High-Quality Visible & Infrared Light Sensors

Overview

This project involves the construction of two fairly high-quality visible and infrared light sensors.  These devices are handy from everything like locating infrared-triggered IEDs, to detecting laser-guided bomb designators, to even building laser-bounce listening devices.  Only the construction of the actual sensor devices and their associated mounting hardware will be discussed here.  The electronics for post-processing the received signal will be covered in several upcoming projects.

The two sensors used for this project are a PerkinElmer (EG&G / Vartec) VTP1188S silicon PIN photodiode and a Fairchild L14G3 phototransistor.  Photodiodes are used when you need to receive a light signal which may be pulsed or carrier-modulated, while phototransistors are slower responding and are used for low-frequency modulated signals while offering a little bit of receive gain.

Photodiodes are configured as a high-impedance, reversed-biased diode whose reverse current corresponds to the amount of light which falls upon it.  The photodiode's very small current output is converted to a voltage by using an op-amp configured as a current-to-voltage converter or transimpedance amplifier.

A phototransistor is just like a regular transistor whose base current responds to light.  The current flowing through the transistor is converted to a voltage by placing a series resistor in the phototransistor's collector or emitter.

Both photodiodes and phototransistors require a small amount of ambient light to bias them to an "on" state.  Their sensitivities are actually quite poor in total darkness.  To overcome this, you can shine a small amount of infrared energy (called dithering) onto the photodiode or phototransistor in order to help properly bias them.  Using DC voltage and resistors to bias the photodiode or phototransistor will also work, but this should be avoided as it adds additional noise to the received signal.

Silicon photodiodes and phototransistors both have a distinct frequency response.  They tend to peak in the high near-infrared region (800-900 nm) and actually have a fairly poor response at the normal "visible" wavelengths below 700 nm.  There is little we can do to overcome this without switching to more expensive or complicated components.

The photodiode and phototransistor in this project will be mounted in some surplus Andrew 1/2-inch Heliax connectors.  You can sometimes find these sold as scrap parts at ham radio swapfests.  A few standard brass plumbing parts will finish off the overall design.

Pictures & Construction Notes

Constructing the body of the photodiode and phototransistor sensor mounts.  Both of them will be the same.

A couple of Andrew 44ASN 1/2-inch Heliax connectors were salvaged for the sensor's bodies.

Solder a 1/2-inch flare to 1/2-inch MIP union (Watts A-277) to the rear of the compression fitting of the Andrew 44ASN connector, as shown above.

To align the two sections before soldering, use a vise to press fit the 1/2-inch flare to 1/2-inch MIP union into the rear of the Andrew compression fitting.

Be sure to sand and flux each part to ease soldering.

Drill a hole for mounting a BNC jack in a 1/2-inch FIP pipe cap (Watts A-819).

A standard BNC jack is shown above, but an isolated BNC jack was used in the final version.

The isolated BNC jack will allow for more options in configuring the photodiode or phototransistor for the post-processing electronics, and is highly recommended.

Drill out the lip in the center of the Andrew 44ASN connector with a 21/64-inch bit.

Solder two wires to the leads of the VTP1188S photodiode.

The use of Teflon wire is highly recommended to help prevent any leakage current from a poor dielectric.

The BLACK wire is to the photodiode's cathode.

The WHITE wire is to the photodiode's anode.

You can order the VTP1188S from Newark, part number 79K2522.

Mount the VTP1188S photodiode as far down the center of the Andrew 44ASN connector as it will go.

Secure the photodiode in place with some (real) RTV sealant.

The entire connector body was sandblasted inside-and-out to help knock down any potential glare from reflections.

Solder the photodiode's cathode and anode wires to the (isolated) BNC jack mounted in the FIP pipe cap as shown.

Screw the FIP pipe cap onto the rear MIP union.  Be sure to only hand-tighten.

Mounting the L14G3 phototransistor will be a little different.

Place the L14G3 in one of those plastic snap-together LED holders which you can find at Radio Shack.  Add a beveled plumbing washer behind the plastic holder, as shown above.

Solder Teflon wires to the collector and emitter leads of the L14G3 phototransistor.

