One-component Metal Detector 2

Published:2011/8/8 4:13:00 Author:Amy From:SeekIC

Rev. Thomas Scarborough

Even if you have made something yourself you often want it to have a professional look. Also, if you know space constraints well before starting out on a project, it is perfectly possible to achieve nice looking results.

Size of a finished project often equates to ’as small as possible’ so in many cases it is sufficient to make sure a circuit board takes up as little space as possible. SMD parts are then a logic choice but unfortunately not always. For example, devices may be available in conventional leaded cases only. Even more problematic are relatively expen­sive ICs like microcontrollers. To prevent damage during solder­ing, these precarious parts are preferably filled in an IC socket, which will also prove invaluable when the IC needs to be replaced or programmed. Still, conven­tional IC sockets increase the height of the finished PCB and are therefore omitted in many cases.

Fortunately there’s a relatively sim­ple solution to this dilemma as with a bit of skill you can make an IC socket that adds very little height indeed. The DIY socket employs individual receptacles supplied on a carrier strip, for example order code 739049 from Conrad Electronics. These are not the same as the SIL socket strips you can buy to make your own IC sockets. The device used here consists of a carrier with pins onto which round receptacles are secured. The carrier only serves to keep the receptacles aligned with the grid for IC pins. The carrier is cut to size and the receptacles pushed into the PCB holes before they are soldered. Next, the carrier can be pulled off laving he receptacles firmly seated in the board and acting like a very low profile IC socket. The empty strips are best dis­carded straight away as they can not be used to make further IC sockets. As we learned the hard way, ICs will not fit...

Line-2-mic for notebook

Modern laptop and notebook PCs almost invariably come with a microphone input but not a line input for audio signals. A reader asked us for a simple circuit that world enable him to connect an audio line signal to the micro­phone input. We do like the easy ones! Because the microphone input is usually monaural, a stereo line signal requires not only attenuating but also summing to turn stereo into mono. A voltage divider comprising just three resis­tors (R3, R4, R5) does the job.

A couple of measurements on a Creative SB Live! Soundcard showed that the microphone input can handle input signals up to about 15 mV when the ’Mic-boost’ function was enabled. Lev­els exceeding 15 mV or so cause clipping. Based on this research we can only assume that sound-cards in laptops respond in a sim­ilar way.

The calculation of the voltage divider is based on a line level of 2 x 2 V (as, for instance, from a CD player). R5 takes a value of 2.2 kΩfor a reasonable micro­phone impedance simulation. This results in an associated series resistor of 680 kΩ which even affords some headroom to cope with slightly higher line levels.

Two capacitors, CI and C2, have been added to the circuit to afford decoupling of any bios voltages that may be supplied by the mic input for condenser microphones, a well as to prevent plops v/hen the plug is inserted. Rl and R2 serve to keep the capacitors dis­charged as well as to lower the input impedance to the standard value of about 47 kΩ.

For the inputs you may apply cinch sockets or a 3.5-mm stereo jack. If the microphone input on your PC is considerably less sensi­tive without Mic-boost of about 20 dB, (i.e., 10 times less) then the resistor values may be adapted. R3 and R4 take 10 times smaller values while the opposite applies to CI and C2. To maintain a nominal input impedance of 47 kΩ Rl and R2 then need to be increased to 150kΩ.

This circuit is of course also per­fect if you want to add an extra line input to a soundcard in a desktop PC.

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