Silicon Chip Online - Ask Silicon Chip

SCART connectors and A/V inputs

I recently bought my young son his own TV. This fits into his entertainment unit to stop him messing around with the cables at the rear.

The problem is that this set does not have audio/video inputs. It does have a SCART connector and I have been told that if I use an adaptor, the TV will switch itself onto the appropriate A/V channel when it receives a video signal.

I tried this on all three A/V inputs but to no avail. I have also been told that I need to send one of the SCART connector inputs high with a +5V feed but no-one will tell me which one and I am not about to guess. Can you tell me which cable in the SCART connector needs to be pulled high to get the thing to switch to the appropriate A/V input? (D. S., via email).

With some TV sets fitted with SCART sockets for AV input, it is necessary to pull pin 16 of the socket up to +5V via a 220Ω resistor before the set will recognise that video is being fed in. If you are feeding in composite video from a video game console, you may also have to pull pin 8 down to ground via a 10kΩ resistor, to make the set recognise that the video is in composite form.

18V version of charger circuit

I found an article on a cordless drill auto-charger in "Circuit Notebook" of your July 2005 edition. I would like to build the circuit but my cordless drill is 18V with a plugpack of 24V @ 400mA. Can the 12V circuit be adapted to cater for an 18V charger? (F. N., via email).

Yes, the circuit could easily be adapted for use with an 18V charger. To do this, upgrade the 470μF 25V filter capacitor to a 35V rating. Also, swap the 1kΩ current limiting resistor in series with LED1 to 2.2kΩ.

The division ratio of the resistive string connected to the A/D input of the PICAXE will also need to be altered. Try replacing the paralleled 15kΩ and 82kΩ resistors with a single 30kΩ resistor. This larger value ensures that the input voltage rating of the PICAXE port pin is not exceeded in normal use.

Some minor changes must also be made to the BASIC program to reflect the changes to the voltage divider and battery pack. This involves changing the threshold values described in the comments within the BASIC program.

Note that the above changes assume that the open-circuit plugpack voltage doesn’t exceed 35V, as this is the maximum rating input of the 7805 regulator.

Basketball Scoreboard display problem

I have recently completed the construction of the Jaycar Basketball Scoreboard Kit and there are a few questions that I would like to pose. First, there is flicker on the scoreboard LEDs. Is this due to the multiplexing at 40Hz?

There is also a problem when hooking up the second Fouls board. With only Fouls board1 connected, the number of fouls increment singularly ie, 1, 2, 3, etc. With Fouls board1 and Fouls board2 connected, the Fouls board2 display increments both digits at the same time, ie, 11, 22, 33, 44, etc while Fouls board1 operates normally. With Fouls board2 connected alone, it operates normally. Any suggestions on this problem? (M. S., via email).

The small amount of flickering on the Scoreboard displays is due to the 40Hz multiplexing. This was done deliberately, as it increases the apparent brightness of the displays.

It’s not easy to suggest what might be causing your strange fault with digits responding together on Foul board 2 but only when both foul display boards are connected. It sounds like some sort of short-circuit or poor connection on one of the 16-way IDC cable connectors, or a dry joint or short in the digit driver section of the Fouls display board.

Apparently, it’s a fault that is causing both digits of the Fouls 2 board to be gated on together, rather than separately (as they should be), so you should look for anything that can cause these digits to be gated on at the same time. This means looking for faults on the Fouls 2 display board itself, on the 16-way cable and its connectors, or at the 26-way connector on the controller board.

Earthing a turntable

I am restoring an old turntable which has a contact switch linked to the tonearm cradle. When the arm is removed from its cradle, the contacts are closed, sending 240VAC to the motor. My problem is that this causes a loud thump through the audio output. I understand that suppressor capacitors may solve this problem but what values should I use and where should I place them?

The power cord into the unit has no Earth lead so there’s no direct earth connection from the player, although the unit has a Ground connection that can be connected to the dedicated GND connection on the back of the amplifier. I’m fairly sure that the player’s audio circuitry is isolated on the chassis from the power circuit. Could it be causing the problem via the amplifier instead? (P. S., Lane Cove, NSW).

