Simple Cat.5 Network Tester

This circuit came from a
need for a “quick and dirty” network tester that could be operated by
one person. All the commercial units I tried required a person at the
other end to check the remote LEDs, as the
transmitters could not be made to cycle through the test continuously to
allow one person to check both ends. It must be noted that this unit
will only check for pair continuity, pair shorts, crossed wires, and
shorts to other pairs. It will not test bandwidth, etc. Operation is
fairly basic.

Circuit diagram:

Simple Cat5 Network Tester

Simple Cat.5 Network Tester

Half of the 4011 quad 2-input NAND gate is
an RS flip-flop (IC1a, IC1b) which controls the other half, IC1c &
IC1d, operating as a clock oscillator. You can either start and stop the
oscillator running by pressing the Start and Stop switches or by virtue
of diode D1 connected to pins 12 & 13, use the Stop switch to allow
manual clocking of the 4017 counter. The 4017 drives one of eight LEDs
and the lines to the RJ45 socket. An output “High” on the 4017 decides
which line is under test, and if the circuit is complete, the test LED’s current is “sunk” by the 4017 and the LED will light.

If the corresponding test LED on the remote fails to light, then there is a short of that pair in the cable under test. If more than one LED lights, it indicates a short with another pair. A dark test LED
on the transmitter indicates that pair is open circuit. “Start” starts
the circuit cycling at a rate determined by the 470nF capacitor and
220kO resistor and “Stop/Step” stops cycling, steps through the lines,
and when stepped so that no channel LEDs are alight, effectively switches the unit off with a standby drain current of less than a microamp.

Author: Craig Stephen – Copyright: Silicon Chip Electronics

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4-20mA Current Loop Tester

This design will
interest technicians who work on pneumatically operated valves and other
4-20mA current loop controlled devices. Although 4-20mA signal
injector/calibrators are available, this one is both cheap to build and
easy to operate. When first powered up, the circuit sinks 4mA of
current. If switch S1 is pressed, the current level slowly ramps up to
20mA, pauses and then ramps back to 4mA. This cycle will continue unless
the switch is pressed again, whereby the output will lock to its
current level. A further push of the switch resumes the prior cyclic
operation. Output2 from the micro (IC1) is programmed to generate a
pulse-width modulated (PWM) signal to drive the current sink transistor (Q1).

Circuit diagram:

4 20mA Current Loop Tester Circuit

4-20mA Current Loop Tester Circuit Diagram

The digital PWM signal is converted to an
analog voltage using a low-pass filter formed by the 1kω series resistor
and a 4.7μF tantalum capacitor. By varying the PWM
duty cycle and therefore the DC signal level out of the filter, the
program can indirectly vary the current flow through the transistor. A
100 resistor in series with the emitter of Q1 converts the loop current
to a small voltage, which is fed into the micro on input1. The program
uses this feedback signal to zero in on the desired current level with
the aid of the micro’s analog-to-digital converter. Details of this can
be seen in the accompanying program listing.

If the PICAXE senses an open circuit, it
shuts down the output and goes into an alarm state, to alert the
operator and protect the circuit under test. The switch can be pressed
to reset operations to the start once the open circuit has been
rectified. The LED flashes a code for various
milestones, as follows: one flash at 4m and one flash to confirm a
switch press two flashes at 12m when ramping up (for the first 5
cycles); three flashes at 20m and continued fast flash sequence for
open-circuit alarm. For portable use, the circuit can be powered from
two 9V batteries, whereas for bench testing, a 12V DC plugpack will

Author: Allan Doust – Copyright: Silicon Chip Electronics


Simple Function Generator

Simple triangle-wave
generators have a weakness in that the waveform of their output signal
normally cannot be modified. The circuit presented here makes it
possible to smoothly alter the waveform of a linearly rising and steeply
trailing saw-tooth signal through a symmetrical triangle-wave to a
slowly trailing, steeply rising linear sawtooth. The wanted waveform may
be selected independently of the frequency, which can also be varied
uniformly from 0.2 Hz to 8 kHz. At the same time, a rectangular signal
with variable duty cycle (also independent of frequency) is available at
the rectangular-signal output of the circuit.

The circuit consists of integrator IC1b, whose output is applied to
comparator IC1c. The output of the comparator is a rectangular signal
The output of IC1b is raised by amplifier IC1d to a level that allows
the full output voltage range of the operational amplifier to be used.
Op amp IC1a provides a stable virtual earth, whose level is set to half
the supply voltage with P1. The smooth setting of the frequency is made
possible by feedback of part of the output of the comparator to the
input of the integrator via P2. This preset is usually not provided in
standard triangle-wave generators. Network D1-R1-D2-R2-P3 makes it
possible to give integrator capacitor C3 different charging and
discharge times.

