radio
applications lies principally in the characteristics of the soundcard
itself. This applies in particular to SDR
(software defi ned radio) programs that turn your PC into a top-class
AM/SSB/CW receiver, assuming your soundcard cooperates. If you want to
experiment with SDR and avoid a lot of
frustration, it is worth checking first whether the PC soundcard you plan
to use is suitable. There are three essential elements to success:

  • the soundcard must have a stereo line-level input;
  • the card must be equipped with an input anti-aliasing filter; and
  • the sample rate must be at least 48 kHz and the card must be able to cope with signals up to 24 kHz.

Many laptops have only a mono microphone input, sometimes also
rather limited in bandwidth. In this case it may be possible to use an
external USB soundcard. Most desktop PCs these
days have an internal integrated soundcard, although some of these do
not feature an anti-aliasing fi lter. Attempts to disable the integrated
soundcard and replace it with a better one often meet with failure;
again, an external USB soundcard is a possible solution.

SDR Soundcard Tester Circuit

SDR Soundcard Tester Circuit Diagram

To avoid guesswork, the best way to proceed is to test the soundcard
using this very small circuit. This will help to diagnose any problems
and will help determine whether the card is suitable for use with an SDR
program. Figure 1 shows a simple square-wave generator built around an
NE555 timer IC. At the output is a 15 kHz signal rich in higher
harmonics. Using this we can determine whether or not the soundcard can
process the harmonics at 30 kHz, 45 kHz and so on. An anti-aliasing
filter at the soundcard input should attenuate all signals above 24 kHz.
The frequency of the test generator is, within limits, dependent on its
supply voltage.

Using an adjustable power supply, a frequency range from 10 kHz to
20 kHz can therefore be covered. There are two RC networks at the output
of the test circuit, a high-pass filter and a low-pass filter, acting
as simple phase shifters. At the basic frequency of 15 kHz these provide
a total phase difference of 90 degrees, corresponding exactly to the
typical situation at the output of an SDR receiver circuit using an I-Q mixer: signals at the same frequency but differing in phase. To test the soundcard we need an SDR program running on the PC as well as the circuit of Figure 1. Suitable software includes SDradio (available for download from

When things are running correctly, the screen should display just
two signals: the wanted signal at 15 kHz and a weaker image at –15 kHz
(Figure 2). Suppression of the image may not be particularly good as the
test circuit does not have very high phase and amplitude accuracy. If,
however, the signals have the same level, there is a problem in the
processing of the two channels: it is probable that the soundcard only
has a monophonic input. If there is no anti-aliasing filter at the input
of the soundcard the spectrum will show a large number of extra lines
(Figure 3): it is easy to work out which harmonic corresponds to which
alias frequency.

The results obtained using an I-Q receiver were grim: frequencies
all the way out to 100 kHz were wrapped into the audible range,
resulting in bubbling, hissing and whistling. In theory it would be
possible to add an anti-aliasing filter to the output of the receiver to
allow use with soundcards that are not equipped with such a filter. In
practice, however, it is not easy to achieve the required sharp cutoff
and symmetry between the two channels. A typical soundcard has a low
pass filter set at 24 kHz which by 27 kHz is already attenuating the
signal by some 60 dB. This is only practical using digital fi lters; an
adjustable analogue circuit to achieve this performance would be so
complex that the simplicity benefits of SDR receiver technology would entirely evaporate.

Author: Burkhard Kainka – Copyright: Elektor Electronics 2007

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Touch Control Mute Switch Circuit Schematic with 555 IC

Here is another simple circuit to mute the volume of Audio devices
through simple touch. It exploits the action of the flip-flops in the
timer IC 555 to reduce the volume of the Audio amplifier.

