Introduction: How to Build a Simple Twilight Photometer

This is not an original project, it is a crude version of Forrest M. Mims' MAKE magazine photometer found here;

A twilight photometer is a very accurate light sensor, used at early dawn or twilight. Its purpose is to measure the amount of aerosols in the stratosphere.

When the sun is down below the horizon it no longer directly illuminates the ground, most remaining light is a result of photons reflecting of clouds and small particles in the sky. This is known as twilight, the effect also happens just before the sun rises in the morning. By measuring the brightness of the sky around ten minutes before sunset or 45 minutes after sunrise it is possible to estimate the amount of aerosols in the sky.

Results from a homemade photometer are a global indicator of the amount of aerosols in the sky, It cannot be used to, for example measure the difference between pollution near a highway and pollution further away from it. The results from a basic photometer such as this are best used to measure differences in refracted light from aerosols throughout a year, and possibly comparing different years to each other or detecting spikes in aerosol quantities due to volcanic activity. Here is an example of results from a twilight photometer made following this guide. The photometer can be used to actually calculate approximate aerosol density, but only in ideal circumstances, this means locations without any light pollution or electromagnetic interference such as near most observatories or in a desert.

Aerosols are minute particles suspended in the atmosphere. Three kinds of aerosols are known, volcanic aerosols, desert dust and man made aerosols. The effects of these particles are almost universally bad for the environment, they cause sunlight reflection, ozone damage, desert expansion and more. Effort is being made to reduce the amount of aerosols in the atmosphere and that is where twilight photometers are used. Keeping track of the amount of aerosols in the sky is important because the full effects of these small particles is not yet understood completely.

This is instructable is part of a school project.

Step 1: Materials (Preperation)

Materials for the twilight photometer;

  • A Breadboard for prototyping (optional)
  • LED (preferably one with clear epoxy and a wavelength of 660-880 nm)
  • A TLC271BIP operational amplifier
  • 2X 40-gigohm resistor
  • 47 pF capacitor
  • 0.01 µF capacitor
  • 4700Ω resistor
  • 2x 9 volt battery
  • 4 switches
  • A darkened tube with a diameter just larger than the LED you are using
  • A metal case to house the photometer
  • Several small diodes (3-6 depending on resistance)
  • Copper wire (optionally red and black for easy of use)
  • Soldering equipment
  • A perforated board
  • A drill
  • Drill bits of varying sizes, 3-12mm
  • Power supply
  • Multi meter
  • A small file
  • Isolating tape
  • A constant light source
  • Cables for the multi meter and power supply
  • Female standard cable plugs
  • Small saw

Step 2: Exact Matererials (Preperation)

Before building the actual circuit let's go through the exact materials.

For the main circuit:

  • A TLC271BIP operational amplifier, here is a specification sheet as well as a list of authorized retailers and alternate parts with a similar function.
  • 40-Gigohm resistors, if necessary the resistors can be swapped with any resistors between 30- and 60 GOhm.
  • With both the 47pF capacitor and the 0,01µF capacitor, it's important to get high quality parts because electrical fluctuation in one of these have a big effect on the results.
  • A black/darkened tube, this tube needs to be just larger than the LED to be used in the build (an LED is about 5mm, the tube needs to be about 6mm). Using a tube this size results in less unwanted light entering the back of the LED. For best results, make sure that the inside of the tube is not reflective to avoid a difference in light getting to the LED at different angles.
  • 4 switches, at least one of the switches should be a three way switch.
  • A metal case. This case should be big enough to house all the components and have space left over for cable management. This should be made of metal to block electromagnetic interference, this means that if you want to use the photometer in an area with a large amount of electronics it may be prudent to use a case with thicker walls. Otherwise a simple tin case salvaged from something else will probably do the trick.
  • LED, an LED is used instead of a normal silicon photosensor because it is more accurate, consistent and most of all because LED's only detect a narrow band of wavelengths depending on the type of LED used. Most LED's have a peak detection slightly below their peak emission wavelength. The desired light to detect is red, wavelength +/- 650nm, this means an LED of wavelength 660-880 is optimal for detecting reflected light from aerosols.

For the extra battery controller circuit:

  • 4700Ohm resistor, this resistor is in place to avoid short-circuiting, the resistance is should be around 4500Ω, higher if you plan on using more powerful batteries.
  • Diodes, A 16 volt zener diode is ideal in this situation. It will lower voltages higher voltages to 16 volts. Alternatively multiple smaller normal diodes can be used each of these will lower the voltage 0.1 -1 volts depending on the diode, however using this method will not only lower anything above 16 volts but also anything below it, effectively wasting some battery charge. Lowering the voltage is important because the TLC271BIP Op-amp can not handle more than that.

Step 3: Prototype Cicuit (Preperation)

By prototyping the circuit it's easy to discover defective components and get a feel for the electronics that will be used. If you feel confident about your components and your knowledge of electronics you can skip to 'Building the circuit'.

