Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

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 pushbuttons 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.

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.

Ashley Dawson,
Warrandye, Vic. ($35)

Phantom supply for lapel mic adaptor

This modification to the Lapel Microphone Adapter for PA systems (January 2004) will allow the unit to operate with the standard 48V phantom supply available on some audio mixers.

Resistors R1-R4 form a simple voltage divider network to reduce the standard 48V phantom supply to 9V to power the adaptor circuit. Zener diode ZD1 provides voltage regulation and capacitor C1 provides audio decoupling.

The original normally-open (NO) relays are replaced with changeover (DPDT) types to protect the phantom supply from a short circuit. The two original 6.8kΩ audio balancing resistors have been changed to 22kΩ each to prevent excessive current being drawn from the phantom supply.

Both the output and input connectors can be changed to mini XLR sockets for convenience but the stereo phone jacks can still be used.

Alan Morrow.
Reservoir, Vic. ($30)

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).

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.

J. Begg,
Heidelberg, Vic.

LED chaser provides three game functions

This circuit is essentially a light chaser but it can also be set to provides heads or tails (Two Up) or a Dice (die). It also has a speaker to simulate the sound of a spinning roulette wheel.

Note that the dice and heads/tails features can be deleted if required and rules for the games created to suit individuals; eg, betting can be used or the numbers recorded and then totalled to get the highest score per game.

IC1 is a 4046 phase locked loop (PLL) but only the voltage controlled oscillator (VCO) portion of the chip is used to provide the clock pulse for IC2, a 4017 decade counter/divider. In roulette wheel mode, switch S3 is pushed to start the game. This charges the 10μF capacitor at pin 9 and as the capacitor discharges, the output frequency is slowly reduced to slow the rate of the chaser LEDs driven by IC2.

In chaser mode, switch S2 is closed to provide a fixed frequency output from IC1. This can be varied over a wide range with potentiometer VR1. Transistor Q1 is also driven by the oscillator output of IC1 and it drives the speaker. Trimpot VR2 varies the sound level while switch S4 turns it off.

Switch S5 selects Die or other (chaser/roulette). In Die mode, pin 6 is connected to the reset, pin 15, so that the circuit only counts to 6 whereas in the other modes it counts to 10 and displays all LEDs.

Pin 12 drives transistor Q2 and two LEDs to provide the Heads/Tails function.

John McCuaig,
Caloundra, Qld. ($40)

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.

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.

Barry Freeman,
Morphett Vale, SA. ($40)

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