Vertical sync for the Component Video to RGB Converter

Some video projectors with RGB component video inputs have separate inputs for vertical and horizontal sync, so these signals need to be separated before feeding them into the projector. This means that in order to drive these projectors from the Component Video to RGB Converter (SILICONCHIP, May 2004), a small amount of circuitry must be added to the converter to provide a separated vertical sync output.

Note that the existing composite sync output can be used to drive the horizontal sync input of the projector.

The additional circuitry is very straightforward, as you can see from the circuit diagram. It consists of an LM358 dual op amp, plus a 75W resistor and two 100nF monolithic ceramic capacitors. These can all be mounted on a small piece of Veroboard or similar and wired directly to an RCA socket (CON9) mounted on the converter’s back-panel. The extra socket can be positioned between the existing composite sync and red video output sockets.

As shown, only one half of the LM358 is used. It is connected as a unity gain buffer with its non-inverting input connected to pin 3 of the LM1881 sync separator (IC5) on the converter board.

This pin provides the vertical sync output signal. The only other connections between the additional circuitry and the converter board are for the ±5V supply and earth rails.

SILICONCHIP.

LED noughts & crosses

Here’s a twist to the age-old game of noughts and crosses. Instead of pen and paper, it uses nine 10mm tri-colour LEDs arranged in a 3 x 3 grid. One player has nine red buttons while the other player has nine green, set out in identical grids.

The aim, of course, is to make three LEDs in a row glow the same colour – red or green! Pushing a red button causes the LED in the equivalent position on the grid to glow red in colour. Likewise, pushing a green button lights the equivalent green LED.

If a player pushes a button for a LED that is already glowing red or green, then that LED changes to yellow, exposing the false move! All the LEDs are then turned off, ready for the next game, by pressing the "Clear" button.

For simplicity, the circuit shows only one tri-colour LED and a pair of opposing buttons. This circuit fragment must be repeated another eight times to create a complete 3 x 3 grid. A brief press on a button fires the associated SCR and turns on the LED. The common (cathode) lead of all LEDs is connected to the 0V rail via the normally-closed contacts of the "Clear" pushbutton (S3).

The SCRs are available from RS Components (stock no. 183-7695), on the web at www.rsaustralia.comor phone 1300 656 636. PC-mount tactile switches from Jaycar (Cat. SP-0720 & SP-0724) were used on the prototype.

A. J. Lowe,
Bardon, Qld. ($35)

Audio level threshold control

This circuit was originally designed for use in detecting discharges from individual neurons, where the infrequent discharges are difficult to separate from dominant background noise. It may also prove useful in other applications that need to detect infrequent low-level audio signals against a noisy background.

The audio input signal is buffered by op amp IC1 before being applied to the opposing inputs of comparators IC4 & IC5. Positive and negative offset voltages are generated by VR1 and IC2 and fed to the other two inputs of the comparators.

Essentially, the comparators act to produce a negative voltage at their commoned outputs (C) whenever the audio signal exceeds either the positive or negative offset voltage. The signal at "C" is inverted by transistor Q1 to produce "D". These two signals are used to control a pair of CMOS switches (S1 & S2), which either pass the audio signal to the output or short it to ground.

The signal from the CMOS switches is buffered by IC3, which in conjunction with the 10kΩ resistor and 10nF capacitor filters out the switching artefacts.

In practice, the offset voltage is adjusted until there is little or no breakthrough of the noise background at the output. Thereafter, only audio signals exceeding the threshold are passed. Inevitably, this produces some crossover distortion but this is of little consequence compared with the benefit of the quiet background.

Graham Jackman,
via email. ($40)

Cheap pump controller

This simple but effective circuit can be used to control water level in a container. The prototype is used to pump water out of a bucket that collects condensation from a home air-conditioning system.

The design is based around a 555 timer (IC1). Although the timer in configured as a mono-stable, it lacks the usual timing capacitor from pin 6 to ground. Instead, a metal probe inserted in the water provides a current path to a second, grounded probe.

When the water level in the container reaches a third ("high") probe, the trigger input (pin 3) is pulled low, switching the 555 output high and energising the relay via transistor Q1. Once the water level drops below the "low" probe, the threshold input (pin 6) swings high, switching the output (pin 3) low and the relay and pump off.

The two 100kΩ pull-up resistors can be replaced with larger values if more sensitivity is required (eg, if the 555 doesn’t trigger). A switch (S1) can be included to bypass the relay for manual emptying.
The "low" probe should be positioned so that the pump doesn’t run dry. The high level probe is placed at the level that you want the pump to start. Since the water is held at ground potential, you must use stainless steel or copper wire to slow corrosion.

With water fountain pumps available for less than $10, this circuit offers a cheap alternative for those who have an air-conditioner on an internal wall and don’t want to be continually emptying the bucket on humid days.

Adrian Hudson,
Calamvale, Qld. ($35)

DC-DC converter powers USB

Older laptop computers without built-in USB ports can be retrofitted with card-based (PCMCIA) USB ports, where needed. These cards generally require a separate 5V supply for the USB and this can be conveniently supplied using four "AA" NiCd or NiMH cells. The nominal 4.8V output is a little shy of the specified 5V but in most cases, this should not be a problem. However, the use of rechargeables does mean having to carry an extra charger in your kit.

To allow the use of alkaline cells and therefore eliminate the need for the charger, the "3V to 9V DC-DC Converter" project (SILICON CHIP, March 2004) can be pressed into service. With a little trimming, the converter PC board will fit neatly into the space of two "AA" cells in a 4-cell battery holder.

The accompanying diagram gives the trimming details for each end of the board, as well as the modified component overlay. The modifications involve deleting two diodes (D1 & D2), the 270Ω resistor (R4) and a 100μF capacitor, and relocating three capacitors.

The 100nF output filter capacitor must be moved down below the trim line, and the 220μF and 470μF capacitors removed and replaced with smaller 10V units. Use low-ESR capacitors for the replacements (eg, Jaycar RE-6300 & RE-6302) and mount them horizontally rather than vertically. Note that several new holes will need to be drilled, including two for the 5V output leads.

The battery holder is modified by removing the cell separator furthest from the switch and the unneeded battery contacts. The single battery contact with the red wire is then relocated to provide the positive contact for the batteries.

A couple of plastic nuts can be glued underneath the board to act as stand-offs. The board can be held in place with a small piece of foam sandwiched between the board and the lid. Alternatively, use a couple of blobs of hot-melt glue.

The major parts required are the DC-DC Converter kit (stocked by most kit suppliers), a switched battery holder (Jaycar PH-9282) and a DC plug for the USB card.

The above idea would also prove useful in PICAXE and similar low-power microcontroller applications where a portable, regulated +5V supply is needed.

Barry Hubble,
Moulden, NT.
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