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Got a technical problem? Can't understand a piece of jargon or some technical principle? Drop us a line and we'll answer your question. Write to: Ask Silicon Chip, PO Box 139, Collaroy Beach, NSW 2097 or send an email to silicon@siliconchip.com.au

Courtesy light circuit cycling on and off

I recently assembled an interior light delay kit (SILICON CHIP, June 2004). I wired it up with a test light, switch and 12V power supply on my bench before I tried installing it into my vehicle.

The problem I am having is that after dimming the light turns back on, then times out, then dims again etc. All the polarised components are installed correctly and I have used the proper capacitors for 12V operation. Reversing wires to the switch makes no difference. Any suggestions? (D. S., Calgary, Canada).

• For the circuit to drive the lamp to cycle on and dim and then back on again, capacitor C1 must be being discharged each time. That normally only occurs if the door switch closes again or if 12V is removed.

Alternatively, there may be a problem when using a test light to check the circuit operation. That’s because the test lamp may not have a sufficiently low resistance which might be because it consists of a LED and series resistor instead of a light bulb.

The circuit does rely on power being supplied via the cold filament. So to test operation correctly, you may need to connect a 100Ω 5W resistor in parallel with your test light. This resistor would also be required if the courtesy lights in the vehicle are LEDs instead of incandescent lamps.

Using the SportSync as an audio delay line

I was wondering if the SportSync Delay for Digital TV (SILICON CHIP, May 2011) could be modified to work as an audio delay line or a voice pitch shifter?

I’m an analog man myself and do not know much about digital circuitry. But based on Figs.1 & 2 in the May 2011 article, I wonder if this could be done in a future article of SILICON CHIP? (K. S., Scranton PA, USA).

• Yes, the SportSync project could be used as a generic audio delay line as long as 40kHz, 12-bit mono quality is good enough. The sample rate could be increased but the hardware isn’t present for a second channel and the micro’s internal ADC/DACs limit the voltage resolution. While the voltage resolution is nominally 12 bits, when you take noise into account the performance is more like 11 bits.

The software could be modified to provide smaller delay adjustment steps and better repeatability as well as a shorter minimum delay setting.

For use as a voice pitch shifter, with different software it could be configured to perform any DSP task as long as 40MIPS is enough CPU power. The same sound quality proviso as stated above would apply.

What causes chip failure on credit cards

I received my first credit card with an embedded chip around two years ago. In that time the chip has failed three times and I have been sent a replacement card each tim.

I have used cards with the normal magnetic strip for over 30 years and have never had a failure; they are replaced every three years or so at the expiry date. The card is always stored in my leather wallet and the copper colouring of the embedded chip does show some signs of wear as it turns to a silvery colour but looks in good condition.

Considering the amount of information that can be stored digitally in the chip it seems ludicrous that they should fail so easily. Can you enlighten me as to why I am experiencing these problems as no amount of questions directed at the card providers gives me any plausible explanations? (M. T., Donvale, Vic).

• We really have no idea as to why these credit cards fail but we would be prepared to bet that it is due to a failure of the encapsulation protecting the chip.

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