Simple infrared remote control extender

This ultra-simple remote control extender is ideal for use with a hidden video recorder. The recorder is a Panasonic NV-SD200 and is used as part of a camera surveillance system. A PICAXE-08-based circuit is used to detect events and control the recorder. It also flashes a LED near the monitor to indicate the number of events since last viewing.

Strangely, the NV-SD200 model refused to work with a number of commercial infrared remote control extenders, hence the need for this design. As a bonus, it uses less power than a traditional extender (no plugpacks) and the remote can still be used in the normal manner.

As shown, an additional 5mm infrared LED is mounted directly in front of the equipment to be controlled. This is cabled back to a convenient location near the monitor and terminated in a 3.5mm plug.

To modify the remote control unit, break the circuit to the anode of the existing infrared LED and wire in a 3.5mm headphone socket. In most cases, the LED will be accessible without dismantling the circuit board. The purpose of the socket is to allow the existing infrared LED to operate normally when the jack is unplugged.

If the socket won’t fit inside the case, then a very short flying lead with a moulded in-line socket can be used instead. By using light-duty figure-eight cable, the transmitting LED could be 30m or more from the hand-held remote control without problems.

Ron Russo, Kirwan, Qld.

Improved stability for Dr Video The Dr Video Mk 2 video stabiliser (SILICON CHIP, June 2004) works well but in my application, there was a problem with the clamping level during blanking. A "kick" in the level across each field was noticed, as well as a slight slope under each video line. It appeared that sampling (via IC2b) was occurring at times in the vertical blanking period when the black level was not "clean". This was fixed by adding an AND gate and op amp for signal buffering, as shown. The extra parts can be mounted on prototyping board and should include a 100nF decoupling capacitor between +5V and ground. Any unused logic and op amp inputs should be tied to ground. Nick Graham, Sydney, NSW.

Cordless drill auto-charger

The ridiculously low price of battery-powered drills means that like myself, many readers will have been tempted to buy one or more for the toolbox. The recently advertised price of $17.99 for some 12V models is less than the retail price of the NiCd batteries alone!

A series resistor is all that’s used to limit output current in the charger base supplied with these models. This works OK if you remember to turn the unit off once the batteries are fully charged (after about five hours) but it’s too easy to forget this important step.

It’s possible to considerably improve the function of the charging circuit for little cost, eliminating the need to switch off the unit and extending battery life. This can be achieved by modifying the original charger base and adding a PICAXE-based control circuit, as shown in the above circuit diagram.

The circuit can be constructed on a small section of prototyping board and installed inside the charger housing. Modifications to the existing circuit are limited to adding a 470mF filter capacitor across the plugpack input rails and breaking the circuit to the negative battery contact and the anode of the LED. Light-duty hook-up wire is then used to connect the two sections together, as shown.

The PICAXE microcontroller (IC1) controls the charging by switching the negative side of the battery pack with a logic-level Mosfet (Q1). It also drives the existing "power" LED to indicate battery status.

An analog-to-digital input (ADC1) of the PICAXE is used to sample the battery voltage via a resistive attenuator and RC filter. The result from the readadc command allows the program to roughly determine the charge state of the pack. A flat battery pack results in the maximum 4-hour charge, whereas intermediate terminal voltages result in either a 1-hour or 2-hour charge.

When a battery pack is inserted, the LED is flashed for the number of hours that it will be charged. Once charging is complete, the Mosfet is pulsed only briefly at several second intervals and the ADC input read.

The result is used to detect when the pack is removed, allowing the software to reset itself for the next charge cycle.
Note: due to space constraints, we’re unable to reproduce the PICAXE BASIC program for the charger here, but it can be downloaded free from our downloads section found on the left menu.

Clive Allan, Glen Waverley, Vic.

RGB-to-component video converter fix

Some set-top boxes have RGB video outputs, whereas wide-screen TVs typically have colour difference (Y, Cr and Cb) inputs. The "RGB-to-Component Video Converter" (SILICONCHIP, October 2004) provides a simple solution to this problem.

I constructed the converter from a kit of parts but could not get it to produce a picture on my TV. Examination of the three colour difference signals with an oscilloscope revealed that none had the necessary sync pulses that should occur during the blanking intervals. This explained the lack of a picture, as some sets present a blank screen if sync is missing.

