Using Peltier Effect cooling in a PC

This may be completely insane but I have an idea for a project. In the March 2003 issue, you used a Peltier Effect device for a "tinnie cooler". Then in April 2003 someone designed a "one-off" silent PC with no fans.

Why not take the CPU fan out of your PC and bolt on a Peltier device? There might even be sufficient spare capacity on the normal +12V rail to run the Peltier device. Is it worth doing the sums about thermal output, heat load, dissipation limits or is this just another stupid "perpetual motion" idea; ie, the heat from the power supply is greater than what the Peltier can "soak up"?

Still, the Peltier device may act as an efficient "heat pump" to move the high intensity hot spot CPU heat to a more diffuse easier to dissipate power supply? Any thoughts?(C. B., via email).

Your idea is a tempting one if you're particularly plagued by the noise of these high speed fans on modern processors. However, you would then need a fan to cool the Peltier device and so the net result is that you would not really be any better off, even supposing that the power supply has a spare 50W available.

Valve preamp for stereo system

I read with interest about the mono preamp using the 12AX7 valve in the November 2003 issue. I wanted to use this as a preamp front end for my home stereo which has two old Grant monoblock valve power amplifiers. However, there is no volume control specified in your preamp.

Is there an easy way of incorporating a volume control (preferably ganged for two preamps for stereo operation) into the circuit? I don't need tone controls or other switching and I will mainly drive the system with a CD, so an RIAA preamp is not required. Mind you, a highly specified RIAA circuit would make a good project if you haven't already run one in your magazine.(K. C., via email).

If your monoblock amplifiers have an input sensitivity of 1V or better, there is no need at all for the valve preamplifier. Just use a 10kΩ (log) volume control on the output of the CD player. The valve preamp is primarily intended for use with musical instruments which have low output signals.

Rain gauge modifications

I'm going to build the Electronic Rain Gauge described in the June 2000 issue of SILICON CHIP but would like to measure the rain in inches, with a resolution of 0.1-inch. I'm sure I can modify the tipping bucket to measure 0.1-inch but I can't figure out how to modify the software counter. Can you help me with this?

The average annual rainfall in my area is around 13 inches per year and very rarely do we get more than 25mm or one inch per day, so I don't really need to be able to record 250mm of rain at a time.(B. B., Moonta Bay, SA)

You do not need to alter the software as the unit only counts each time the bucket tips. So if the bucket tips when it fills with 0.1-inches of rain rather than 1mm of rain, then the reading will be in 0.1-inch. So 100 on the display will be 10 inches. If you want the decimal point to show, connect a 150Ω resistor from pin 5 of DISP2 to ground.

The calibration will require some 2.54 times the amount of water in order to tip the bucket.

Heart rate monitor queries

I am having trouble getting the Heart Rate Monitor (SILICON CHIP, November 2001) to work. I originally thought that the problem was due to insufficient signal reaching the processor board but on looking at the circuit more closely I have found mistakes in the wiring diagram or schematic diagrams. I can't be sure which is correct and which is not.

VR1 is supposed to be connected to pin 3 of IC2c but is actually connected to pin 2. The parallel combination of the 1MΩ resistor and the .033μF capacitor is supposed to be connected across pins 1 & 3 of IC2 but actually is connected between pins 1 & 2. The negative leg of the 10μF electrolytic capacitor next to the 10kΩ resistor is connected to the 4.5V rail. Surely the positive leg should be connected to the 4.5V rail?

I have modified the circuit to match the schematic diagram but the circuit still doesn't work. Maybe I have got something else wrong but I can't see the problem.

Am I correct in what I have just detailed? If so, has the PC board been corrected?(N. P., via email).

Pins 2 & 3 of IC2 are transposed although the PC board overlay is correct. The 47μF capacitor is correctly shown. The most likely problem would be the infrared detector and emitter setup. Check the connections and make sure the construction of the finger pickup is the same as that described in the article.