IC NE555 is designed in the toggle mode. Its lower and upper comparator
inputs are connected to the touch plates which can be membrane switches
or two pieces of conducting plates. The inputs of comprators are
stabilized through R1 and R2 to avoid floating. When the touch plate
connected to pin 2 is touched momentarily, output of IC1 goes high and
T1 conducts. The centre tap of the volume control is connected to the
collector of T1. So when T1 conducts current going to the amplifier
drains through T1. This reduces the volume. IC1 remains latched in this
position with LED on. When the touch plate connected to pin 6 is touched
momentarily, output of IC1 goes low and T1 turns off. This restores the

Schematic of the Touch Control Mute Switch Circuit


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Electronic dog repellent project

Electronic dog repellent project
Join the forum discussion on this post

The electronic dog repellent circuit diagram below is a high output ultrasonic transmitter which is primarily intended to act as a dog and cat repeller, which can be used individuals to act as a deterrent against some animals. It should NOT be relied upon as a defence against aggressive dogs but it may help distract them or encourage them to go away and do not consider this as an electronic pest repeller.

The ultrasonic dog repellant uses a standard 555 timer IC1 set up as an oscillator using a single RC network to give a 40 kHz square wave with equal mark/space ratio. This frequency is above the hearing threshold for humans but is known to be irritating frequency for dog and cats.

Since the maximum current that a 555 timer can supply is 200mA an amplifier stage was required so a high-power H-bridge network was devised, formed by 4 transistors TR1 to TR4. A second timer IC2 forms a buffer amplifier that feeds one input of the H-bridge driver, with an inverted waveform to that of IC1 output being fed to the opposite input of the H-bridge.
For more electronic dog repeller circuits check the related links bellow.

Cat and dog repellent circuit diagram

Dog repellent circuit schematic

Where you can buy it?

  • DAZER II Portable Ultrasonic Dog Deterrent from Amazon – (~$25)
  • StreetWise Ultrasonic Dog Repeller – starting from $18
  • Ultrasonic Dog Dazer Deterrent – starting from $20

This means that conduction occurs through the complementary pairs of TR1/TR4 and TR2/TR3 on alternate marks and spaces, effectively doubling the voltage across the ultrasonic transducer, LS1. This is optimised to generate a high output at ultrasonic frequencies.

This configuration was tested by decreasing the frequency of the oscillator to an audible level and replacing the ultrasonic transducer with a loudspeaker; the results were astounding. If the dog repellent circuit was fed by a bench power supply rather than a battery that restrict the available current, the output reached 110dB with 4A running through the speaker which is plenty loud enough!

The Dog and Cat repellant was activated using a normal open switch S1 to control the current consumption, but many forms of automatic switching could be used such as pressure sensitive mats, light beams or PIR sensors. Thus it could be utilise as part of a dog or cat deterrent system to help prevent unwanted damage to gardens or flowerbeds, or a battery powered version can be carried for portable use. Consider also using a lead-acid battery if desired, and a single chip version could be built using the 556 dual timer IC to save space and improve battery life.

555 datasheet


Voltage Doubler Circuit with 555

This dc voltage doubler circuit produces a voltage that is twice its voltage supply. This is useful when a higher voltage level is needed out of a single lower voltage power supply. Since the current consumption levels are low in such cases, the circuit can be built with minimal resources.

Voltage Doubler Circuit Schematic

555 voltage doubler circuit schematic

The electronic circuit is basically a square wave generator using the common LM555 timer IC. It is followed by a final stage made of transistors T1 and T2. The actual doubler circuit is made of D1, D2, C4 and C5 components.

The 555 voltage doubler timer IC works as an astable multivibrator and generates a frequency of about 8.5 kHz. The quare wave output drives the final stage made of T1 and T2. This is how the doubler works: by a low amplitude of the signal, transistor T1 blocks while T2 conducts. The minus electrode of the capacitor C4 is grounded and charges through D1. By a high amplitude of the signal, transistor T1 conducts while T2 blocks. However, capacitor C4 cannot discharge because it is blocked by D1. The following capacitor C5 is therefore charged with a combined voltage from C4 and the power supply (12V input).
On standby, the circuit delivers around 20 volts The maximum load must not exceed 70 mA. The actual output voltage is around 18 volts giving an efficiency rating of 32 %. On lower current ratings, the voltage is higher. If a stable voltage lever is desired, a 3 pin voltage regulator IC can be added at the output. The regulator IC’s own current consumption must be added to the total current consumption which must not exceed 70 mA.
For more voltage doublers check the related posts bellow.

555 datasheet