Datalogging Seismograph
A Light Beam Detector Circuit

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.

earlier cardboard screen
circuit diagram

Download Circuit diagram in pdf
Detector Circuit

Electronics enthusiasts can probably build the detector circuit without any plans but the following instructions will make it easier for the average circuit builder.

Components:most of the component values I have chosen for the circuit are not critical and the soldering is only difficult near the 8 pins of the op amp socket. 
Capacitors:3 x 470 microFarad electrolytic: the negative end has a - sign
The capacitor at the end of the circuit (it filters out voltage ripples in the power supply) can be any value from 100 to 10000 microFarad but must have a voltage rating well above the power supply voltage (e.g. 25 volts).
70 cents each
Resistors:¼ watt resistors 5% tolerance. Depending on the type of LDR you purchase you might have to reduce the value of the 10k resistor attached to the LDR to 5k or 2k. Use a digital multimeter to measure the resistance of the LDR in "bright daylight" but NOT in direct sunlight. The Dick Smith LDR ($3.00) has a bright daylight resistance of about 5k and works best with a 10k resistor (when the aluminium screen is half way across the LDR). If the resistance of the LDR in bright daylight is only 2k then use a 5k resistor. One type of LDR from Oatley Electronics had a resistance of only 1k and so needed a 2k resistor. Resistors and LDR can be soldered into the circuit either way around.
Variable resistor. Adjusts the gain of the op-amp. 100k or 200k or 250k
5 cents each
Light Emitting Diode; LED:Use a bright or superbright red or clear LED. Remember that the long leg of the LED is the positive lead. $1.00
Op-amp:741, TL071, LF351, OP27 etc. 8 pin. Use a socket for the op-amp so that you don't damage the op-amp with poor soldering. Buy two sockets just in case. $4.00
Power supply plug pack:You MUST use a DC supply for this circuit; any DC output plug pack between 6 and 15 volts will do. Most cheap plug packs (called "unregulated power packs") actually produce more volts output than the label says when the current is very low. The plug pack used for the circuit board photos was an "el cheapo", 7.2 volt model. Our detector circuit only uses about 30 milli Amps of current, so the plug pack really produces 10 volts at this low current. Cheap plug packs also have an output with lots of "ripple". This means that the output voltage (10 volts) flicks up to 12 volts and down to 8 volts and back up 50 times every second. The Detector Circuit has a 100 ohm (anything between 10 ohm and 200 ohm would do) and a filtering capacitor to smooth out the ripple BUT the voltage could be up to 12 volts; from a 7.2 volt plug pack. This is why the filter capacitor must have a high voltage rating. Good quality "regulated power packs" will give out the voltage on the label. They cost more than twice as much as el cheapo designs. $8.00
Power Supply Socket: Must match the output plug on the power supply plug pack. Be careful; there are several VERY similar looking 2.1mm and 2.5mm diameter plugs and sockets. Take your plug pack along when you buy the socket. Use a multimeter to determine the positive and negative output lugs on the socket. You will have to pass the plug pack lead through one or two holes in the floor of the display case so you cannot permanently solder the plug pack output wires directly into the circuit board; plugs and sockets are essential. $2.00
Printed Circuit Board: We hope to eventually have a printed circuit board manufactured for detector circuit. A blank circuit board however is very useful. Most electronics suppliers have several suitable "prototype" boards for op-amp circuits. The board in the pictures was from Dick Smith Electronics H 5605 You might have to cut the board so that it fits in the jiffy box. $4.00
Banana plug sockets: are used for the output sockets from the Detector Circuit; most Data logger voltage sensors have banana input plugs
  • One black banana socket is the zero / negative output.
  • One red banana socket is the low output positive.
  • A second red banana socket is the high output positive.
  • $3.00
    Jiffy Box:any plastic box that will comfortably house the board, sockets and variable resistor, and will allow the LED to poke out through the side of the box will do. Remember that it will have to fit in the display case beside the seismograph bar. About 130mm x 70mm x 40mm will do. $4.00
    Large Display Voltmeter: this option can give a very visible output from the Detector Circuit if wired in parallel with the data logger sensor.

