Hey,
I was set a project to measure a variable of my choice, I chose to use an infrared led like those used in floodlight sensors. I do have a few limits listed below:
1) using a dc power supply set at 5V ≤ 250mA.
2) Include an output proportional to the variable level being monitored, clamped to a range of 0 to 3.3v
3) Include an alaram to indicate the variable exceeding a set limit (in this case the LED sensing a person etc)

So, my problem, I have little idea where to start, I do have to order the IR LED myself and then create the circuit too, so any little nudges from you guys would be amazing, feel free to ask any questions that you may need to know and I will try to help you as much as possible. This is my weakest area and I don't like asking for help that much but I will take any suggestions and help on board.

Thanks

 

It would be difficult to use an LED for the purpose you describe because the signal would be pretty small and amplifying it to a useful level with a suitable signal-to-noise ration would be difficult. Those heat-sensing floodlights use pyroelectric detectors. These are devices whose resistance drops when their temperature chances and are used by focusing infrared in a field of view onto a piece of pyroelectric material. When a person walks into view, the resistance drops and light comes on.

The Wikipedia article:
https://en.wikipedia.org/wiki/Passive_infrared_sensor

Murata's catalog with some application circuits:
http://www.murata.com/~/media/webrenewal/support/library/catalog/products/sensor/infrared/s21e.ashx

A place to buy some sensors
http://th.mouser.com/Semiconductors/Sensor-ICs/_/N-6gixy?Keyword=pyroelectric&FS=True
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You can also use a phototransistor to measure IR -not as sensitive as the pyroelectric sensor you mentioned. That is very simple:

Choose a circuit from this web page and add an infrared pass filter to reject visible ambient light:

http://home.cogeco.ca/~rpaisley4/PhotoDetectors.html

 

Thanks for getting back to me, I've decided to look at both options, and have since bought a small kit to make a PIR, but I would also like to look into the photo transistor option, using the circuits that you should shown how would I get the circuit to run from a 5v supply.

 

Thanks for getting back to me, I've decided to look at both options, and have since bought a small kit to make a PIR, but I would also like to look into the photo transistor option, using the circuits that you should shown how would I get the circuit to run from a 5v supply.

simply get an op-amp that runs on lower voltages. Look for "logic-level" op amp when searching on Digikey or other supplier sites.

You also want to change the resistor on the LED to 220 ohm instead of 1k ohm

If you are really stuck on 5 volt operation, you can also use standard logic chips, us an inverter or a buffer (depending on whether you want LED on in dark or light conditions.