Digital Main Voltage Indicator

Continuous monitoring of
the mains voltage is required in many applications such as manual
voltage stabilisers and motor pumps. An analogue voltmeter, though
cheap, has many disadvantages as it has moving parts and is sensitive to
vibrations. The solidstate voltmeter circuit described here indicates
the mains voltage with a resolution that is comparable to that of a
general-purpose analogue voltmeter. The status of the mains voltage is
available in the form of an LED bar graph.
Presets VR1 through VR16 are used to set the DC voltages corresponding
to the 16 voltage levels over the 50-250V range as marked on LED1
through LED16, respectively, in the figure. The LED
bar graph is multiplexed from the bottom to the top with the help of
ICs CD4067B (16-channel multiplexer) and CD4029B (counter).

The counter clocked by NE555 timer-based astable multivibrator
generates 4-bit binary address for multiplexer-demultiplexer pair of
CD4067B and CD4514B. The voltage from the wipers of presets are
multiplexed by CD4067B and the output from pin 1 of CD4067B is fed to
the non-inverting input of comparator A2 (half of op-amp LM358) after
being buffered by A1 (the other half of IC2). The unregulated voltage
sensed from rectifier output is fed to the inverting input of comparator
A2. The output of comparator A2 is low until the sensed voltage is
greater than the reference input applied at the non-inverting pins of
comparator A2 via buffer A1.

When the sensed voltage goes below the reference voltage, the output
of comparator A2 goes high. The high output from comparator A2 inhibits
the decoder (CD4514) that is used to decode the output of IC4029 and
drive the LEDs. This ensures that the LEDs
of the bar graph are ‘on’ up to the sensed voltage-level proportional
to the mains voltage.The initial adjustment of each of the presets can
be done by feeding a known AC voltage through an auto-transformer and
then adjusting the corresponding preset to ensure that only those LEDs that are up to the applied voltage glow.

It is advisable to use additional transformer, rectifier, filter, and
regulator arrangements for obtaining a regulated supply for the
functioning of the circuit so that performance of the circuit is not
affected even when the mains voltage falls as low as 50V or goes as high
as 280V. During Lab testing regulated 12-volt supply for circuit
operation was used.)


Heart Rate Monitor

Strictly speaking, this
simple circuit shouldn’t work! How could anyone expect an ordinary light
dependent resistor photo cell to ‘see’ through a fingertip in natural
daylight and detect the change in blood flow as the heart pulsates? The
secret is a high gain circuit, based on a dual op amp IC which can be
either the low power LM358 or the JFET TL072. The LDR
is connected in series across the 9V battery supply via a 100k resistor
(R1) and the minute signal caused by the blood pulsing under the skin
is fed to the non-inverting (+) input, pin 3, of IC1a via a 0.µF

Pin 3 is biased by a high impedance voltage divider consisting of
two 3.3M resistors. The feedback resistors to pin 2 set the gain to 11
times. The output of IC1a is fed via a 0.47µF capacitor and 220k
resistor to IC1b. This is configured as an inverting op amp with a gain
of 45 so that the total circuit gain is about 500. The output of IC1b is
used to drive an analog meter which may be a multimeter set to the 10V
DC range or any panel meter in series with a resistor to limit the
current to less than its full-scale deflection. The prototype used an
old VU meter with a 47k resistor fitted in series.

Heart Rate Monitor Circuit

Heart Rate Monitor Circuit Diagram

Note that the unit was designed to use the Dick Smith Electronics light dependent resistor (Z-4801). Other LDRs may require a change in the value of resistor R1. A light source such as a high brightness LED
is not required. All that is needed is a reasonably well-lit room,
preferably natural daylight, to produce a healthy swing of the needle.
Only when the hands are very cold does it make it a little more
difficult to accurately count the pulses. To check your heart rate,
carefully position your thumb or finger over the LDR
and count the meter fluctuations for a period of 15 seconds. Then
multiply the result by four to obtain your pulse rate. The circuit can
not be used if you are walking or running, etc.