Use a .01μF or .022μF 250VAC metallised polypropylene capacitor across the switch contacts.

The turntable chassis should be earthed, ideally via its own 3-core power flex but you need to make sure that the cartridge output leads are fully isolated from the mains earth otherwise you will get severe hum from the resulting earth loop.

Balancing The Unbalanced

Most of SILICON CHIP’s audio projects are designed for hifi systems and hence are "unbalanced" (ie, signal and shield). By contrast, most professional PA systems use balanced lines (signal+, signal-shield) because of their superior noise rejection, especially for long cable runs.

I am wondering if there is any way to convert balanced signals to unbalanced signals to use on projects such as the subwoofer crossover (EA, September 1994) or for the many amplifiers recently published? Would this be possible with a transformer or a differential op amp, or is there a better way?

If you could convert a balanced signal to unbalanced, it would be possible to take advantage of the noise rejection of a balanced line for a long cable run and provide an unbalanced input to take advantage of the cheap and useful projects available. (A. W., via email).

We published a PC board with balanced and unbalanced stages in the December 1989 issue. These stages used LM833 op amps and were originally intended for the Studio Series third-octave and half octave graphic equalisers published in the same year. They can be used wherever you need balanced-to-unbalanced or unbalanced-to-balanc-ed conversions.

The PC board is coded 01112891 and can be obtained from RCS Radio Pty Ltd. Phone (02) 9738 0330.

Demister car radio antenna

Does SILICON CHIP have a project for an antenna amplifier for a car radio? I’ve managed to zap the amplifier in my car’s system and thought making one would be pretty cheap compared to spare part prices.

My car is a Nissan 200SX and the amplifier sits above the rear passenger’s head, close to the aerial which is in the rear window. (K. M., Bathurst, NSW).

We published a rear window demister car radio antenna adaptor in the December 1988 issue of SILICON CHIP. This is not an amplifier but an interface circuit to allow the demister array to pick up RF signal and direct it to the car radio.

Ballast required for UV steriliser

I run a business in water filtration and have bought a UV steriliser light from an importer. While the light fitting is good, the ballast transformer is rubbish and actually smokes and gets very hot.

Inside the plastic box, it appears to be a transformer. The mains Active connects to one side of the transformer and the other side goes to the light fitting. The Neutral is connected directly to the other side of the light fitting.

The label on the ballast reads: UV Lamp Ballast; Input AC 240V/50Hz; Amps 147mA; Output AC 44V, 6 watts. The label on the light reads: UV output 253.7nm; Lamp current 0.162A, 6 watts.

I would like to use something smaller, electronic (switchmode?), encapsulated, that can be connected inside a junction box (not running hot). As it is used in hire equipment, and has to run continuously all year, approval for use in Australia would be good. (N. J., Mackay, Qld).

The ballast is not a transformer; it is a choke (inductor) and only has two terminals, as you have noted. There is no electronic equivalent that we know of but it might be possible to substitute a fluorescent light ballast (say an 18W unit), as these are designed for continuous use. You would need to make sure you obtain the correct voltage across the lamp, otherwise it will be damaged.

Bridging the Mighty Midget amplifier module

Is the Mighty Midget amplifier module (March 2002) bridgeable? The article talks about the bridging of the internal power device but there is nothing on whether two modules can be bridged. (T. O., via email).

Since the two outputs of the TDA-1562Q are themselves bridged and both swing in anti-phase with each other, it’s not possible to bridge to another TDA1562Q. If you did connect one speaker between two modules, the remaining two outputs would be swinging in "mid-air" doing nothing?

Using Luxeons in the Spacewriter

Could you suggest modifications to the Spacewriter described in your May 1996 issue to allow it to drive seven 1W Luxeon LEDs? (A. C., via email).

The Spacewriter could drive 1W LEDs, with the following modifications:

(1). Change the power supply regulator to a 7805, fitted with a heatsink.

