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Tuning the ThereminAfter a lot of time and help from some university technicians, I finally got my Theremin (SILICON CHIP, August 2000) working – it’s magnificent! However there are a few queries that I have on modifying the instrument. First, I find it very difficult to tune – as you point out in the instructions, the tuning alters with the lid off the box. It also seems to alter quite dramatically, depending on whether I take a line out to an external amplifier or whether I use the small speaker in the box. Is there a way of adding a switch to turn on/off the internal speaker so that results are a bit more consistent when using the line-out only? Second, is there a way of setting up the tuning so that the usable melodic range is a bit more spread out in space? I find that the most tiny movement towards the antenna, in some registers, can change the pitch hugely, making melody playing a very tricky business! Are there any tricks for setting up the tuning to do this? I also find the tuning (and volume) a bit temperamental. When I
haven’t used it for a few days then switch on, I often find that I need to take
it apart and re-tune it. Is there a way of adding external adjustments for the
T2 and VR2 controls? Tuning is critical and can be affected by external connections and interactions. Note that any nearby metal surfaces can affect tuning. It is recommended to place the unit on a stand to keep the Theremin away from any surfaces that may affect it. The loudspeaker can be switched out using a switch if required. VR2 could be a multiturn unit to improve adjustment control. It is not recommended to have external adjustments. The Theremin does have a wide frequency range and so this makes it difficult to have complete control over the output frequency. Very fine finger movements should be used for small variations rather than hand movements. Proximity sensor for rubbish binCould you please tell me what type of sensor is used in the
Jaycar Intelligent Rubbish Bin and where I could purchase one? This type of
sensor is used in many applications such as hot-air hand-dryers and in some
public places for turning on taps. Maybe you could print a circuit/project in
the magazine, as this type of control could be used in many applications around
the home. There is no sensor as such. The bin probably uses a proximity switch circuit which reacts to body capacitance. A proximity switch circuit was featured in the August 2000 issue.
Stereo version of the Studio 350I have just purchased the Studio 350W power amplifier, as featured in the January & February 2004 issues. I was wondering if it was possible to connect two amplifier kits to the one power supply. Would this be possible without losing any performance from the amplifiers and would the components in the power supply be able to handle the extra current? Also, if this could be done, would the amplifiers be connected
in series or parallel? If you intend using the stereo amplifier in the home, the existing power supply with its 500VA transformer will be quite adequate. The amplifiers would be connected in parallel to the supply. Increasing hysteresis in the CoolmasterI have built the Fridge Temperature Controller from the June 2005 issue, to use as a thermostat in a bar fridge. I am a refrigeration mechanic and while it works well, its differential is too close. It is cutting in at 1.9°C and out at 0.3°C; a difference of only 1.6°C. Thus, the off-time of the compressor is not long enough for the gas to equalise and the compressor overloads every time it tries to start up. Unless one knows how fridges work, this may not be noticed but it doesn’t do the compressor much good; it will run quite hot. I have a data-logger connected to the fridge so I know exactly what’s going on. I tried fitting a large heatsink to the sensor to try to damp the temperature change but this didn’t work. I have now wrapped the sensor in insulation to slow the temperature change. Is there a way to change the value of one of the components to
lengthen the time between cut-in and cut-out? Typically, 5°C would be a good
figure. You should be able to increase the differential between cut-in and cut-out temperatures by increasing the value of the 100Ω resistor to 150Ω. You can also reduce the value of the 33kΩ resistor to 27kΩ. Both of these changes will increase the positive feedback around the comparator and hence increase its hysteresis. Telephone exchange simulator wantedI am trying to find an article or kit on how to hook up two
telephones without hooking them into the Telecom exchange. The kit I have in
mind was powered by an external power supply and used two standard telephones.
Have a look at the Telephone Exchange Simulator featured in the February 1998 issue. Wind the Micromitter coils correctlyI purchased and built the Micro-mitter kit from the December 2002 issue and I am having a couple of problems. First, in the test and adjustment section, it mentions to set the voltage at TP1 to around 2V by adjusting L1. All I seem to get is 0.61V, no matter what adjustment I make. Second, even though I only get 0.61V at TP1, I still manage to
get an output when I connect a stereo signal to the RCA input but the output is
mono not stereo. Can you please help? The problem most people are having with this project is with the adjustment of the PLL voltage for the RF oscillator. The cause is usually that coil L1 is not wound in the correct direction. You must wind it in the direction shown in the photo, with the coil close to the PC board and the turns touching each other. In addition, the bottom of the former should be flush with the bottom of the PC board. If this is not done, the coil’s inductance will be too high.
