Cable Tester Uses Quad Latch

This circuit was designed to allow microphone cables or other cables to be easily tested for intermittent breaks that can often be difficult to find using a multimeter. The circuit can test cables with up to four cores. Both switches used in the circuit are momentary contact push-buttons and it can run from a 9V battery, in which case the 7805 regulator can be omitted. To test a cable, connect it between the two sockets and press switch S2 which resets all four latches in IC1, setting them low. This turns on all four LEDs.

Circuit diagram:
Cable tester uses quad latch circuit schematic

A good connection for each core of the cable will mean that the relevant Set inputs of the latches (pins 3, 7, 11 & 15) will be pulled high and the appropriate LED will remain on. A broken connection in the cable will result in the relevant Set input being pulled low by the associated 10kΩ resistor and the so the LED will be off. Because the circuit latches, it is easy to pinpoint even the smallest breaks by simply flexing and twisting the cable up and down its length until one of the LEDs turns off. To test different types of cables, simply connect appropriate sockets in parallel with or in place of the XLR sockets.
Author: Ashley Dawson - Copyright: Silicon Chip Electronics
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Video Tracer For Trouble-Shooting

This circuit was designed as an aid to installers and maintainers of video systems. It is basically a video sync separator (IC1) followed by a LED and buzzer driver (IC2, Q1 & Q2). In use, the device is connected to a video cable and if there is video present, the LED will flash at about 10Hz. If there is no video, the LED flashes briefly every couple of seconds. A buzzer can also be switched in to provide an audible indication. The buzzer is particularly useful when tracing cabling faults or trying to find a correct cable amongst many, where it is difficult to keep an eye on the LED.

Another use for the buzzer option is to provide a video fault indication. For example, it could be inserted in bridging mode, with switch S1 in high impedance mode (position 2) across a video line and set to alarm when there is no video present. If someone pulls out a cable or the video source is powered off, the alarm would sound. IC1 is a standard LM1881 video sync separator circuit and 75Ω termination can be switched in or out with switch S1a. The other pole of the switch, S1b, turns on the power. The composite sync output at pin 1 is low with no video input and it pulses high when composite sync is detected.

Circuit diagram:
Video tracer for trouble-shooting circuit schematic

These pulses charge a 100nF capacitor via diode D1. When there is no video at the input, oscillator IC2b is enabled and provides a short pulse every couple of seconds to flash the LED. The duty cycle is altered by including D2, so that the discharge time for the 10μF capacitor is much shorter than the charge time. The short LED pulse is used as a power-on indicator drawing minimal average current. When video is present at the input, IC2b is disabled and IC2d is enabled. The output of IC2d provides a 10Hz square wave signal to flash the LED. The buzzer is controlled by switch S2. In position 2 the buzzer will sound when there is video at the input and in position 1 the buzzer will sound when there is no video at the input.
Author: Leon Williams - Copyright: Silicon Chip Electronics
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Using AC for LED Christmas Lights

This circuit uses low-voltage AC to drive a string of 50 or so bi-color LEDs (two LEDs connected in inverse parallel). Power to the LEDs is controlled by the Triac and the two optocouplers which have their photo-transistors effectively connected in inverse-parallel. Depending on which optocoupler is turned on, the Triac applies positive, negative or both half-cycles to the LEDs and so the colours can be red, green or in-between. Switch S1 is used to select the pulses from two oscillators which are formed by the NAND gates in IC1 (4011B). This provides a variety of LED flash patterns, depending on the setting of S1.

Circuit diagram:
Using AC for LED Christmas lights circuit schematic
Author: Matthew Peterson - Copyright: Silicon Chip Electronics
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TV Relative Signal Strength Meter

This circuit was designed to assist the installation of TV antennas. The signal is monitored using a small portable TV set and this circuit monitors the output of the TV's FM detector IC via a shielded lead. To initially calibrate the meter, adjust trimpot VR2 to zero the meter. Trimpot VR1 is a sensitivity control and can be set for a preset reading (ie, 0dB) or can be calibrated in millivolts. Rotating the antenna for a minimum reading on the meter (indicating FM quieting) gives the optimum orientation for the antenna.