    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.

    circuit + LED and LDRStep 1
  • The top line across the circuit board will be the positive and the bottom line will be the negative/zero. Use a multimeter to find the positive contact on the power supply socket.
  • Disconnect the power pack before you solder. A 100 ohm (brown, black, brown, gold) resistor connects the positive of the power supply socket to the positive on the board. The black insulated wire is from the negative of the socket to the negative of the board.
  • The capacitor connects from positive to negative on the board. The minus sign on the capacitor MUST, MUST, MUST connect to the negative. Avoid bending the wires too close to the body of the capacitor.
  • The LED connects from the positive line through a 1k resistor (brown, black, red, gold) to the negative. The longer leg of the LED is soldered to the positive line.
  • The LDR connects from the positive line through a 10k resistor (brown, black, orange, gold) to the negative.
  • The red insulation is stripped from thicker wire and prevents short circuits.
  • Step 2
  • Connect the power pack, turn it on and the LED should come on.
  • Connect a digital multimeter (about $15) as shown and measure the voltage in bright daylight. The black (negative) lead for the multimeter for our tests ALWAYS connects to a wire soldered to the negative line of our circuit board. YOU MUST SET THE MULTIMETER TO 20 OR MORE VOLTS DC BEFORE YOU CONNECT IT. The voltage should be as high or maybe even higher than the voltage rating of the plug pack (It read 8 volts for the 7.2 volt plug pack that I tested.) Cover the LDR with your fingers and the voltage should drop to about half.
  • If the voltages read as negative volts then you have the wrong output pins on the plug pack socket.
  • If the LDR test works then the plug pack and LDR are probably OK.
  • If you can see the LED flicker then the capacitor might be the wrong way around or have a "cold joint"….. resolder it.
  • If nothing works check the plug pack socket; you might have chosen the wrong output connection pins.
  • If the LED is not on check that you have the long leg to the positive line, good soldering and the correct resistor.
  • The capacitor acts like a tiny rechargeable battery. The LED stays on for a few seconds after you turn off the power. ONLY continue if these tests work. If not ask for help…… maybe an Industrial Arts teacher if other Science staff can't help?
  • testing LDR response

    STEP 3

    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;
  • The two capacitors are connected + to + and then connect through a 10k (brown, black, orange, gold) to Pin 2
  • The two 10k (brown, black, orange, gold) resistors (from the positive line to the negative line) act as a voltage divider. From their junction a wire connects to Pin 3. The op-amp uses the voltage at Pin 3 as a reference level. It compares it with the voltage at Pin 2 which in our circuit will change as the light level on the LDR changes when a quake occurs. Pin 4 should be connected to the negative line and Pin 7 should be connected to the positive line for this photo but I forgot to solder in the wires before I took the photos.
  • The variable resistor connects from Pin 6 to Pin 2. It controls the amplification of the op-amp. None of the pictures show it well but one of the wires to the variable resistor connects to an end connector pin while the other wire MUST connect to BOTH the end connector pin AND the centre pin. It doesn't matter which wire goes to Pin 6 or to Pin 2.
  • The output from Pin 6 connects to a 3.3k resistor (orange, orange, red, gold) and then a 1k resistor (brown, black, red, gold) to the negative line. This gives two possible output levels.
  • opamp socket voltage
    Measure the voltage at each of the pin holes in the socket BEFORE you put in the op-amp. The voltages should be roughly
    • Pin 1 zero…… not connected
    • Pin 2 between about 2 volts and about 10 volts depends on light level (less than pin 7)
    • Pin 3 about half of pin 7
    • Pin 4 ZERO ..connected to negative line
    • Pin 5 zero .. not connected
    • Pin 6 very low depends on setting of variable resistor
    • Pin 7 high . connected to positive line
    • Pin 8 zero.. not connected
    If there a major difference go back and check your soldering.
    circuit high output voltageStep 4
  • Turn off the power until the LED goes out.
  • Bend the pins of the op-amp inwards slightly so they aim into the holes of the op-amp socket and gently push it into the socket. (The pins are made to bend out slightly so that they can be picked up by machine and the pressure of the machine will bend them in to the correct width to fit in the holes.) MAKE SURE that you don't bend over any pins.
  • With the OP-amp in place reconnect the power and measure the output voltage from Pin 6. In this case the output is 4.84 volts. When the LDR is covered the voltage rose to over 8 volts. This is where the high output is connected.
  • circuit low output voltageStep 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.

    Try to position the detector so that the shadow line of the aluminium screen is about half way across the LDR. With the "blow test" the bar should move just enough for the shadow to cover all and then none of the LDR. Don't necessarily have the amplifier set at maximum. If it is every movement of the bar will be off scale.

    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.

    For further information contact Dave Dobeson

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    Last Update: Monday, 30-Apr-2012 15:05:06 AEST