Humongous DC supply wantedI have a major dumb question for you guys, which with the amount of electrical and electronic work I do, makes me seem like a complete idiot, but here goes. I do a lot of car stereo work, mainly way overpowered systems for DB Drag and SPL competition cars and I need to rig the stands for a few show cars at this year’s December AutoSalon car show in Sydney. As part of this, I am going to need a ridiculous supply of 13.8V DC to run each of these cars. I have access to 3-phase mains and will be using a distribution board to run single phase but I need to be able to build power supplies that can supply roughly 300A or so (that is not a typo) per car! In my limited experience with building power supplies, I am assuming that I should run toroids, bridge rectifiers and smoothing caps and then run a bunch of these in parallel to get the required amount of amps all up. The way I plan to set it up is to run maybe one to four "sets" of toroids and associated gear in a 19-inch rack-mount case and then have a full rack of them. I am also thinking about running those IEC panel sockets that have inbuilt EMI/RFI suppression to give some protection from the noise on the mains line. Once all of these power supplies have been joined in parallel by something like 4-gauge or 2-gauge cable running from case to case in piggyback style, I then intend running roughly five metres of 2-gauge cable to the car. Sitting underneath the car, I plan to have more rack cases with roughly 12 1-farad car audio capacitors and then have that running into the battery bank of each car. I would just like to know what you guys think I should do, how I should wire the individual power supplies, what gear I should use, what voltages the toroids need to be, what voltage and farad ratings I would need to have for the smoothing caps, etc. Currently I’m using the 40A switchmode power supplies from
Jaycar but they just aren’t enough and at roughly $500 each, running enough of
them in parallel to amount to roughly 300A per car would just be utterly
ridiculous price-wise. What you are talking about is a power supply with about 5kVA input, for each car. The only practical way to do this is to use a 3-phase transformer coupled with a big 6-diode bridge rectifier and a large bank of electros. Using 3-phase will give much lower ripple but you will still need a big bank of electros. Having said that, this is not an area where we can help in the design. How long to rejuvenate a batteryI found the Lead-Acid Battery Zapper in the July 2005 issue to
be interesting. One question though: how does one know when the battery is
rejuvenated (after using the project)? Good question. It could take several days or more of zapping to get a result. Then you need to test whether the battery will accept a charge; ie, a charge that takes a reasonable time for the battery voltage to come up to 14V or so. Then you need to check the battery’s capacity by giving it a known load such as a 60W headlamp bulb and checking how long it takes to discharge to 11V or so. As stated in the article, not every battery will be capable of being rejuvenated. Valve RIAA preamplifierI have assembled two of your valve preamps for instruments
(high gain) because I want the preamp for phono gain. They will be driving a
valve power amplifier but I require RIAA compensation. Can you provide a circuit
diagram for what I require and where it goes. I have tried the Internet but have
not found what I need. We do not have an RIAA version of the preamp and we doubt whether this preamp would be quiet enough for a magnetic cartridge. Have a look at our RIAA preamp from the March 2002 issue. It is an op amp design. If you are really set on producing a valve RIAA preamp with 12AX7s, we have gone back through the archives and found a preamp back in the June 1954 issue of Radio & Hobbies. We can supply a photostat copy of this article for $8.80 including postage. Multiple cycles for the FlexitimerCan the Flexitimer featured in the August 2005 issue be made to
run for more than 99 minutes by increasing the number of cycles to run
concurrently? The Flexitimer can effectively be made to run for longer than 99 minutes and 59 seconds by setting the number of cycles to greater than 1 (the default). However this only applies if you have only Time A programmed, not Time B as well. This is because when you only have Time A programmed, extra cycles result in Time A being effectively restarted immediately after its has ended (until the end of the last cycle). So the effective time for Time A becomes the programmed time multiplied by the number of cycles you have programmed. Finally, we recently have modified the software to give the unit a continuous cycling mode – see Notes & Errata this month.
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