First, have a look at the circuit. Start by placing the TLC271BIP operational amplifier in the middle of the breadboard, make sure it is aligned so that the pins are not connected with each other (horizontally in most cases). Now place the other components, LED, 47 pF & 0.01 µF capacitor and the two 40 Gigohm resistors. Start connecting the components as demonstrated in the circuit, optionally you can follow the photographs for a more step by step installation of the components and connecting wires.

Important things to remember;

  • Make sure you use the dedicated positive and negative lanes on the breadboard and connect your components to that instead of connecting them directly to a power source.
  • Some breadboards have a disconnection half way through for their upper- and lowermost lanes, make sure to connect those if you plan to use those.
  • Change the layout of the circuit once or twice to find a logical and compact setup that can be replicated when making the actual circuit.

Now you should have a fully functioning circuit, supply it with 5-15 volts and connect a multi meter or other voltage testing device to the multi meter connector points. The LED should be functioning as a light sensor. To test this simply see if there is a difference in the output voltage by shining a bright light at the LED, the voltage should increase to about the same as the supply voltage.

Step 4: Building the Circuit

To start, decide on the size of board you want to make and put the components down in the places previously decided on the breadboard. Do not solder the LED to the board, it should be connected to through wires instead so it can be repositioned later. Also, instead of connecting the IC1 (OPamp) directly to the plate, place it in a socket with pin 2 sticking out, it is connected to the LED. As the gain is extremely amplified, any interference from the board or dust on this pin will make measurements less accurate. The parts should be inserted in the copper-less side of the board. Once again make sure that the components aren't connecting in any undesired way through the copper layer on the bottom of the perforated board. Now the resistors, capacitors and the OPamp are in place, they need to be connected. This can be done two ways;

The first is to create a breadboard style construction on the perforated board, creating a positive and negative line and connecting the parts to these. This way makes it easy to copy any previous design over, keeping everything clear. It also leaves enough room for external connectors such as the power input, resistance switch and measuring points. The disadvantage is that it takes up more space.

If you are working with less space or with limited supplies, connecting parts to each other directly may be more appropriate. This second method is also illustrated in the images above. It takes more thought and precise planning but also saves a great amount of space. To make a circuit in this style, leave no space between the components and use solder to connect them. Make sure you leave enough room to connect appropriate external wires. Four pairs of wires should be coming out of the circuit. Two wires for power input (these are the ground and V+ wires), two wires for measuring the output voltage (these should be ground and a wire between IC1 pin 6 and the resistors indicated by arrows). Two wires for the resistance switch (one connected between R1 and R2 and one connected at the other end of R2), the final pair of wires is for the LED (connected to ground and IC2 pin 2) this should be the only pair of external wires connected to anything.

When all wires connecting the circuit to other parts of the photometer are soldered in, no exposed wires on the top of the board should be visible except for the two wires connected to the the second pin on IC1. make sure everything on the underside is soldered correctly and cut of all the protruding wires to avoid shorting the circuit. Cut out the board with a small saw or a dremel leave about a one hole clearance around the board to avoid the copper on the underside touching anything conductive like the side of the case or an un-insulated wire.

Important things to remember;

  • Before putting any components on the board, check if there is any oxidization on the copper coated side. If the copper is not entirely shiny, use high-grit (400+) sandpaper to remove any imperfections as they might cause the wires to not connect with the plate or introduce unnecessary resistance.
  • Any wires connecting to other parts such as the wires connecting to the LED should be insulated correctly, this means no exposed copper, the connection between the LED and the connector wires should be covered with heat shrink tubing.
  • Make sure to mark the six wires coming out of the board with labels like V+ V-, Multi meter and resistor switch. This will save a lot of headache later on.

Step 5: Powering the Circuit

Powering the photometer is important because an unstable or unpredictable power supply can change the results or even cause components to stop working. If you have access to a power outlet and a power supply at the place you want to do measurements consider connecting the power wires from the main circuit directly to a connector on the outside of the device. This way you can use an external power supply to power the photometer without making any new circuits or doing more complicated wiring. If you want be more mobile with the photometer, things get more complicated.

More voltage results in a more accurate output, however the IC1 can only handle up to 16 volts. The best way to do this is by using two 9 volt batteries. These will output 18 volts when put in series, this should be lowered by using a circuit consisting of a few small diodes and a 4700Ohm resistor. The diodes will lower the voltage by about 2 volts (depending on how many are used) and the resistor is in place to prevent short circuiting.

Because the power circuit is less complicated than the main circuit, it should be easier to build. It may still be useful to build it on a breadboard first to find the most compact or efficient layout. A simple way to solder the diodes and resistor is to place them all next to each other and solder them in a zig-zag pattern. To clarify the wiring mark the three places connections need to be made. These are the start of the diode chain (start), between the last diode and the resistor (middle) and on the other side of the resistor (end). Just like the previous circuit, cut off excess wires and make sure everything is soldered correctly.