LED torch without inverter The LED torch in the November 2003 issue looks like a very exciting project to build. I would like to know if it is possible to run the Luxeon LED from three D cells (4.5V) to eliminate the step-up DC-DC converter? (C. N., via email). The LED can be driven from a 4.5V source, although a 3.3Ω 1W series resistor would be required to limit the LED current to a safe value. However, the LED brightness would vary considerably with battery voltage. It would run at full brightness (1W) when the batteries were fresh (4.5V) but would be pretty useless as the battery voltage dropped below 3.5V (ie, 1.17V per cell). Overall, the performance would be poor.

Query on DC-DC inverter

With reference to the valve preamp in the November issue, I would like you to answer a couple of questions regarding the power supply. I am unsure what the transformer is doing in the circuit. I know what a transformer does but why are the coils connected in series? Could you use two inductors in series? If I was to wind the coils using wire that was larger than the wire specified but with the same amount of turns, what effect would this place on the circuit? What would altering the size of the ferrite E cores do to the output? (A. G., via email).

The two coils in T1 are in series but they are on the same core so they constitute an auto-transformer to step up the input voltage. So you can't just use two separate inductors.

There is little point in using thicker wire to wind the inverter transformer since the output current is quite low. The inverter core could probably be reduced in size but then the PC board would have to be changed.

Programming PICs with analog inputs

I am interested in the Parallel Port PIC Programmer and Checkerboard described in the March 2001 issue and I am wondering if it is capable of programming and testing PICs with analog inputs like the 16F628A? If not, is there a circuit or kit that you recommend? (A. M., via email).

Although the PIC Programmer & Checkerboard was not intended for use with the F627/8, it can be used with these new pin-compatible devices with a small modification.

You'll need to install a resistor between pin 10 of the PIC socket (IC2) and ground. The purpose of this resistor is to ensure that the RB4/PGM pin is at a logic low level during programming, so preventing inadvertent selection of the F627/628 LVP (Low Voltage Programming) mode.

Choose a value of about 100kΩ so that it doesn't interfere too much with the 10kΩ pullup resistor. Also, make sure that DIPSW6 pole 5 is open during programming.

Although there is no direct support for testing analog circuitry on the board, each PIC pin is accessible via header pins. It shouldn't be too difficult to hook up your own circuits to these pins for prototyping.

Make sure that you've read the "Updating the PIC Programmer & Checkerboard" article on page 79 of the July 2003 edition.

PIR sensor for flexible keypad alarm

I have just built the Flexible Keypad Alarm featured in the April 2003 issue and it tests out OK but I am a little unsure of how exactly to connect it up to my PIR detector. It is a normally-open configuration but when I connect it up not a lot happens. I am sure that I have done something incorrect as the test procedure yields all the required results. (N. P., via email).

PIR sensors have relay contacts which can be either normally open (NO) or normally closed (NC) or a combination of both. Check the PIR operation and the closing or opening of contacts with a multimeter set to measure ohms. Closed contacts will show zero or low ohms and open contacts will show open circuit or high ohms.

The contacts then connect to the keypad alarm at the instant or delayed input and to the common or ground supply for the keypad alarm.

12V DC motor for Linn Sondek turntable

I have a Linn Sondek LP12 turntable and I read that the stock 2-pole 240VAC motor is not much good. I figure that a 12V battery powered DC motor and controller should do the trick but I am at a loss to find a suitable motor and speed controller. Any advice would be greatly appreciated.

The more I read about AC motors and 240VAC to DC controllers for turntable motors the more I believe that a good old car battery will do a better job. If you so advise me, I will convince my partner that a car battery in the lounge room is acceptable! (N. M., Albury, NSW).

Using a DC motor with a speed control can be a problem when used with turntables since hash from the brush motor can induce noise into the pickup leads. The PWM switching used on most speed controllers may also cause interference. Furthermore, if you want to precisely set the turntable speed to 33.3RPM, the speed controller really needs to have tachometric feedback from a winding on the motor.

We have not described a speed controller with tachometric feedback but if you want to try a simple controller, have a look at the Mini Drill Speed Controller described in the January 1994 issue.

Just running the motor from a 12V battery is very hit and miss and car batteries in lounge rooms are a definite hazard. If the 240V motor in your turntable still works, there is no good reason to swap it.