USB-Powered PIC Programmer

This simple circuit can
be used to program the PIC16F84 and similar “flash memory” type parts.
It uses a cheap 555 timer IC to generate the programming voltage from a
+5V rail, allowing the circuit to be powered from a computer’s USB
port. The 555 timer (IC1) is configured as a free-running oscillator,
with a frequency of about 6.5kHz. The output of the timer drives four
100nF capacitors and 1N4148 diodes wir-ed in a Cockroft-Walton voltage
multiplier configuration.

USB Powered PIC Programmer Circuit

USB-Powered PIC Programmer Circuit Diagram

The output of the multiplier is switched through to the MCLR/Vpp pin of the PIC during programming via a 4N28 optocoupler. Diodes ZD1 and D5 between the MCLR/Vpp pin and ground clamp the output of the multiplier to about 13.6V, ensuring that the maximum input voltage (Vihh) of the PIC
is not exceeded. A 100kΩ resistor pulls the pin down to a valid logic
low level (Vil) when the optocoupler is not conducting. The circuit is
compatible with the popular “JDM” programmer, so can be used with supporting software such as “ICProg” (see

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Cat And Dog Repellent

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.

Cat And Dog Repellent Circuit

Cat And Dog Repellent Circuit Diagram

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


Transcutaneous Electrical Nerve Stimulator (TENS)

A Transcutaneous Electrical Nerve Stimulation (TENS)
device is, put bluntly, a machine for giving electric shocks. The
author was prescribed such a device on loan by his orthopaedic
specialist. The unit has a large number of programmes, of which he used
only one. Measuring the signals at the output of the device in this mode
revealed damped oscillations at a frequency of approximately 2.5 kHz,
with a repetition rate of approximately 100 Hz.

Transcutaneous Electrical Nerve Stimulator Circuit

Transcutaneous Electrical Nerve Stimulator Circuit Diagram

How hard can it be to make such a device ourselves? The simple circuit uses a CMOS
555 timer to produce a brief pulse which feeds a 1:10 miniature
transformer. Together with a 4.7 nF capacitor the transformer makes a
parallel resonant circuit: the resonance leads to a considerable
increase in the output voltage. The pulse width can be adjusted using a
potentiometer, here shown combined with the on-off switch. Wider pulses
produce higher output voltages. Since a peak voltage of up to 200 V can
be produced, the transformer must have adequate insulation: Conrad
Electronics type 516260-62 is suitable. A low-cost phono socket at the
output gives reliable connection to the electrode cable.

The adhesive electrodes shown in the photograph (disposable and
permanent types are available) can be obtained from pharmacies and
medical suppliers. They generally have connectors compatible with 2 mm
banana plugs, and so it is possible to make up the necessary cable
yourself. To treat responsive parts of the body, such as the arm, the
potentiometer need not be turned up far to obtain the necessary
sensation. Less sensitive parts, such as the knee or foot, need a rather
higher voltage and hence a correspondingly higher potentiometer

Author: Klaus Rohwer – Copyright: Elektor Electronics Magazine


Adjustable High-Low Frequency Sine Wave Generator

This circuit uses the
versatile MAX038 function generator. Although in this circuit some of
the advanced characteristics of this IC are disabled, you can generate
Sine, Triangle, Square waves (adjusting A0 and A1 pins see datasheet on if you want other waves, use a switch). The signal is amplified through a TCA0372 (from ONSEMI) Power opamp with current capability up to 1A and bandwitch up to 1 MHz.

Adjustable 122Khz Sine wave generator

Adjustable 122Khz Sine wave generator

I selected this particular frequency (122 Khz) because i needed a cheapo ESR-o-meter
for my electrolytic capacitors to monitor their health as they have to
discharge tens of amperes in less than 2 ms. At 122 KHz capacitive
reactance is very low, and inductive reactance isn’t so high, so forcing
a current (es 200mA, using a precision resistor) through a capacitor
and reading AC voltage drop accross it gives me an estimation of ESR (Vdrop/current). Of course inductive and capacitive reactance are still present, but negligible.