Simple Function Generator Circuit

Simple Function Generator Circuit Diagram

This arrangement enables the output signal at A1 and the duty cycle
of the rectangular wave signal at A2 to be varied. Varying the
amplification factor with P5 has no effect on the frequency set with P2.
The slope of the signal edges, the transient responses, and the output
voltage range (rail-to-rail or with some voltage drop) depend on the
type of op amp used. The TL084 used in the prototype offers a good
compromise between price and meeting the wanted parameters. The circuit
is best built on a small piece of prototyping board. The circuit draws a
current of not more than 12 mA.

Brief parameters:
Provides triangle-wave, sawtooth or rectangular signal
Waveform variable independently of frequency (triangle wave and sawtooth)
Duty cycle of rectangular signal can be set independently of frequency

Test and measurement
Pulse-width control

Summary of preset action:
P1 – sets virtual earth to a level equal to Ucc/2;
P2 – sets the frequency;
P3– sets the waveform;
P4 – sets the hysteresis of the comparator (frequency and amplitude of the triangle-wave signal)
P5 – sets the amplification of the triangle-wave and sawtooth signals.


Thyristor Tester

The circuit in the
diagram is a very handy tool for rapidly checking all kinds of thyristor
(SCR, triac, …). In case of a triac, all four
quadrants are tested, which is done with S3, while in case of a
standard thyristor, a positive power supply and trigger current need to
be set, which is done with S1. The value of resistors R1 and R2 is
chosen to obtain a current of about 28 mA, which is more than sufficient
for most thyristors. The hold current is determined by R3, and is 125
mA, which is more than adequate to keep the thyristor in conduction
after it has been triggered. Since D1 is a red, low-current LED, and D2 a green, low-current LED, it can be seen in a wink in which quadrant the thyristor conducts.

Thyristor Tester Circuit

Thyristor Tester Circuit Diagram

Testing is started with S2, and the circuit is reset with S4 after
the test has been concluded. Three short lengths of circuit wire
terminated into insulated crocodile clips on connector K1 will be found
very convenient for linking any kind of thyristor to the circuit. Mind
correct connections, though: in the case of a triac, MT1/A1 is linked to
earth, the gate to S2 and MT2/A2 to R3; in the case of a standard
thyristor, the anode is linked to R3, the cathode to earth, and the gate
to S2. If, in a rare case the trigger current needs to be altered, this
can be done by changing the value of resistors R1–R3 as appropriate.
The trigger current may also be made variable by the use of a variable
power supply. If that is done, make sure that the dissipation in the
resistors is not exceeded.

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Wire Continuity Tester by LM709

Wire Continuity Tester :
While detecting discontinuities on a circuit board, it is probable to
include resistors, semiconductors or other elements in measurements.
This situation may cause wrong results. On the other hand sometimes the
voltage or current of the multimeter may defect some circuit components.

Our circuit overcomes this inconvenient conditions. The circuit
determines greater than 1 ohm values as discontinuity. Measure voltage
is not more than 2mV. So no kind of diode, IC or other component is
bypassed. Maximum current output of the circuit is about 200uA.

Indicator of the circuit is a LED. Voltage
supply may be two 9 Volt batteries. Voltage adjustment of the amplifier
is done by P1 potentiometer. To do this process, first short circuit
the probes and then turn P1 until LED brights. When you separate the probes again LED will fade out. This is a cheap and very useful circuit. You can build it on a PCB to use more easily.


Panner Waveform Generator

This device is a
microprocessor controlled waveform generator that can be used for
driving a voltage controlled stereo panner for music applications.
Panning is simply the movement of a mono audio signal between the left
and right channels of a stereo sound system.The circuit can also be used
to drive other voltage controlled modules that are used in analog music

The output of the waveform generator is a 0-10V DC control voltage.One of eight waveforms can be selected using an 8 position BCD
switch. Waves include: ramp, triangle, sine, cos(x)*sin(5x), damped 4
cycle sine wave, and pseudo random. Other waveforms may be substituted
by changing the assembly language waveform tables.

The waveform generator runs in an asyncronous manner, int can be
syncronized with other devices by use of an external (5V logic) sync
signal or a manual sync trigger button. The waveform is generator is
clocked by an oscillator circuit, the oscillator has three range
selections, and an LFO rate control for fine
speed adjustments.The output waveform can be smoothed with an adjustable
low pass filter. This can be used to remove the stair steps in the
output waveform for a more rounded waveform.