LED indicators for Sunset Switch

I have built the Sunset Switch which appeared in the June 2003 issue and it is working perfectly. However, can LEDs be added to indicate the timing set for the switch; eg, 15 minutes, one hour, two hours, etc? Is it also possible to reduce the timing by changing a resistor value?(H. D., Mumbai, India).

There is no easy way to add LED indicators to show the timing selected unless a 2-pole rotary switch is used to perform the switching instead of the DIP switch. One pole would set the time function and the second would switch the LEDs.

The timing can be altered by changing the 10mF capacitor on pin 9. A smaller value reduces the time.

SC480 Amplifier Blows Fuses I built the SC480 amplifier from a Jaycar kit; my first ever effort. It was working great and the voltage readings were fine too. After a couple of hours testing it blew an M205 3A fuse. I don't know if that means anything but I am getting another fuse. I will buy another kit probably. I have learnt a lot from what I did with the present kit and eventually I might find the present problem. I thought the instructions in the articles for the SC480 were very comprehensive and good for someone like me who has had no experience building kits.(J. K., via email). Commonly, this sort of problem is involved with Q7 and VR1. If Q7 goes open circuit or is not biased properly via VR1 and the associated resistors, the current through the output transistors goes sky high and blows the fuses. You can check this diagnosis by re-installing the 560W 5W resistors and then using a short clip lead to short between the collector and emitter of Q7. If this brings the quiescent current down to zero, you've found the problem. That done, carefully check your soldering around this part of the circuit and only after you've done that should you consider replacing parts.

How a Vbe multiplier works

I have recently built the SC480 amplifier (January & February 2003) and would dearly like to know how the Vbe multiplier is doing what it is doing.

Could you please tell me to where I might find in-depth information on how it is able to multiply the Vbe voltage by the ratio of the resistors in parallel with the transistor? (D. B., via email).

The principle of the Vbe multiplier is quite simple. The current through the base and emitter resistors is made large enough to swamp the effects of the transistor's base current. Therefore, if the transistor is to turn on, it must have (say) 0.6V across the base-emitter resistor and for this to happen it must also have a proportional voltage across the collector-base resistor.

In this way, the transistor maintains a constant current through the collector-base and base-emitter resistors and therefore maintains a constant voltage between collector and emitter.

In a typical setting (for Q7 in the SC480), if VR1 is set to 100 ohms, the voltage between collector and emitter will be Vce = Vbe x (470 + 100 + 100)/(100 + 100) = (0.6 x 670)/200 = 2.01V.

In practice, VR1 is adjusted not to produce a particular voltage across Q7 but to produce the required quiescent current through the output transistors.

Remote control extender for VCRs

I am interested in building the Remote Control Extender For VCRs described in the July 1996 issue. I'm not sure if you can help me but would you know if it is possible to take out the infrared transmitting LED in the kit and replace it with 10 IR transmitting LEDs.

I would be placing each of them on separate wires and running them individually to each component in my home theatre system. This is necessary because I have all of my equipment in a cupboard and while the door is closed the transmitter cannot reach each unit.(C. R., via email).

You can drive at least three IR LEDs provided that a separate 220Ω resistor is connected in series with each LED and the original 220W resistor at the collector of Q1 is shorted.

For more LEDs, you can duplicate the circuit using another transistor driven via its own 2.2kΩ resistor from the outputs of IC2a and IC2d. This second transistor can drive another three LEDs as before.

WARNING! SILICON CHIP magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should be considered dangerous or even lethal if not used safely. Readers are warned that high voltage wiring should be carried out according to the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed or injured while working on a project or circuit described in any issue of SILICON CHIP magazine. Devices or circuits described in SILICON CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. SILICON CHIP also disclaims any liability for projects which are used in such a way as to infringe relevant government regulations and by-laws. Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Trade Practices Act 1974 or as subsequently amended and to any governmental regulations which are applicable.

You can buy products mentioned in this article here : KC5346 : KIT - SC480 AMP 50W MONO TO3