The 122 khz 2V p-p sine wave is generated by the MAX038 IC, its
frequency can be calculated by the formula Freq (MHz) = Iin(uA) / C6
(pf) . Iin = 2,5V / R1 (25Kohm default). So the freq is 0,122 MHz . The
resistor is for small adjustments, don’t go under 10000 Kohm or above
40000 Kohm because the accuracy will drop. If you want multifrequency
just use the multiposition switch with 820 pF, 8,2 nF , 82nF , 820 nf
for 122Khz range 12,2Khz range 1220 Hz and 122 Hz. Fine tuning can be
done adjusting R2 , the frequency can vary from 1,7x (Vfadj = -2,4) to
0,3x (Vfadj = 2,4) of the main frequency (when fadj is at 0V).

The sine wave output is feed into a TCA0372 1/2 opamp to achieve a
gain from 1 to 5 (2V p-p, 10 V p-p), adjust the potenziometer and into a
TCA0372 2/2 opamp buffer stage also present on the same IC.

Adjusting the frequency needs a frequency counter, so this circuit
should be used on conjunction with a freq couter. The max current is 1A,
but i would suggesto to not go above 0,5A to remain accurate. Needs a
computer power supply with 12V,5V,-5V,-12V,GND to be operated, if you
don’t have one just use a multivoltage mains transformer (15 watt is
enough) diode bridges (low current 1-2 Amps), smoothing capacitors
10000uF 16V, and voltage regulators such as LM7905 and LM7912.


Pipe Descaler

For many years now,
magnetic (or electromagnetic) water descaler devices have been showing
up on the shelves of Home Improvement and other DIY
stores all over Europe. Despite the numerous studies completed on that
subject, by manufacturers as well as by various consumer associations,
none have been able to conclude on the efficiency of commercial pipe
descalers in a decisive manner. Since electronic devices of this type
are relatively expensive (especially when we discover what they are made
of!), we decided to offer this project to our readers.

For the price of a few tens of pounds, you will be able to evaluate
the state of your own faucets, pots, and other pipes. The device we’re
offering as a project is identical to top-of-the-line items found on
sale; in other words, it includes the bi-frequency option because it
seemed that would be the best way to fight lime scale deposits. An
initial astable oscillator, based on a traditional 555, labeled IC3,
functions at around 10 kHz when the only capacitor C6 is operating; in
other words, when T1 is blocked. The latter is controlled by another
astable oscillator, based on IC1 this time, but which functions at about
1 Hz.

Pipe Descaler Circuit

Pipe Descaler Circuit Diagram

When T1 is turned on by IC1, capacitor C4 is effectively in parallel
with C6 which divides the frequency produced by IC3 by two, i.e. to
about 5 kHz. In order to have high amplitude signals, the power supply
operates with a mid-point transformer utilized in an unconventional way,
with simple half-wave rectification. The first half of the secondary
delivers 15 VAC which, after being rectified, filtered and regulated by IC2, supply stable current of 12 VDC to supply power to the oscillators.

The entire secondary makes it possible to have available, after rectification, approximately 40 VDC
which is used to supply power to coils L1 and L2, wound around the pipe
systems on which the assembly will work. To do that, IC3 is followed by
high-voltage transistor T2 (a BF457 or equivalent) which chops this
high voltage to 5 or 10 kHz frequency depending on the state of IC1. LED
D3 lights up to signal that the power supply is present. Coils L1 and
L2 are simple inductors made from insulated flexible wire, with about
ten windings each.

They have to be wound around the pipes carrying the water to be
‘treated’ and are spaced about ten centimeters from each other. Neither
the material of the pipe system, nor its diameter, should have any
influence on the efficiency of the device. Paradoxically, these coils
have one end in the air, which may surprise you as much as us but we
indicated at the beginning of this article, that our goal with this
project is not to explain the principle but rather to allow you to make
the same device as those sold in stores, so that you can perform your
own tests.

Author: Christian Tavernier – Copyright: Elektor Electronics


Simple Telephone Central Project

This simple telephone switching system is designed by Dr. Mustafa Kemal PEKER for training purposes. All responsibility is belong to user. Schematic and code is under GNU public licence. Cannot be used as commercially.


Circuit diagram


1) System contains 1 subscriber and 1 central
2) Central cannot call.
3) When subscriber hangs up, Central rings.
4) When central answers, ringing stops.
5) Conversation ends when one side closes the phone.

For simplicity you may use an old AT computer power supply which includes
12v and 5v. Have fun…..


