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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

Faulty LED indication in Speed Alert

I built the Speed Alert project as published in the November and December 1999 issues of SILICON CHIP. The kit was a dream to build and functions well except that LED1 (visual alarm) stays on permanently.

I have checked all solder joints etc and cannot find any faults. It seems that there is ground potential on pin 6 of the PIC all the time. The audible alarm works every 10s (as it should) but is quite faint. (R. B., via email).

• The alarm LED (LED1) is driven via the 5V dimmed supply at the emitter of Q4 through the 560Ω resistor and to pin 2 of IC1. If pin 2 is low then LED1 will light.

Normally pin 2 is at 5V unless the alarm is on. The only way LED1 could be alight continuously is if there is a short to ground for pin 2 (or a low resistance to ground). Alternatively, check that LED1 is oriented correctly.

9V to 27V DC
converter wanted

I am interested in your DC-DC Converter, as published in the June 2003 issue. Its maximum output is 20V.

My requirement is for a DC-DC converter which has a 9V input and a 27V output. Do you have a suitable circuit? (R. R., Rowville, Vic).

• The DC-DC Converter can be altered to deliver 27V by changing the 1.2kΩ resistor in series with trimpot VR1 to 470Ω. Note that the output current would be limited to about 450mA.

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