Email Address:

Lost your password?

This is the legacy website; please use the new website.

Ask Silicon Chip

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

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.

Share this Article: 

Privacy Policy  |  Advertise  |  Contact Us

Copyright © 1996-2021 Silicon Chip Publications Pty Ltd All Rights Reserved