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Circuit Notebook

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.

Speaker-headphone switch for PCs

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If you need to use a headset with your PC, then you will know how frustrating it is continuously swapping over speaker and microphone cables. This is even worse if the PC is parked in a dark corner and the hard-to-read writing on the sound card sockets is covered in dust.

This simple switch box eliminates all these problems. It sits on top of the desk and connects to the PC with stereo one-to-one cables. On the rear of the box are sockets for the PC speaker and microphone connections and the existing speakers. On the front of the box are the sockets for the headset microphone and headphones, an input for an external microphone and two switches.

One switch is used to direct the sound card output from the PC to either the existing speakers or the headphones. The second switch connects either the headset microphone or the external microphone to the input socket of the PC sound card. The switches used were 3 position 4 pole rotary switches with the last pole unused and adjusted for 2-position operation. All sockets were stereo 3.5mm types.

This multiple switching arrangement is very flexible and is especially handy if you want to use an external microphone while monitoring with headphones. The ground wire as well as the left and right wires are all switched to prevent noise that could otherwise be induced into the microphone input through joining separate earths. For the same reason, a plastic case is used so that the earths of the sockets are not shorted together as would happen with a metal case.

You will require two additional short stereo extension cables to connect the box to the PC.

Leon Williams,
Bungendore, NSW. ($35)

Simple Cat.5 network tester
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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. Half of the 4011 quad 2-input NAND gate is an RS flipflop (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 220kΩ 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.

Craig Stephen,
Cromwell, NZ.

Using AC for LED Christmas lights.

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This circuit uses low-voltage AC to drive a string of 50 or so bi-colour LEDs (two LEDs connected in inverse parallel). Power to the LEDs is controlled by the Triac and the two optocouplers which have their phototransistors 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.

Matthew Peterson,
Manukau, NZ. ($40)

DC motor speed controller
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This circuit takes advantage of the voltage drop across bridge rectifier diodes to produce a 5-position variable voltage supply to a DC fan or other small DC motor. It is not as efficient as a switchmode circuit but it has the virtues of simplicity and no switching hash.

The four full-wave bridges are connected so that each has two pairs of series diodes in parallel, giving a voltage drop of about 1.4V, depending on the load current. The rotary switch should have "make before break" contacts which should be rated to take currents up to about an amp or so. For higher currents, higher rated bridge rectifiers and a suitably rugged rotary switch (or solenoids) will be required.

If you want smaller voltage steps, you could use the commoned AC inputs on the bridge rectifiers to give intermediate steps on the speed switch.

Stephen Butcher,
Masterton, NZ. ($30)

Short circuit protection for balanced supply rails

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This circuit was designed to protect a dual rail power supply from shorts across the two rails. It uses an optocoupler to monitor each
supply rail, with the internal LEDs powered from ZD2 and ZD3 and the associated resistors. While the LEDs are on, the optocoupler's internal transistors are both turned on which ensures that transistor Q1 is on and relay RLY1 is energised. If either rail is short-circuited, the associated optocoupler is turned off, robbing Q1 of base current and the relay then drops out to disconnect the supply rails.

Operation is restored by pressing the reset button. The value of ZD1 and the associated resistor should be chosen to suit the supply and relay coil voltages.

Mark Arnold,
Wurtulla, Qld. ($40)

Tablet reminder uses watch module
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This device is used as a reminder to take medicine every day. This device actually contains a crystal watch and a 4001 quad 2-input NOR gate with two of the gates (IC1a & IC1b) wired as an RS flipflop.

The watch is set to "tablet time", usually mornings, when an alarm is activated with a high signal fed via diode D1 which sets the RS flipflop and enables the oscillator comprising gates IC1c & IC1d. This drives the LED with a 10% duty cycle.

The 10nF capacitor resets the watch alarm when positive voltage appears on pin 3 of IC1. The circuit consumes only 50μA with a 3V battery.

Rasim Kucalovic,
Liverpool, NSW. ($35)

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