Easy-simple IR Detector

Posted 06-08-2018

A lot of hobby photographers are using IR cameras, or retrofitting common cameras to do infrared photography.  If the camera is a retrofit, then the photo hobbyist may not know the level of IR before a shot is taken.

This easy junk box build is a way to discover at least a ball-park guestimate of how much IR is present before a shot is taken.

The nice thing about this detector is that it uses a photodiode that reaches all the way down to 1.7 um (micro-meters) in the Near IR / Mid IR boundary area. Some newer cameras may be able to take photographs in that band.

Many silicon photo-diodes are good for only about 1.0 or 1.1 um, which means that IR shots are taken at the edge of the device capability. The Luna photodiode (SD003-151-001) is about six bucks, and has its responsivity centered far lower than the edge limit of cheaper silicon non – InGaAs chips. The Luna is an Indium Gallium Arsenide chip. So, if the detection is taking place at the center (or not far from it) of the device capability, then the result should be better (I’d hazard a guess on this).

Figure 2:  The IR output from a tungsten 25 watt stove range hood is shown.

I put the simple circuit into a small Hammond enclosure (8 bucks on Amazon).  The breakdown of parts was no bank buster:

  • (1) SD003-151-001  photodiode (900 nm – 1.7 um)  $6 at Digikey.
  • (1) 2N3904 NPN transistor (from BGMicro, maybe 50 cents?)
  • (1) 9V battery ($1 at dollar store)
  • (1) Hammond enclosure ($8 from Amazon reseller).
  • (1) 0-30 volt panel meter module (come in packs of 5 on Amazon, works out to about $1 each).
  • Resistors, pot, fuse, switch from my junkbox (free).

Grand total?  $16.50

So, one could find a box on Amazon or Ebay for about that, but it wouldn’t feel as nice (as one that was built by the hobbyists himself) – and it very likely would not go down to 1.7 um – cuz it’d use the cheaper silicon chip.

Figure 3: Hardly anything on either of the two little proto boards inside.

So, there’s no calibration setup.  This is a do it, remember it, repeat it sort of operation.  I get a ball park idea of how the IR I read from my gadget corresponds to a result.

 Figure 4: 40 watt tungsten bulb reading.

The potentiometer does help when reading extreme values (that’s the “variable resistor” shown with the knob in the center of the box).  Extreme values?  When I point the device directly at the sun, the reading goes over-scale.  So, I don’t do that.  But, in very bright sunlight, if I point it high up in the sky (but not at the sun) – it’ll go to the saturation of the device (the NPN) – which is a little below the battery voltage, so it reads around 8.2 or so volts for just about anything in the upper sky.

It’s amazing how much IR the sun delivers to the surface of the earth.  This little toy has been a revelation.  Pointing the gadget directly at a 40 watt incandescent bulb at three feet gives about 4.5 volts on the meter reading for the pot setting I usually use.  But, outside on a sunny day, one can get that much reflection from the road asphalt!  By the way, not everybody’s junk box has a 50 kohm potentiometer.  So, one could fashion a circuit with a 56k, 22k, and 20k potentiometer, although that would give less vernier control:

If I turn the pot down a little (the knob) – then I can get a non-saturated sky value that I can compare to other readings. But, it’s still horse shoes and hand grenades.

Figure 5:  “Zeroed out” – because the sensor’s on the back, and covered.

So, the sensor is rated from 800 nm to 1.7 um, but that’s just the spec. The skirts of the responsivity curves may show just a tad more bandwidth, if my eyes haven’t deceived me. The graph looks more like 750 – 1750 or 1775 nm to me.  So, to cut out any near-visual light that may creep in on the skirt, I bought an IR filter (which blocks light with shorter wavelengths than 890 nm).  This costs another $7, but it’s not on the cost sheet cuz it’s optional LOL. The filter, fitted to the rear of the box, where the photodiode sensor is located, is shown in the following figure:

Figure 6: The IR filter (known as low pass filter) – for 890 nm cutoff.

The filter cuts all of my readings down about 10 percent.  But, I don’t know if that’s the result of bandwidth cutting, or just that the glass has attenuation. But, I feel good that there’s no visible light getting to the photo diode.  While the NPN based device is all well and good, it’s my “coarse” device.  So, for edition II, I’m going to try to figure out a way to increase the sensitivity, and noise figure of the gadget.  Towards that end, I’ve built another detector into a wallet gift box (it’s a Croft/Barrow box) – and no, they only sell the wallets in the boxes, and have nothing to do with me.  But, the boxes make darn good project boxes, if you can manage not to cut your fingers on the edges of any holes you make.  So, maybe it’s not my advice to use them.  To each his own:

Figure 7:  A start on the second edition.

The second edition uses an op amp in place of the NPN transistor.  I built a prototype, but managed to wire the op amp into the circuit backwards, frying it.  So, stay tuned for more about that …

Note the old-style meter on the box.  I love those old things!  About a buck or two at BGMicro, last time I checked.

It’s fun to check the IR output of things around the house.  For instance, most of my LED bulbs (the type used for incandescent bulb replacements for table lamps, etc) – have almost no IR output at all. This is probably not healthy for the eyes, long term, because the eyes do in fact “see” infrared radiation. It’s just that the infrared part is not used for image processing by the brain.  In some other animals, the non-image IR is used for navigation.

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