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More current for white LED torch I recently purchased a kit for the 6 white-LED torch (pub­lished May 2001) from Dick Smith Electronics. I notice that the LEDs are rated for a continuous forward current of up to 30mA (and can tolerate spikes of up to 200mA for less than 10 micro­seconds) but that the circuit delivers 18.5mA. You say that in practice the current is slightly higher than this but you don’t specify by how much. Am I likely to destroy the LEDs if I replace the 27Ω resistors in series with the LEDs with 20Ω or 18Ω resis­tors, giving a current of 25mA and 28mA respectively to each LED? I would like to make the torch over them which you have disturbed, allowing corro­sion to proceed. Possible tip: put clear nail varnish over the screws that have been disturbed. Surround sound amplifiers I recently built the surround sound decoder and digital delay board from the May 1999 issue of “Electronics Australia” and would like to build suitable amplifiers for the centre and rear surround channels. The rear channel usually has two speakers in parallel which would result in a 4Ω load. My main amplifier is capable of 50 watts RMS per channel. Could you advise me of the power output required for the rear and centre channels that would be a suitable mix for my current main amplifier? (Y. C., via email). • Have a look at the 50W module described in the March 1994 issue of SILICON CHIP. Kits are available from DSE, Jaycar, etc. We can supply the March 1994 issue for $7.70, including postage. Glue gun temperature control I am using a 10W 240VAC glue gun to make candles and I find it gets far too hot for the wax I am using. What is the best way to control the temperature? (A. Q., Adelaide, SA). www.siliconchip.com.au as bright as possible but I do not want to put the LEDs in jeopardy. (S. H., via email). • We do not recommend driving the LEDs with more current than that afforded by the 27Ω resistors. This is because the turn on voltage for the LED could be lower than the typical 3.5V, causing a higher current flow. The only realistic way to measure and reset the LED current to a higher value is to measure the voltage across each resistor and the replace the resistor with a value which will provide the desired current. Also the inverter driving the LEDs is not suitable for driving all six LEDs at 30mA as it does not have this much power output. MicroZed Computers HAS MADE THESE NAMES FAMOUS IN AUSTRALIA micro Engineering Labs, Inc. NOW WE INTRODUCE • Have a look at the article entitled “Power Control With a Light Dimmer” in the October 1996 issue. Although this article described power control for soldering irons, the idea is equally applicable to glue guns. Leakage on engine immobiliser I am enquiring about the Engine Immobiliser described in the December 1998 issue. The immobiliser board has about 0.4MΩ across collector and ground of Q1 when in the idle state. This is enough to affect the points and not allow starting when the unit is not turned on. Everything works well with the overall kit and all is OK once the standing 12V is removed from the board. Is there a correction for this or do I need to look at my own handiwork? (B. M., via email). • Try shorting the base and emitter of Q1. If this eliminates the 0.4MΩ, it suggests that you may have leakage in the earlier stages or across the PC board itself. Notes & Errata LP Doctor, January 2001: in order to avoid residual mains voltage across the transformer when the power switch is off, the associated .01µF/250VAC capacitor should be reduced to SC .001µF/250VAC. Basic Micro offer ATOM: a Stamp2 lookalike with extended capabilities available in an IC or OEM format. A development board with sockets for IC and OEM version with a breadboard space instead of solder pads. Basic Micro also offer BASIC compilers, programmers and a range of development boards for 18, 28 and 40 pin PIC Chips. MicroZed Computers PO Box 634, Armidale, NSW 2350 (296 North Cooke’s Rd) Tel: (02) 6772 2777 Fax: (02) 6772 8987 Mob: 0438 277 634 http://www.microzed.com.au January 2002  93