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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

Parts for 1000:1 prescaler

I have a question about your article on the 1000:1 UHF Prescaler For Frequency Counters (SILICON CHIP, October 2006), which was reprinted in the January 2009 issue of Everyday Practical Electronics.

I can’t seem to find the panel switch for power (DPDT toggle, right-angle PC board solder mount with threaded bushing). I’ve seen this kind of switch before but cannot lay my hands on one that matches (4mm lead spacing, etc).

I wonder if you can give me an example of a manufacturer and part number for this switch. I can’t wait to actually assemble this project but I have a strong desire to pre-fit the parts to make sure I have drilled everything right, before I assemble the board!

I need similar advice on sources and part numbers for the SMA connector, though I can probably adapt the board to a different part if need be. (P. S., Washington, DC).

• The mini DPDT PCB-mount switch used in the UHF prescaler is the one still stocked by Australian retail chain Jaycar under their catalog number ST-0365. It is available from them direct via their website at

The reverse PCB-mounting SMA socket used in the same project is also stocked by Jaycar, catalog number PS-0596. This can also be ordered direct from the Jaycar website.

24V version of Digital Lighting Controller

Your Digital Lighting Controller (SILICON CHIP, October & November 2010) is designed for 240VAC operation. What changes would be required to operate the Triacs on 24VAC, as most Christmas lights operate on 24V not 230V. (J. S., Rockingham, Qld).

• We have not tried it but the slave module should be able to switch 24VAC for LEDs. There are some minor disadvantages of doing it this way though.

First, for the same amount of lighting power, the unit will need to switch roughly 10 times as much current. For LEDs, this will probably be OK as LED strings usually don’t require a lot of current. Keep in mind though that the total current for all the LEDs will be flowing through the power input connector.

Another disadvantage is that the voltage loss across each Triac is a more significant proportion of the total. This reduces the overall efficiency.

Practically speaking though, we don’t think either of these problems will prevent you from using the slave module to control 24VAC. You will just need to use different connectors at both ends.

We don’t think the Triac latching current will be an issue but if it is, you can set the master module to drive the Triacs for the full conduction period (this was explained in the article published in November 2010).

Having said all that, we intend publishing a new slave module design to suit LEDs running off 12VDC or 24VDC. This will use Mosfets and will be significantly more efficient for low voltage control than Triacs controlling 24VAC. The project is scheduled for next month (October).

Valve guitar preamplifier

I have built the valve preamplifier featured in the November 2003 issue and I would like to modify it to use as a guitar and mike preamp with a volume control. Could you please advise me on the necessary modifications? (P. C. via email).

• Very few modifications are required for your application. If you want a volume control, this can be fitted in place of the 1MΩ output bleed resistor, with the pot wiper connecting to the output connector.

Note that if your volume control has a lower resistance than 1MΩ, the value of the output coupling capacitor will need to be increased in order to maintain the bass response. For example, if you use a 500kΩ pot, you should ideally connect a second 220nF 630V capacitor in parallel with the existing capacitor.

If you want to increase the preamp gain a little, change one of the 33kΩ feedback resistors to 47kΩ or 68kΩ. Note that doing this will inevitably reduce the frequency response and also increase the noise and distortion to some extent.

Multi-channel mixer wanted

I am enquiring as to whether you have ever published a project to build a stereo audio mixer. I’m looking for a unit that I can use at home and occasionally take out to parties, with inputs for say two laptops, two microphones and two auxiliary sources.

My basic requirements include the following: 6-channel stereo; two balanced/unbalanced mic inputs; four unbalanced laptop/MP3/aux inputs with bass, mid and treble controls and possibly master tone controls; a headphone output; and an amplifier output (possibly balanced).

If you haven’t done one already, would you consider such a machine as a future project? I’d be quite sure that there would be a demand for such a thing. My children love playing around with music at home and one of these would be really useful. (P. W., Auckland, NZ).

• Our last stereo mixer project was in November & December 1996 and had eight input channels mixing down to two. It was very comprehensive but not cheap. While all the parts would still be available, you would have to make your own metalwork.

Upgrading the 20A speed controller

The 12/24VDC 20A Motor Speed Controller described in the June 2011 issue is great. Is there a way I can upgrade this kit to 40A? I do not mind making the unit box bigger or can I just upgrade the Mosfets? (J. N., via email).

• The circuit as designed is rated for up to 20A. To upgrade it to 40A, you would require wider PC tracks for the current carrying conductors and a 40A diode (eg, a 1N6097 50A diode). The fuseholder would also have to be changed to suit 40A fuses.

The two Mosfets should be suitable for 40A but may require better heatsinking or you can add more in parallel, along with extra gate resistors.

In short, the whole PCB would need to be re-designed to do the job properly.

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