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
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
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
You will require two additional short stereo extension cables
to connect the box to the PC.
Bungendore, NSW. ($35)
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. 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
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
"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.
Using AC for LED Christmas lights.
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.
Manukau, NZ. ($40)
DC motor speed controller
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
Masterton, NZ. ($30)
Short circuit protection for balanced supply rails
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
Wurtulla, Qld. ($40)
Tablet reminder uses watch module
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.
Liverpool, NSW. ($35)