(2). Change the 9V battery to six C or D cells (in order to supply 1A).

(3). Remove the 10Ω series resistor from the 5V supply to the LEDs and change each 15Ω LED series resistor to 4.7Ω 1W.

(4). Change transistors Q1-Q7 to logic level Mosfets (eg, ZT-2271 from Jaycar; STP30NE06L)

(5). Increase the 10μF supply decoupling capacitor for the LED supply to 1000μF 16V.

(6). Thicken the relevant PC tracks with solder to cope with the higher LED currents.

Courtesy light delays immobiliser

I recently purchased the Courtesy Light Delay kit (described in the June 2004 issue) and fitted it to my 2005 Nissan Patrol. It works very well although I did modify the installation by connecting the bridge rectifier to the accessories switch rather than to the parking lights. This means that the lights go out instantly when you start the car rather than having to switch on the lights.

The issue I now have is that the remote central locking system/immobiliser monitors the courtesy light circuit and if it detects that one of the doors is open, it will not arm the system. The courtesy light kit puts a voltage back into the courtesy light circuit which is interpreted by the immobiliser that one of the doors is open. So I have to wait for the lights to go out before it will lock the doors, which means that I have to stand next to the car or lock it manually.

Do you have a suggestions as to how the overcome this situation as I quite enjoy having a delay on the interior lights? (A. C., via email).

You could use a separate door switch for the immobiliser "door closed" detector or set the courtesy delay to a shorter time so you do not need to wait so long before leaving the car.

Jacob’s Ladder cannot climb

I built the Jacob’s Ladder project from the September 1995 issue and have a few questions about it. I had it running on a power supply and everything was fine until someone "juiced" it up way over 12V. The 10Ω resistor got a bit black and the thing still works but it’s not going up the ladder much now; it tries but it mostly burns out at the start.

Might the 555 timer chip be dead? I can’t see anything else on the board that looks damaged. (M. S., Penrith, NSW).

The 10Ω resistor was burnt because it would have had to supply excess current into the 16V zener diode (ZD4) when the supply voltage was increased. This resistor should be replaced, along with ZD4. It is also possible that the 75V zener diodes have been damaged and they are now reducing the maximum coil voltage. The remaining parts should be OK.

Smoking an Output Filter At 80kHz

I recently built the SC480 power amplifier, as featured in the January & February 2003 issues of SILICON CHIP, and have been testing it on the bench. I am viewing the results on an isolated scope and using a function generator for the input source at 1.6V amplitude (3.2V peak-to-peak) and all is well up to 60kHz input.

However, when I increase the input frequency to about 80kHz, the 6.8Ω 1W output resistor starts to smoke! Is this a result of the load being resistive and not running it through a speaker system or something else? Will using a 2W resistor resolve this issue?

Second question: why doesn’t SILICON CHIP use Australian Standards for schematic symbols (ie, resistors, capacitors, fuses, etc)?
(S. S., via email).

We are not surprised that 6.8Ω resistor is smoking. The amplifier was never intended to be tested at full power at such a high frequency.

If you have a look at the output filter and consider its behaviour at very high frequencies, it is no wonder the 6.8Ω resistor is smoking. At 80kHz, the 150nF capacitor has an impedance of 13Ω while the 6.8μH choke now has an impedance just over 3.4Ω.

So instead of the 6.8Ω resistor being effectively short-circuited by the 6.8μH inductor (as it is at normal audio frequencies), it is being subject to about 30% of the amplifier’s output signal. At full power, this amounts to about four or five watts across a resistor with a 1W rating. Even going to a 2W resistor would not improve matters much.

In fact, none of our amplifiers are intended to deliver their rated power above 20kHz, even though their frequency response at 1W may be 60kHz or more. Testing at high power at such high frequencies will not only burn out the 6.8Ω resistor but may also cause the output transistors and their drivers to overheat. The output filter would have to be changed and so would the output stages and the heatsinking, in order to make the amplifier reliably deliver high power at 50kHz or more.

