Our first 'robot' circuit: What is this Component?

Let's Make Robots
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We've been working on a lot of basic circuits, just trying to understand basic electronics.  Almost all of our components are salvaged from old electronics, so it has been slow going, but we are learning a lot!

After a day or so of experimenting and research, we finally were able to make a circuit that acts more or less as an infrared detector, with obvious (hoped for) application to our upcoming robot project.  The movie (not great quality, sorry) shows it in action.  However, we are still not 100% sure what one of the components is.  Here is a picture of the real live circuit, and the diagram:

In the picture, the dark looking LED on the left is the IR emitter, and the other one is the 'detector'.  We wrapped the detector (apart from the tip) in black tape to shield it from ambient light, but without the tape it is clear.  The 'detector' has a flat side and we assumed that this is the cathode -- it is the top lead in the picture, directly connected to the negative terminal of the power supply (which we made from an old 5V 'wall wart').

As indicated by the question marks in the circuit diagram, we are not really sure whether the IR detector component is a photodiode or a phototransistor.  We think it is a photodiode, but we are given pause by the fact that it was labelled as a phototransistor. 

We did a few tests with our cheap multimeter.  The tests were run with the component alone (not connected to the circuit), both with and without IR light from the IR LED shining on it.

With the positive lead on the multimeter connected to the anode (see above) and the negative lead connected to the cathode, we got these results.

Resistance. with IR light: 2.3 Ohms; without IR light: 2 Ohms

Current.  no noticeable current with our without IR light (but our ammeter does not appear to be very good at all)

Voltage.  with IR light: it appears to be zero or slightly below zero on the voltmeter; without IR light: it appears to be slightly above zero (the motion was noticeable, but not a lot)

With the positive lead on the multimeter connected to the anode (see above) and the negative lead connected to the cathode, we got these results.

Resistance. with IR light: 0 Ohms; without IR light:  effectively infinite (the meter maxed out)

Current:  with IR light: a very small noticeable current; without IR light: no noticeable current

Voltage:  with IR light: slightly above zero; without IR light: it appears to be slightly below zero (the motion was noticeable)

 

Any ideas what this thing is?  Photodiode?

https://www.youtube.com/watch?v=nAJC8ZKdvFs
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Excellent experiment, not so much for documentation

The experiment in the video demonstrates how the PD1 could be used. But I would not have guessed that from your diagram. Are you sure you wired it exactly like that? I think the anode of LED2 is directly connected to R2.

The mystery thingey is obviously sensitive to some sort of light. It need not be Infra Red specifically. In a darker environment, you could try a range test. This would demonstrate that the PD1 has a greater effect when more light hits it. Once you have established the maximum distance where it will still demonstrate some effect, you could change LED1 for different kinds of LEDs. Across the entire spectrum. This would demonstrate what colours will set off PD1 and at what distances. The colour that can affect PD1 at the greatest distance is probably the colour PD1 likes the most.

A potmeter in series with LED1 might have the same effect as varying the distance, but is not as demonstrative.