An array of 8 LEDs is included for
displaying where the current panning position is located, these are
spread evenly across the device front panel for an interesting visual

Power Requirements:
+5V DC approximately 200mA
+12V DC
approximately 25mA
-12V DC @ approximately 25mA
Output Levels: 0-10V DC
External Sync Input: TTL level (0-5V) DC, triggers on rising edge

The NE555 timer is wired as an asyncronous rectangular wave clock
oscillator. It creates pulses that are used by the microprocessor to
step through the selected waveform. Coarse adjustment to the 555 is made
by selection of three timer capacitors. Fine adjustment is performed by
changing a variable resistor.

The LFO clock signal is fed into the
microprocessor’s interrupt pin. Each pulse causes the software to
advance to the next waveform value in a stored table and send the value
to the input side of the DAC, resulting in a new analog value on the output of the DAC.
When the end of the table is reached, the software loops back to the
beginning of the table. The sync input goes to pin PC3. This signal is
either manually generated with a pushbutton, or externally generated
with a TTL level signal. When the sync input goes high, the microprocessor resets the to the beginning of the selected waveform table.

One of eight waveforms is selected from eight tables using the 3 bit BCD select switch that is wired to PC0, PC1 and PC2. If a suitable BCD switch cannot be found, three SPDT toggle switches can be substituted.

The microprocessor is set to use its internal clock, the 15K
resistor on the Xtal pin is part of the oscillator circuit. The internal
oscillator only provides low clock accuracy, this is fine since the
timing is gated by the LFO clock. For stable LFO operation, good quality capacitors should be used for the three LFO range
parts. The microprocessor reset circuit involves a resistor, diode, and
capacitor. This produces a slowly rising reset signal at power-up, and a
quickly falling signal in the event of a brief power outage.

The eight waveform display LEDs are connected to the microprocessor’s PB0 through PB7 outputs via some 330 ohm current limiting resistors.The MC1408P8 DAC receives the waveform level values from the microprocessor’s PA0 through PA7 outputs. The DAC converts the digital values into one of 256 analog levels. The output of the DAC is fed through the LM741 low pass filter stage for waveform smoothing. The LPF value is selected by changing the value of the capacitor across the feedback path.

Power is supplied to the circuit from a triple voltage “wall wart”
or another source of regulated +5, +12, and -12V DC power. Standard
bypass capacitors are connected across the three power supply lines.

The control program pan.asm is written in 6805 assembly language, it
needs to be assembled and loaded into an MC68705P3 microcontroller IC.

The prototype of this circuit was built on a hand-wired perforated
circuit board using wire-wrap technology. The analog components were
installed in DIP headers and were connected
with wire-wrap wire. 8-way resistor networks were used for the 10K and
330 ohm resistors. The controls and input/output connections were
mounted on a rack mount metal chassis, the wiring was connected to the
circuit board with a single dual inline header.

Connect the waveform generator to the voltage controlled panner circuit,
wire up the mono audio input and stereo audio outputs to the panner.
Select a waveform with the 8 position switch. Tune the LFO
range and rate controls for the desired panning speed. Adjust the low
pass frequency switch for the desired smoothness. Play with the various
controls until a good panning sound is heard.

It is advisable to observe the control voltage output on an
oscilloscope in order to become familiar with the effect of the various
controls on the output signal.

This effect is featured on my Soundtrack for a Low-Budget Sci-Fi Movie audio CD.


Mains Frequency Monitor

Here is a simple
frequency counter designed to monitor the 240VAC mains supply. It as a
frequency range of 0-999Hz, so it could also be used with 400Hz
equipment. Standard TTL/CMOS logic is employed
for the counters and display drivers, while an ELM446 (IC1) generates
accurate 1Hz pulses for gating. This device utilizes a 3.579545MHz
crystal for its timebase, as commonly found in TV and video circuits and
even on old PC motherboards.

Mains Frequency Monitor Circuit

Mains Frequency Monitor Circuit Diagram

Copyright: Silicon Chip Electronics Magazine


Water Level Indicator Using 7-Segment Display

This water-level indicator uses a 7-segment display, instead of LEDs,
to indicate the water level (low, half and full) in the tank. Moreover,
a buzzer is used to alert you of water overflowing from the tank. The
circuit shows the water level by displaying L, H and F for low, half and
full, respectively. The circuit uses five sensors to sense the
different water levels in the tank. Sensor A is connected to the
negative terminal (GND) of the power supply. The other four sensors (B through E) are connected to the inputs of NOT gate IC 7404. When there is a high voltage at the input pin of the NOT gate, it outputs a low voltage. Similarly, for a low voltage at the input pin of the NOT gate, it outputs a high voltage.