You can safely cut off pin-2 (base) of the L14G3.

You can order the L14G3 from Digi-Key, part number L14G3-ND, or from Mouser, part number 512-L14G3.

Radio Shack has a very similar phototransistor which will also work, part number 276-145.

Again, drill out the lip in the center of the Andrew 44ASN connector with a 21/64-inch bit.

The new L14G3 assembly will need to be press fit into the connector's body.

Make sure to isolate the metal case of the phototransistor from the sides of the connector.

Align the L14G3 assembly into the center of the Andrew 44ASN connector and finish tightening the compression fitting.

There is no need to use any RTV sealant on this mount.

Finished photodiode and phototransistor visible and infrared light sensors.

Optional 1/2-inch pipe hanger clamps were added to help mount the sensors.

Protective shields for each of the light sensor were made from the shells of old PL-259 connectors with some tape over the end.

Experimental mounting idea.

Use the nuts from salvaged SO-239 or N panel-mount RF connectors.

Sensor in action.

It's a good idea to use high-quality Teflon coaxial cables (and connectors) to connect to the sensors in order to avoid any excess leakage current.

Here's a simple (optional) optical attenuator which can be made using two polarized lens and a salvaged CATV 75-ohm hardline connector.

The CATV 75-ohm hardline connector consists of two main pieces.  The front-end assembly, which slides inside the hardline coax itself, and the rear compression fitting which secures everything.

The CATV hardline connector was sandblasted to help clean it up a bit and then the polarized lens were epoxied over the openings of each connector assembly.

That silver-colored metal piece, which slid into the dielectric of the hardline coax, was removed on the front-end assembly.

It looks are little rough, but it turned out to work quite well.

Assembling the homebrew optical attenuator.

A shell from an old PL-259 connector was cut down in order to attach the optical attenuator to the light sensor.

You'll also need to drill out the center conductor on the CATV hardline connector using a 3/8-inch (or larger) drill bit.

Finished optical attenuator.

Rotating the compression fitting on the optical attenuator rotates the outside polarized lens while the inside polarized lens remains stationary.

This provides a simple means to attenuate the amount of light radiation reaching the final light sensor.

Example of a simple infrared dithering circuit.

The infrared emitter is from Radio Shack, part number 276-142.  The dark-colored diode in the package is the emitter.

A salvaged panel-mount F connector will be used to hold the infrared emitter.

Solder the infrared emitter to the F connector like so.

The infrared emitter's anode is on the center conductor, the cathode is to the shield (ground).

A 0.1 µF capacitor should be soldered from the infrared emitter's anode to ground.

Mount the infrared emitter near the phototransistor or photodiode you wish to bias.  Be sure it is not blocking its view.

An external potentiometer (conneted via the F connector) will control the current going through the infrared emitter.  This will determine the final amount of bias on the phototransistor or photodiode.

A piece of black art foam was added on the bottom of this PVC cap to reduce any stray reflections.  A similar black rubber o-ring was added around the sensor mount's shiny securing nut.

Example of an optional fiber optic probe adapter.

This is useful for sniffing out infrared triggered devices, like alarm systems and IEDs.

It's made from a Nite Ize, Inc. "AA Fiber Optic Adapter" and a salvaged male N compression connector for RG-8 coax.

First thing to do is remove the fiber optic probe from the rubber flashlight adapter.

You may wish to touch up the ends of the fiber optic probe with a very fine grain sanding stick.  This will help improve the fiber optic's light transmission ability slightly.  Since this isn't real fiber optic cable, there will be a fair amount of attenuation to any light signal passing through it.

Slide the fiber optic probe through the front of the male N connector like so.

Quickly solder the brass tab of the fiber optic probe to the center ring of the male N connector.

Try not the melt the plastic of the fiber optic!

It was probably better to glue the probe to the connector instead of soldering it.

Completed fiber optic adapter connected to a light sensor.

When attaching the fiber optic probe adapter, be sure the secured end of the probe doesn't smash into the photodiode or phototransistor inside the light sensor.

A small rubber grommet was added to the open end of the male N connector to further secure the fiber optic probe.