Unless you have a specific need to operate the amplifier supersonically, we recommend you don’t do it.

We don’t use Australian Standards drawing symbols because as far as we are concerned, they are not as easy to follow as our own symbol library. Our drawing symbols are also more in line with those used on the circuits of most consumer electronics equipment.

Inverter for car fridge

I would like to know if the output of the CFL inverter described in the September 2004 issue can be reduced to 240V DC. The reason for this is that I would like to see if I could use it for a fridge in my camper, mainly while driving long distances.

The fridge does not hum so this tells me it’s a heater (evaporative) type and 240V DC would not affect the circuit in the fridge. I can’t quote the output power of the fridge to see if the inverter can handle the load, because it has no plate that I can see.

Also, how much load will this have on the car battery if it is continuously being charged by the alternator while driving. (M. B., via email).

You can reduce the output to 240V by increasing the 6.8kΩ resistor to 11kΩ. We don’t know how much your fridge would draw at 240V but would guess between 50W and 100W. This would not be a problem when your car is being driven.

Battery indicator for R/C aircraft

We have a problem in finding a low voltage indicator circuit for flying radio-controlled aircraft. We need a device in the aircraft to indicate that the onboard battery is at full charge (5.6V) and when the voltage falls to about 5.2V. Below 5.2V, the battery can sustain flight but not the operator’s ability to control the craft, even at fairly close range. As the aircraft can cost anything up to $1500, we do not want to see it nose-dive or fly into the sunset! (J. G., Cudal, NSW).

We have published two low battery indicators in Circuit Notebook, in June and November 2003. The June 2003 circuit may be more relevant.

Electric brake control for a caravan

I am looking for an electric brake controller circuit for a small caravan. I built a similar circuit for a model train controller for my kids a few years ago and it worked very well but unfortunately, I have lost the circuit. It used a variable pulse width control with a 555 IC and a diode (for 360° control). I completed much of the control box a few years ago, with a pendulum and magnet sweeping over 10 reed switches, and mounted a 2N3055 on the diecast box to control the 6A @ 12V the brakes draw. (N. W., via email).

Check out the pulse width modulation generator in the Duty Cycle Meter project as published in our "Performance Electronics for Cars" handbook. A very similar PWM controller is also included in the Nitrous Fuel Controller in the same book. This last-mentioned project includes a Mosfet to drive a solenoid load and may be ideal for the brake control.

The PWM adjustment is made with a potentiometer that could be used to swing in response to braking. A kit for the Nitrous Fuel Controller is available from Jaycar.

Accuracy of speed sensors

How accurate are the speed sensors on 2000 Detroit Diesel in large trucks? (T. S., Troutville, VA, USA).

We assume you are talking about road speed. It would be better than 5% but it will depend on the rolling tyre diameter and this will decrease significantly with tyre wear. Speedo accuracy is typically much worse, usually high by 10% or more.

Fuel Mixture Kit Displays Awry

I put the Fuel Mixture kit together and found it does not function. The directions state to check the voltage on pins 4 & 14 of IC1 for 5V before installing the chip. This was present. There was 5V present on another two or three pins as well.

I put the two circuit boards together and applied power and grounded the sensor wire. This produced a momentary reading of "14." on the two left LEDs. After a second, the reading is a partial 0. I say partial because the centre LED shows an incomplete zero. The right LED (the one that would be tenths of a volt) does not light up and the LED bar does not light up at all.

I have R1 installed for bar mode, and did not install R2 or R3, as this was for unleaded use using A/F ratio.

Last, the brightness of the LEDs does not change when I cover the photocell. I realise this is hard to troubleshoot and there are as many possible problems as there are solder connections. However, if there is something you can suggest, I would be most appreciative.

(J. M., via email).

We suspect that transistors Q2 and Q5 are the incorrect types. Q2 is a BC327 and Q5 a BC337. This would prevent the third display from working and the brightness adjustment from working.

Check the pin connections to DISP2. Perhaps there is a bad solder joint on one of the pins.