Computer data cable tester

You don't need access to fancy test gear to check and fault-find computer data cables. Instead, this simple device will give a quick indication of the status of point-to-point cable wiring; ie, crossovers, shorts and open circuits.

It works like this: IC1a & IC1b (part of a 4069 hex inverter IC) form an oscillator which provides a 1Hz clock signal. Its output is buffered and inverted by IC1c and IC1d (connected in parallel) and clocks IC3, a 4017 decade counter.

IC3's O4 output is connected back to its reset pin (pin 15). As a result, IC3 toggles its O0 - O4 outputs high (and low) in sequence to test the four data pairs and the ground (GND) connection (if required).

In operation, IC3's outputs drive the indicator LEDs and inverters IC2c-IC2f. Note that there are four indicator LEDs for each pair. LEDs A & C connect to the local end of the cable, while LEDs B & D connect to the remote end.

The inverted outputs provide the return paths for the relevant "mate" of each tested pair. As a result, if the cable is OK, LEDs A, B & C will all light for each tested pair.

The "D" LEDs at the remote end indicate if there is a crossover in the cable, while the "C" LEDs indicate which pair is being tested. As well, two extra LEDs are connected in series with the GND lead (one at either end) to indicate its status. These LEDs are driven by the O3 output of IC3 via 470W resistors.

Finally, note that the LEDs must be high-brightness types.

A Liehr,
Kallangur, Qld. ($50)

Inductive speed sensor for cars Here's a way to avoid winding the pickup coil for the inductive speed sensor used in projects such as the Speed Alarm, Gearshift Indicator, etc. All you need to do is carefully break open the plastic case of a small relay and cut away the armature mechanism, so that you are left with the relay coil and core. If you are careful you can retain the terminating pins that are moulded in the plastic former to solder the shielded cable to. The relay coil can then be mounted on a strip of aluminium which can then be used as a mounting bracket. Fishing rod binding thread can be used to fix the coil to the bracket, after which you can use binding varnish to protect it after the wires have been soldered. You can then dip the whole lot in epoxy for further protection or just use a small piece of heatshrink. Rick Goodwin, Moonah, Tas. ($25)

Digital thermometer with LCD readout

Unlike the SILICON CHIP design described in December 1998, this digital thermometer obtains its supply from a single 6V battery. In addition, this design includes its own metering circuitry and doesn't have to be plugged into a DMM.

As shown, IC1b is used to amplify the thermocouple output and this drives IC2, an ICL7106 counter/LCD driver. IC2 in turn drives an Hitachi L1331CC 3.5-digit LCD. Alternatively, an LCD panel meter could be used here with just a few minor changes.

IC1a and D1 function as a voltage regulator and this provides a reference voltage to the negative end of the thermocouple and to pin 6 of IC1b. D1 establishes a 0.65V reference on pin 3 of IC1a, while VR1 sets the gain and thus the reference voltage from pin 1.

The prototype thermometer is wall-mounted and uses four AA alkaline cells to ensure long battery life. The counter is wired for a 20V range and calibrated against a known voltage by adjusting VR3.

The calibration procedure for the temperature sensing section is very simple. First, VR1 is adjusted to obtain a 1.500V reading on the INLO input of IC2 (pin 30). VR2 is then adjusted until the LCD readout matches the reading from an accurate reference thermometer (eg, the LCD should show 022 for a temperature of 22°C).

K. J. Benic,
Forestville, NSW. ($40)

Low battery indicator This circuit indicates the remainAing battery life bAy varying the duty cycle and flash rate of an LED as the battery voltage decreases. In fact, the circuit actually indicates five battery conditions: (1) a steady glow assures indicates that the battery is healthy; (2) a 2Hz flicker (briefly off) indicates that the battery is starting to show age; (3) a 5Hz 50% duty-cycle flash is a warning that you should have a spare battery on hand; (4) a brief flicker on at a 2Hz rate indicates the battery's last gasp; and (5) when the LED is continuously off, it's time to replace the battery. IC1 is wired as an oscillator/comparator, with a nominal fixed voltage reference of about 1.5V on its pin 2 (inverting) input (actually, it varies between about 1.7V and 1.4V depending on the hysteresis provided via R6). This reference voltage is derived from a voltage divider consisting of resistors R4 & R5, which arAAe connected across the 5V rail derived from regulator REG1, and feedback resistor R6. Similarly, IC1's pin 3 input (non-inverting) is connected to a voltage divider consisting of R1 & R2 which are across the 9V battery. Using the component values shown, the circuit will switch LED1 from being continuously on to flash mode when the 9V battery drops to about 6.5V. Subsequently, LED1 is continuously off for battery voltages below 5.5V. Naturally, you can tweak the resistor values in the divider network for different voltage thresholds as desired. In operation, the circuit oscillates only when the sampled battery voltage (ie, the voltage on pin 3) is between the upper and lower voltage thresholds set on pin 2. Capacitor C3 provides the timing. Above and below these limits, IC1 simply functions as a comparator and holds LED1 continuously on or off. Finally, to precisely set the "dead-battery" threshold, make R4 adjusable to offset the variations in regulator tolerance. Ashish Nand, Melton South, Vic. ($35)

