|Circuit components||Building the circuit||August 2005 update||September 2005 update|
The simplest detector that I have been able to design has an LED light beam shining towards a Light Dependent Resistor (LDR). When the seismograph bar moves during an earthquake the amount of light hitting the LDR will alter very slightly. A single "op-amp" circuit with about ten components soldered onto small PC board changes this to a voltage and then amplifies it to give an output that changes as the bar swings. The changing voltage is then sent to a data logger to turn it into a digital signal to input into the computer. My pictures show two possible arrangements for the LDR and LED. One has the LED facing up from the Jiffy Box and the LDR held by the two yellow banana plugs facing downwards towards the LED. Later designs show the LED and LDR sticking out through holes in the Jiffy Box and bent so that they face each other. The electrical circuits however are identical.
A second possible detector system (favoured by most internet designs) uses a very fine coil of wire that generates a few microvolts when a magnet on the seismograph bar "moves" during an earthquake. A series of op-amps amplify the signal and feed it to the data logger. Although these circuits do work very well they are much more complex to build and the complexity might discourage many teachers. If you want to try this I will probably put my circuit on this site later in the year. (The best source of the coils is aquarium air pumps)
Some data logger Physics packages come with a force sensor. It might be possible to use a force sensor in the diagonal wire to measure the change of tension as the vertical motion of the ground that can occur as part of the P and S waves during an earthquake. The force sensors are much less sensitive than the light beam or magnetic detectors and would probably only work in New Zealand, Indonesia or Japan.
Download Circuit diagram in pdf
Electronics enthusiasts can probably build the detector circuit without any plans but the following instructions will make it easier for the average circuit builder.
Detector Circuit Building and Testing.
If you are not an electronics expert we have arranged a step by step, set of instructions to help you successfully build the circuit. If you have made a "Dick Smith Funway 2" kit then you can build this one.
Use a fine clean soldering iron. Wipe it every few minutes on a wet sponge to keep it clean. Use fine resin core solder. Clip excess leads with fine side cutters. If you make a total disaster of one bit of soldering you can simply move the components further along the board and try again. Lie the tip of the hot iron against the board and wire touch the solder onto both the wire and board so that the solder melts on the hot wire and board not just on the hot iron. Good solder joins have smooth, shiny solder that flows along the wire AND the copper tracks on the board. A "cold joint" or "dry joint" occurs if the solder does not melt properly.
Solder in the mounting socket for the op-amp. Pin 1 of the op-amp will be at the lower left hand end of the socket.The pin arrangement is
Since pins 1, 5, and 8 are not being used in this circuit, if your soldering is messy you can leave these pins unsoldered. Remember that from underneath the numbering will be reversed.
If you get solder across several tracks don't worry. Get the spare socket and solder it in to the right of your "practice socket". You will have to lengthen some other connections but the circuit will still work.Solder in the rest of the components;
|Measure the voltage at each of the pin holes in the socket BEFORE you put in the op-amp. The voltages should be roughly|
|Step 4||Step 5|
The low output is taken from between the 3.3k and 1k resistors; it is close to ¼ the high output voltage.
Assemble the circuit board into the jiffy box as shown in other photos. Bend the LDR and LED to face each other. About 1cm gap is perfect. The box will stand upright so the output banana sockets; black from the negative line, lower red from the low voltage output and upper red from the high voltage output, will be in a single vertical line. The solder tags for the banana sockets are large and conduct heat away quickly. Make sure that the solder runs properly onto the tag and does not produce a "cold joint". The variable resistor does not need a knob but looks prettier with one.