Mains Slave Switcher

There are many
situations where two or more pieces of equipment are used together and
to avoid having to switch each item on separately or risk the
possibility of leaving one of them on when switching the rest off, a
slave switch is often used. Applications which spring to mind are a
computer/printer/scanner etc or audio amplifier/record deck/tuner
combinations or perhaps closest to every electronics enthusiast’s heart,
the work bench where a bench power supply/oscilloscope/soldering iron
etc are often required simultaneously.

The last is perhaps a particularly good example as the soldering
iron, often having no power indicator, is invariably left on after all
the other items have been switched off. Obviously the simplest solution
is to plug all of the items into one extension socket and switch this on
and off at the mains socket but this is not always very convenient as
the switch may be difficult to reach often being behind or under the
work bench. Slave switches normally sense the current drawn from the
mains supply when the master unit is switched on by detecting the
resulting voltage across a series resistor and switching on a relay to
power the slave unit(s).

This means that the Live or Neutral feed must be broken to allow the
resistor to be inserted. This circuit, which is intended for switching
power to a work bench when the bench light is switched on, avoids
resistors or any modifications to the lamp or slave appliances by
sensing the electric field around the lamp cable when this is switched
on. The lamp then also functions as a ‘power on’ indicator (albeit a
very large one that cannot be ignored) that shows when all of the
equipment on the bench is switched on.

The field, which appears around the lamp cable when the mains is
connected, can be sensed by a short piece of insulated wire simply
wrapped around it and this is amplified by the three stage amplifier
which can be regarded as a single super-transistor with a very high
gain. The extremely small a.c. base current results in an appreciable
collector current which after smoothing (by C3) is used to switch on a
relay to power the other sockets. Power for the relay is obtained from a
capacitor ‘mains dropper’ that generates no heat and provides a d.c.
supply of around 15 volts when the relay is off.

Circuit diagram:

Mains Slave Switcher Circuit

Mains Slave Switcher Circuit Diagram

The output current of this supply is limited so that the voltage
drops substantially when the relay pulls in but since relays require
more current to operate them than they do to remain energized, this is
not a problem. Since the transistor emitter is referenced to mains
Neutral, it is the field around the mains Live which will be detected.
Consequently, for correct operation the Live wire to the lamp must be
switched and this will no doubt be the case in all lamps where the
switch is factory fitted. In case of uncertainty, a double-pole switch
to interrupt both the Live and Neutral should be used.

The sensitivity of the circuit can be increased or decreased as
required by altering the value of the T2 emitter resistor. The sensing
wire must of course be wrapped around a section of the lamp lead after
the switch otherwise the relay will remain energized even when the lamp
has been switched off. The drawing shows the general idea with the
circuit built into the extension socket although, depending on the space
available an auxiliary plastic box may need to be used.

The circuit itself is not isolated from the mains supply so that great
care should be taken in its construction and testing. The sensor wire
must also be adequately insulated and the circuit enclosed in a box to
make it inaccessible to fingers etc. when it is in use.

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Infra-red Receiver

This very simple
infra-red receiver is intended to form an infra-red remote control
system with the simple infra-red transmitter described in this site. The
system does not use any kind of coding or decoding, but the carrier of
the transmitter is modified in a simple manner to provide a constant
switching signal. Since the receive module, IC1, switches from low to
high (in the quiescent state, the output is high) when the carrier is
received for more than 200 milliseconds, the carrier is transmitted in
the form of short pulse trains. This results in a pulse at the output of
the receiver that has a duty cycle which is just larger than 12.5%. The
carrier frequency used in the system is 36 kHz, so that the output
frequency of IC1 is 281.25 Hz.

Circuit diagram:

Infra Red Receiver Circuit

Infra-Red Receiver Circuit Diagram

This signal is rectified with a time constant that is long enough to
ensure good smoothing, so that darlington T1 is open for as long as the
received signal lasts. A drawback of this simple system is that it may
pick up signals transmitted by another infra-red (RC5) controller. In
this case, only the envelopes of the pulse trains would appear at the
output of T1. This effect may, of course, be used intentionally. For
instance, the receiver may be used to drive an SLB0587 dimmer. Practice
has shown that the setting of the SLB0587 is not affected by the RC5
pulses. The receiver draws a current of about 0.5 mA.