555 timer circuit with variable on/off times

This circuit enables the on/off times of a 555 timer to be independently varied over a wide range. This is not possible with a conventional 555 circuit with the RC network being charged from the positive supply rail and discharged via pin 7.

Instead, the capacitor at pins 2 & 6 of IC1 is charged and discharged from the output at pin 3. Furthermore, the charging and discharging circuits are different, being isolated by diodes D1 & D2. Therefore the capacitor at pins 2 & 6 is charged via diode D2 and trimpot VR2 and discharged via D1 and trimpot VR1.

With this arrangement you can have very long on times combined with very short off times and vice versa, or you can adjust the duty cycle to exactly 50% and so on.

This circuit also employs a second 555 timer (IC2) as an inverter so that complementary pulses are available, if required. If not, delete IC2.

A. Davies,
Canberra, ACT. ($30)

High-current battery discharger If you have a motley collection of 12V batteries in varying states of health, this simple circuit will allow you to easily check their capacity. It's basically a high-current discharge load which is controlled by the NiCd Discharger published in the November 1992 issue of SILICON CHIP. A subsequent circuit published in Circuit Notebook, September 2000, showed how to add a clock timer to this discharger, so that the discharge period could be measured. This involved increasing the existing 10μF capacitor across LED1 to 100μF, to enable it to supply the brief current pulses required by the clock mechanism. The discharger's "clock connection" now controls a BC457/BD139 Darlington transistor pair (Q1 & Q2) via a 1kΩ resistor. These in turn activate a car headlamp relay to switch in a preselected lamp load (one of three). With 12V selected, the prototype unit stops the discharge at 11.4V which corresponds to a cell voltage of 1.9V (this is a pretty good indication of a discharged 12V battery). The loads consist of three automotive lamps, selected to provide discharge rates to suit the battery being tested. These lamps should be fitted to sockets, so that they can be easily swapped for other lamps with different wattages, if required. That way, the discharge current can be varied simply by changing the lamp wattage. By the way, this circuit will also work with 6V batteries, provided the relay holds in. This gives an "end-point" voltage of about 5.7-5.8V. Reg Carter, Ballarat, Vic. ($25)

High current low-dropout regulator

This circuit was designed to allow a laptop computer to be powered from a solar power setup. The computer requires 12V at 3.3A.

The circuit is a linear regulator with Mosfet Q4 as the series pass device. A 100kΩ resistor provides Q4 with a positive gate-source voltage. Any tendency for the output voltage to exceed ZD1's voltage causes Q2 to turn on. This turns on Q3 which reduces Q4's gate voltage and thus reduces the output voltage. Note that Q2's base-emitter voltage stabilises at about 0.35V. This combined with the zener voltage gives an output of 12.4V.

If a more precise output is required, first select ZD1 so that its voltage rating is at least 0.4V less than the required output voltage. You can then "trim" to the required output voltage by installing a resistor in series with ZD1.

Q2's base-emitter voltage and the 680W base resistor set the current through ZD1 to 0.5mA. This means that the output voltage will be boosted by 0.1V for each 200Ω of resistance in series with ZD1.

Zener diode ZD2 ensures that Q4's maximum rated gate-source voltage is not exceeded. Mosfet Q1 provides reverse polarity protection.

Note that Q4 requires a heatsink since it will dissipate about 10W under worst-case conditions. No heatsink is required for Q1.

At 3.3A, the regulator reduces the output voltage by just 0.2V. This can be further reduced by paralleling Q1 & Q4 with additional Mosfets.

Andrew Partridge,
Kuranda, Qld. ($35)

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