There will be five wires connecting to this board, one (positive) wire connected to 'start', one (positive) wire connected to 'middle' and three (negative) wires connected to the 'end'. Don't connect the wires to anything else, this will be done later in step 7.

Important things to remember;

  • Soldering the diodes in the 'zig-zag' patern means soldering them in place directly next to each other, with the conducting direction reversed compared to the diode before each one. Now solder the diodes together as if they where laying in a straight line, front to back.
  • The diodes need to be placed in the reverse direction. The conductive direction is indicated with a white ring around the diode. Place the diodes with the ring towards the resistor if normal diodes are used. If instead a zener diode is used, place it in the opposite direction.

Step 6: Building the Case

The case protects the circuitry inside from electrical noise caused by electromagnetic radiation. Almost all electricity based machines produce an electromagnetic field, more powerful devices have a stronger EM field. This field is can be ignored for most devices because the resulting power fluctuations aren't noticeable. A twilight photometer however, uses a very small current, this current is easily influenced by EM waves. The small current and subsequently the fluctuations are all amplified greatly. Electromagnetic radiation is a combination of electric and magnetic fields, the magnetic field is not dangerous to the photometer because magnetic fields don't (electrically) influence devices that are not moving relative to the field. The electrical portion of the EM-radiation is the biggest problem, this kind of field exerts force on electrons in conducting materials like the copper in a circuit resulting in unwanted electrical 'noise'. To protect a vulnerable circuit, a different conductive material should be placed between the circuit and the source of the EM radiation. The electrical field induces a current that causes displacement of charge inside the shielding metal which cancels it out. Any conductive material can be used, a simple tin case works as well as a solid iron case but is much easier to work with.

To start off, drill a hole for the light shielding tube this hole should be slightly larger than the tube itself, which in turn should be slightly larger than the LED you want to use. Drill this hole near an edge so the tube is not taking up unnecessary space. next drill two switch sized holes near each other. If you are using switches with a square profile use a small drill bit and use a small file to create a hole with the correct dimensions. These switches will control the power circuit on/off and the 9/18 volt setting. The one/two resistor switch and the on/off switch for the entire device can be placed arbitrarily. Next drill holes for the connection points, the first pair should be near the first two switches. These will be the external power inputs, placing them near the power circuit makes wiring easier. Next drill holes for the power out connectors, these will be connected to a measuring device.

Important things to remember;

  • If you do not want to use internal batteries, the only necessary switches are the on/off switch and the resistor switch.
  • The method of attachment may vary depending on the switches you use a switch with a face plate may only need to be inserted, however a switch without a face plate or screw on mechanism may need to be hot-glued in place.
  • A larger switch for the power on/off is not necessary but does look and feel cool!

Step 7: Connecting Everything

At this point you should have a case with four switches, two pairs of connection points, a hole for the light sensor tube, a power circuit with 5 wires sticking out and a main circuit with 8 wires attached. First join both circuits together by connecting one of the V- wires to the 'end' point on the power circuit. Now place the two circuits inside the case together with the two nine volt batteries to determine the fit and the space left for wiring. If you are working with a small case and have little space left for wires, consider shortening these. This means less chance of random electrical problems caused by wires connecting to the wrong surfaces, it does however reduce the margin of error so be careful. Make sure to connect the circuit plates to the switches before connecting the switches back to batteries and power in and out connectors. It is very important to insulate all open conductive surfaces, this means the inside of the case, the place switches attach to wires and miscellaneous connection points. Insulate the small wires with heat shrink tubing and insulate larger surfaces with electrical tape to prevent shorting out the hardware.

If you are not planning on using the photometer without an adjustable power supply, only two switches are necessary; the on/off switch and the resistance switch. The 'power in' connectors should also be wired differently, the positive connector should connect directly to the on/off switch, the negative connector should connect to one of the negative (V-) wires on the main circuit.

Important things to remember;

  • If space is limited and two switches are placed above one and other, put the switch with the most wires above the switch with less to make soldering easier.
  • Using two color wire is great for identifying problems, use a red for positive wires and black for negative wires. By using different colors for negative and positive, the project not only becomes less cluttered for you but also makes recognizing the different paths and connections easier for anyone else interested in the function of the device.

Step 8: Finishing Touches

The photometer is almost done. But before testing it in the field a few things need to be done, first insert the LED in the tube and make sure no light is able to reach it from the back (shouldn't be a problem unless light is able to enter the case in this area). Also make sure the circuits are not being pressed into the case when closed. This could result in electrical shorting circuiting, even when properly insulated, or damage to components form physical stress. After making sure everything is connected correctly one last time, the photometer should be ready to use. Bring it outside, connect a power source or turn on the batteries and start testing!