When the tank is empty, the input pins of IC 7404 are pulled high
via a 1-mega-ohm resistor. So it outputs a low voltage. As water starts
filling the tank, a low voltage is available at the input pins of the
gate and it outputs a high voltage. When the water in the tank rises to
touch the low level, there is a low voltage at input pin 5 of gate N3
and high output at pin 6. Pin 6 of the gate is connected to pin 10 of
gate N9, so pin 10 also goes high. Now as both pins 9 and 10 of gate N9
are high, its output pin 8 also goes high. As a result, positive supply
is applied to DIS3 and it shows ‘L’ indicating low level of water in the
tank. Similarly, when water in the tank touches the half level, pins 4
and 5 of AND gate N8 become high.


Circuit diagram

As a result, its output also goes high and DIS2 shows ‘H’ indicating
half level of water in the tank. At this time, pin 9 of gate N9 also
goes low via gate N4 and DIS3 stops glowing. When the water tank becomes
full, the voltage at pin 1 of gate N1 and pin 3 of gate N2 goes low.
Output pin 3 of gate N7 goes high and DIS1 shows ‘F’ indicating that the
water tank is full. When water starts overflowing the tank, pin 13 of
gate N6 goes low to make output pin 12. The buzzer sounds to indicate
that water is overflowing the tank and you need to switch off the motor
pump. Assemble the circuit on a general-purpose PCB
and enclose in a suitable box. Use a non-corrosive material such as
steel strip for the five sensors and hang them in the water tank as
shown in the circuit diagram. Use regulated 5V to power the circuit.

Source: EFY Mag


DC or AC Voltage Indicator

Detects 1.8 to 230 Volts DC or AC, Minimum parts counting

This circuit is not a novelty, but it proved so useful, simple and
cheap that it is worth building. When the positive (Red) probe is
connected to a DC positive voltage and the Black probe to the negative,
the Red LED will illuminate. Reversing polarities the Green LED will illuminate. Connecting the probes to an AC source both LEDs will go on.
The bulb limits the LEDs current to 40mA @
220V AC and its filament starts illuminating from about 30V, shining
more brightly as voltage increases. Therefore, due to the bulb filament
behavior, any voltage in the 1.8 to 230V range can be detected without
changing component values.

DC or AC Voltage Indicator Circuit

DC or AC Voltage Indicator Circuit Diagram

P1 = Red Probe
P2 = Black Probe
D1 = 5 or 3mm. Red LED
D2 = 5 or 3mm. Green LED
LP = 1220V 6W Filament Lamp Bulb

A two colors LED (Red and Green) can be used in place of D1 & D2.

Source: Red Free Circuit Design


Relay Switch Activated by Tone and Signal

The essence of the
circuit is for the input of tone and signal to provide an activation for
the relay switch.

  • Relay – an electrically operated switch where the current flowing
    through the coil of the relay is creating a magnetic field which
    attracts a lever and changes the switch contacts, thereby making its
    state open or close
  • BC214 – a complementary silicon planar epitaxial transistor used
    in AF small signal drivers and amplifiers as well as for low noise
    preamplifier applications due to its feature of good linearity of DC
    current gain
  • LM741 – a general purpose single operational amplifier with
    features such as offset null, compensated internal frequency, voltage
    range with high input, good stability of temperature, and protected from
    short circuit

The use of relay will allow the circuit to switch from one condition
to another. It can also be referred to as a form of an electrical
amplifier since it is able to control an output circuit of higher power
than the input circuit. There are many types of relays being used in
many electronic and electrical circuits, which include solid-state
relay, Buchholz relay, overload protection relay, latching relay,
forced-guided contacts relay, mercury-wetted relay, contactor relay,
machine tool relay, reed relay, polarized relay, and solid state
contactor relay.


Circuit diagram

The circuit created is sensitive enough to the AC signals in the
input stage, where the signals are ranging above 5 mV. It will also be
sensitive to react with the human voice signals having a range of
frequency from 50 Hz up to 3 KHz. The human voice is a part of the human
sound produced primarily by the vocal cords or vocal folds which in
turn produces a voice frequency that is used for the transmission of

During the absence of an input signal, the state of the 12 V relay RL1 is at OFF
condition as regulated by the 10K Ohms trimmer RV1. The circuit can be
made to react with its sensitivity in points A, B, & C, where a
negative feedback can be placed due to the addition of band pass filter.
The filter will operate only in the 1 KHz range and the circuit will
only correspond at this frequency.

Part list

Part list

The signal and tone activated relay switch were used in a wide range
of fields which includes measuring instruments, audio systems,
communications equipment, and factory-automation equipment. They can
also be found on telephone subscriber circuits for the polarity
reversing switch, testing, and ringing functions.