Circuit diagram:
TV relative signal strength meter circuit schematic
Author: Ted Sherman
Copyright: Silicon Chip Electronics
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Model Theatre Lighting Dimmer

This circuit is the basis for the dimmers in a model theatre lighting system which uses touch globes as the light source. The circuit is based around a 555 timer, driving a Triac. All dimmers share the one power supply and zero-crossing detector. As it will only work if there is a common AC/DC return path, it has a simple DC supply circuit consisting of one 1N4004 diode and one 4700µF capacitor. Transistors Q1 to Q3 comprise a zero-crossing detector whose output is inverted into a negative-going pulse by Q4. This pulse is fed to the trigger input (pin 2) of the 555 IC which then starts its timing period at the beginning of each mains half cycle.

Circuit diagram:
Model theatre lighting dimmer circuit schematic

The length of this period is set by capacitor C2 and the combination of resistors R6 with pots VR1 and VR2. The output of IC1 at pin 3 is then fed to transistor Q5 which inverts this signal to trigger the Triac via a 100# resistor. When the timing period is short, the Triac is turned on early in half cycle and lights are bright. Conversely, when the timing period is longer, the lights are dim or turned off. The main dimmer control is potentiometer VR1. Trimpot VR1 is used to set the range of VR1. With VR1 set fully clockwise (ie, maximum resistance) trimpot VR2 is adjusted until the lights are just turned off. The lights should then be able to be faded over the full range by the control potentiometer.
Author: Barry Freeman - Copyright: Silicon Chip Electronics
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Frequency Multiplier For LF Measurements

When designing bass reflex loudspeaker cabinets, it is necessary to measure the resonance of the speaker to an accuracy of about 1%. To do this, you need an audio oscillator and a frequency counter. However, the typical accuracy and resolution of a frequency counter when measuring frequencies below 50Hz can lead to errors of several percent. The solution to this problem is to use a frequency multiplier and the circuit presented here can be switched to multiply by 10 or 100. It uses a 4046 phase locked loop (PLL) and a 4518 connected as a dual divide-by-10 counter. As shown, the oscillator signal is fed into the comparator formed by IC1a and its output drives the SIGin input, pin 14, of the 4046 PLL (IC2).

Circuit diagram:
Frequency multiplier for LF measurements circuit schematic

The PLL's output is fed to IC3 and divided by 10 or 100, depending on the setting of switch S1. The divided signal is then fed to the COMPin input (pin 3) of IC2. In this way, the PLL is forced to multiply the input frequency by 10 or 100 and this multiplied frequency can be read out with much improved accuracy by a typical digital frequency meter. However, you must then divide the displayed reading by the selected multiplication ratio to get the true frequency. The limitation in this circuit is that the 4046 can only run up to 20kHz so that the input frequency is limited to 200Hz or 2kHz, depending on the multiplication ratio. This is quite adequate for measuring bass reflex cabinets.
Author: J. Begg - Copyright: Silicon Chip Electronics
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Courtesy Light Extender

In essence, this circuit is a 15 to 20-second courtesy light extender for cars. It is activated in the usual way by opening a door but it also samples the negative lock/unlock signals from a car alarm or central locking and does two more things. First, when an unlock signal is received, it turns on the courtesy light for 15-20 seconds before you open the door. Second, when a lock signal is received, it turns off the courtesy light immediately, with no fade-out. This is done to eliminate false triggering of the burglar alarm through current drain sensing. When a car door is open or the unlock relay is activated, the 33µF capacitor discharges through diode D1 and this keeps transistor Q1 turned off.

Circuit diagram:
Courtesy light extender circuit schematic

This allows Q2 and Q3 to turn on and the courtesy lamp is activated. When the door is closed, the courtesy lamps stay illuminated and the 33µF electrolytic capacitor starts charging through the associated 1MO resistor. As the voltages rises, Q1 turns on slowly, turning off Q2 and Q3 which gradually fades out the courtesy lamp. If a lock signal from the central locking system is received, relay 1 closes and charges the capacitor instantly, so the lamp turns off immediately. Relays were used to interface to the central locking/alarm system as a safety feature, to provide isolation in case something goes wrong.
Author: Matt Downey - Copyright: Silicon Chip Electronics
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