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Door minder is insensitive I have recently assembled the “Door Minder” kit as pub­lished in the July 1995 issue of SILICON CHIP. I find that it is not very sensitive and it only works in a small room with the windows shut and if I give the door a hefty pull. I would be most grateful if you could tell me how to make it more sensitive; the preset pot is adjusted to the most sensitive it will go. The article says it works with open windows in an adjoining room. Also, where can I purchase the Philips ETD49/25/16 trans­former components and the TEA100 nicad monitor IC for the Fast Nicad Charger, as published in the September 1995 issue? I refer to the letter on page 7 of the April 1995 issue, on making PC boards by photocopying the original back to front and rubbing with thinners mixture on the back. I have had limited success and find it is best if the photocopy is removed whilst still moist. I say moist and not wet, as too much thinners gets underneath and smudges the pattern. When dry, doing it a second time darkens the picture but use a fresh, new photocopy. monitor the output of IC1 to see that it delivers the correct signal from its output in re­sponse to the arm/disarm switch. How to reduce preamp gain I recently put together the preamp section of your 50W amplifier design and find it very satisfactory. My amplifier and speakers are of very high sensitivity so I need much less gain than is provided. Would you please advise the correct way of changing the feedback components around IC1 to achieve closer to unity gain? I prefer to do this rather than use atten­uators. (K. A., Moss Vale, NSW). • The gain of IC1 can be reduced by increasing the 4.7kΩ resistor at pins 6 & 2. To halve the gain, increase the 94  Silicon Chip However, the result produces a porous copper surface which has to be heavily layered with solder. An improvement can be made by going over the whole pattern with a fine felt tip pen which is water resistant. (D. S., Caloundra, Qld). • We are surprised that your Door Minder is so insensitive. You should check that the 0.1µF and 1µF capacitors on IC1a’s input are the correct values and that the 1µF capacitor is inserted with the correct polarity. Also confirm that the two resistors around IC1a are 47kΩ and 3.9MΩ – a wrong value may have been fitted. Also, did you ground the case of the electret insert? Check that the regulated DC is about 8V and the voltage on pin 1 of IC1a is 3.3V. As the circuit has a gain of 80 it only needs 10mV from the microphone to trigger the chimes. It should not be necessary to increase the gain of the circuit, as it is quite adequate. Perhaps the microphone insert is faulty. You can get some extra gain by decreasing the 47kΩ input resistor to 39kΩ but any more variation could alter the passband of the filter. Regarding the parts for the Fast Nicad Charger, these can be purchased from Jaycar Electronics. 4.7kΩ resistor to 10kΩ. To obtain unity gain, omit the 4.7kΩ resistor. Frigid remote control won’t respond I have just built the UHF remote switch from the December 1989 issue of SILICON CHIP. It is operating perfectly from a distance of 10 metres but only when the temperature is over 20°C. Can you advise how to overcome this problem as the temperature in Victoria is generally under 20°C. (P. L., Spring­ vale North, Vic). • Ever thought of moving to warmer climes? It appears as though one or more of the transistors or possibly one of the ICs is temperature sensitive. As a first step, we suggest you check all voltages in the circuit. Second, check all soldering on the PC boards. Cold solder joints can be temper­ature sensitive. Third, with a can of freezer spray, freeze each semiconductor component to see if it causes the problem. Alterna­ tively, replace the transistors one by one to see if you can effect a cure. Extended leads for a digital thermometer Recently, I bought a digital thermometer with indoor/outdoor display from Jaycar (Cat QM-7210). I intended to extend the outdoor twin lead by a further 12 metres to enable the probe to be sited in the foliage of a bushy tree for a genuine outdoor reading. Doing this increased the leads’ total resistance by about 0.3Ω which, in turn, increased the readout figure by 7°C. I tried counteracting this by adding 0.3Ω in parallel with the probe but then the readout for this probe disappears. I cannot see any internal adjustment to compensate for extra lead length. Have any readers had similar experience along these lines? (M. B., Taree, NSW). • We doubt whether the additional resistance in the probe leads has caused the increase in temperature reading. We are more inclined to think that the long leads may be picking up hash which is adding to the reading. Try connect­­ing a 0.1µF green­cap or MKT capacitor across the probe leads where they enter the case. Note & Errata Dolby Pro Logic Surround Sound Decoder, Pt.1, November 1995: the anode of diode D12 is shown incorrectly joined to the junction of the cathode of D14 and an associated 10kΩ resistor. Instead, D14 and the 10kΩ resistor should connect directly to pushbutton switch S7. Dolby Pro Logic Surround Sound Decoder, Pt.2, December 1995: the resistor connected to pin 21 of IC2 is marked “30O” on the layout diagram (Fig.4, p71). The correct value of this resistor is 30Ω. Five-Band Equaliser, December 1995: the supply pins for IC2 on the circuit diagram (Fig.5, p24) are shown reversed. Pin 4 should go to the +15V rail, while pin 11 should go to -15V. The parts layout diagram (Fig.6, p25) SC is correct.