Depending on the type of power plug pack socket used you might have to unsolder it and resolder it once it is mounted in the box. You can cheat by cutting a slot into the box sitting the socket in the slot.
|August 2005 Update|
I spent some time trying to adapt the detector output to match the Data Harvest data loggers used by the Environmental Education Centres around NSW so that they could set up and use the seismographs. Unfortunately they did not have voltage sensors, only high level (100 000 lux) light detectors. A super bright LED connected from the high output socket to the low output (NOT to the zero voltage socket) socket changes brightness as the seismograph responds. (The longer leg of the LED connects to the high output voltage socket and the shorter leg, near the shaved off edge of the LED body, connects to the low output socket.) The 1k resistor in the output voltage divider (between the low output and zero voltage sockets) is important because it limits the maximum possible current through the LED. The light output was not high enough for the high light sensors but it would work well with 10 000 lux sensors. If you have a 1000 lux sensor then a normal white LED should give a full range response. The graph of an earthquake however might be slightly asymmetrical as the LED output is not linear. This might be a way to have the data logger not actually connected to the seismograph detector so that it is easier to move for other experiments. Some simple environmental monitoring dataloggers do not have the option of voltage input but do have light level inputs and so could be "tricked" to monitor the seismograph.
The LED could also be used as an optical alarm if no data logger is available. A voltmeter like the one in one of the photos can be connected from the high output to the zero output connectors, even with the data logger also attached. I will try to design a Picaxe audio alarm to tell you of an earthquake even if you are not recording it to a computer.
Most amateur seismographs use magnetic detectors and multiple op-amps to provide the signal for the Analogue to Digital converter. I tried several different designs starting with the one in the original Scientific American article from 1979. The circuit below is a hybrid with many parents and grand parents. It is slightly more sensitive than a light detector circuit when used on the same seismograph but is harder to build, has several trim pots to adjust and needs a +/- power supply. The big advantage is that there is no drift when the bar position changes because of stretching and slow thermal changes in the room. The disadvantage of the extra sensitivity is that it amplifies everything including the background "noise". This background noise includes the effects of tides, ocean waves, storms, barometric changes and the movement of the sun and moon as well as short range effects of traffic, people and thermal changes in the building and seismograph itself.
A supermagnet on the bar should be about 3mm from a very fine coil of wire. The wire coil from an old fish tank air pumps is nearly perfect; just make sure that it isn't burnt out. If you can make the magnet go inside the coil the voltage produced will be much greater and you will need less amplification and pick up less interference. Use a shielded lead (earth the outside shield wires) if the distance from the coil to the amplifier is more than 10 cm. You probably don't need to put the amplifier in a shielded box but try to put it as close to the coil as is reasonably possible.
|The diagram which appeared here in the August updates has now been replaced by the one below in the September update.|
The amplifier is built on any of the common prototype boards. A power supply with positive AND negative rails is needed. Anything from +/- 3 to +/- 15 will be ok for the amp. Remember that this will determine the maximum swing of the output voltage to you're A/D converter, so plan your power supply to suit the A/D converter. A 5k trimpot from the output to earth can set the final output. This circuit will work best with A/D converters that use signals swinging above or below zero. I am not sure if the zero adjustment will give enough range to make the output always positive so that you could output to a Picaxe A/D converter.
Many kit suppliers have kits that produce a +/- output from an AC plugpack. They mostly use 7805 and 7905 or 7812 and 7912 or 7815 and 7915 voltage regulators. The power supply can be separate from the amp or can even be built onto the same prototype board as the amplifier. The current drain of the amplifier is only a few milliamps so for testing I have run it from 2 sets of 3 AA batteries. The zero/earth line is taken from where the positive of one set of batteries meets the negative of the other set of batteries. This battery supply lasted for more than a week.
|September 2005 Update|
|The light beam earthquake detectors can respond to changes of light (sunlight, changing room lighting, shadows, and even the 50Hz flicker of lamps) within the room if it hits the LDR. The photo shows a simple "tent" made by bending the spare aluminium lid that comes with the jiffy box. The inside of the tent was sprayed with a dark mat paint to reduce reflections. Black cardboard could also be used as a tent.|
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Last Update: Monday, 30-Apr-2012 15:05:06 AEST