The Oscilloscope

The Oscilloscope:
If you do decide that you are going to research new equipment, design and possibly invent new devices, then an oscilloscope is useful.   Let me stress again that this is not an essential item of equipment and most certainly is not needed until you are quite familiar with constructing prototypes.   It is quite easy to misread the settings of an oscilloscope and the methods of operation take some getting used to.   The low-cost book “How to Use Oscilloscopes and Other Test Equipment” by R.A. Penfold, ISBN 0 85934 212 3 might well be helpful when starting to use a ‘scope.

It is possible to get an oscilloscope at reasonable cost by buying second-hand through eBay.   The best scopes are ‘dual trace’ which means that they can display the input waveform and the output waveform on screen at the same time.   This is a very useful feature, but because it is, the scope which have that facility sell at higher prices.   The higher the frequency which the scope can handle, the more useful it is, but again, the higher the selling price.   Not all scopes are supplied with (the essential) ‘test probes’, so it might be necessary to buy them separately if the seller wants to keep his.   Getting the manual for the scope is also a decided plus.   A low cost scope might look like this:

Magnetic Measurement.   People who experiment with permanent magnets, can make use of an instrument which displays the strength of a magnetic field. Professionally made devices to do this tend to be well outside the purchasing power of the average experimenter who will already have spent money on materials for his prototypes. Here is a design for a simple and cheap circuit, powered by four AA dry cell batteries, and utilising a Hall-effect semiconductor as the sensor:

This design uses an OP77GP operational amplifier chip to boost the output signal from the A1302 chip which is a Hall-effect device. The gain of the DC-connected operational amplifier is set by the ratio of the 1K and 1M fixed resistors shown shaded in the circuit diagram, giving a gain of 1,000.

The circuit operation is simple. The six-volt battery charges the 10 microfarad capacitor which helps iron out any supply line fluctuations caused by varying current draw by the circuit. The 10K variable resistor is used to set the output meter display to zero when the Hall-effect device is not near any magnet. The 1K variable resistor is there to make fine tuning adjustments easier.

When the A1302 chip encounters a magnetic field, the voltage on it’s output pin 3 changes. This change is magnified a thousand times by the OP77GP amplifier. It’s output on pin 6 is connected to one side of the display meter and the other side of the meter is connected to point “A”. The voltage on point “A” is about half the battery voltage. It would be exactly half the voltage if the two 4.7K resistors were exactly the same value. This is rather unlikely as there is a manufacturing tolerance, typically around 10% of the nominal value of the resistor. The exact value of the voltage on point “A” is matched by the OP77GP tuning and so the meter reads zero until a magnetic field is encountered. When that happens, the meter deflection is directly proportional to the strength of the magnetic field.

Electronics Tutorial

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