It was expected that the RGB source would include "sync-on-green", which in the converter circuit would propagate through to the Y (luminance) output for use in the TV. Two popular set-top boxes were tried, but neither provides the sync-on-green function. However, they do have composite video outputs. This little add-on circuit extracts the sync pulses from the composite signal and adds them to the Y output to correct this deficiency.

A fourth RCA input socket can be added to the front panel of the converter to accept the composite signal from the set-top box (or other appliance). The appliance may have a composite output in the form of a separate RCA socket or as part of the SCART connector. Alternatively, the "Y" channel of a Y/C output can be used as the source.

Referring now to the circuit, the composite video signal is first terminated with a 75W resistor and excessive chroma or noise is attenuated with a simple low-pass RC filter, formed by the 560W resistor and 470pF capacitor. The signal is then AC-coupled to the input of an LM1881 sync separator IC.

The separated sync pulses appear on pin 1 of the LM1881, after which they’re inverted by transistor Q1. The result is injected into the Y signal path by feeding it into the input (pin 1) of op amp IC2a on the converter PC board. An 8.2kW series resistor effectively sets the sync level at about 0.3V.

The circuit was built on a small piece of Veroboard (approx. 20 x 40mm) and attached to a vacant area of the PC board with double-sided tape. The project works very well and achieves the desired results, improving resolution and eliminating "crawling" around the edges of high-chroma pictures.

Graham Bowman, Duncraig, WA.

Transistor makes high-power zener High-power zener diodes are expensive and hard to find, particularly above the 10W level. In certain applications, a power transistor can be used as a substitute. The base-emitter junction of an ordinary transistor acts like a zener diode when operated in reverse bias. The actual breakdown voltage varies according to the type of transistor and manufacturer. To obtain a specific reference voltage, a simple test circuit can be constructed as shown in the diagram at right, using a series resistor of about 1kW and a 15V DC supply. Suitably rated silicon diodes can be added in series with the "zener" to increase the overall breakdown voltage, with each diode adding about 0.7V. A Motorola TIP31C transistor was found to have a reverse breakdown of about 9.0V, whereas for a Motorola 2N3055, breakdown was somewhat higher at 11.6V. Editors note: transistors could conceivably make very simple, high-power shunt references. However, unlike zener diodes, their P-N junctions are generally not designed to conduct current "evenly" when in avalanche mode. In other words, it’s impossible to know what the maximum safe level of reverse current would be for a particular device. Michael Ong, City Beach, WA.

Low-voltage remote mains switch

This circuit allows a 240V mains appliance to be controlled remotely via low-voltage cabling and a pushbutton switch.

The mains appliance (in this case, a light bulb) is switched with a suitably-rated relay. All of the electronics is housed in an ABS box located in proximity to the appliance. The pushbutton switch and plugpack are located remotely and can be wired up with 3-core alarm cable or similar. Cable lengths of 20m or more are feasible with this arrangement.

When the switch (S1) is pressed, the input (pin 8) of IC1c is briefly pulled low via the 10mF capacitor, which is initially discharged. The output (pin 10) immediately goes high and this is inverted and fed back to the second input (pin 9) via another gate in the quad NAND package (IC1d). In conjunction with the 1MW resistor and 470nF capacitor, IC1d eliminates the effects of contact "bounce" by ensuring that IC1c’s output remains high for a predetermined period.

The output from IC1c drives the clock input of a 4013 D-type flip-flop (IC2). The flipflop is wired for a "toggle" function by virtue of the Q-bar connection back to the D input. A 2.2MW resistor and 100nF capacitor improve circuit noise immunity. Each time the switch is pressed, the flipflop output (pin 13) toggles, switching the transistor (Q1) and relay on or off.

Note that all mains wiring must be properly installed and completely insulated so that there is no possibility of it contacting the low-voltage side of the circuit.

Bob Hammond, Engadine, NSW.

CONTRIBUTE AND WIN! As you can see, we pay good money for each of the "Circuit Notebook" contributions published in SILICON CHIP. But there’s an even better reason to send in your circuit idea: each month, the best contribution published will win a superb Peak Atlas LCR Meter valued at $195.00. So don’t keep that brilliant circuit secret any more: sketch it out, write a brief description and send it to SILICON CHIP and you could be a winner!