Let's Make Robots!

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

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)

Voltagewith 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)

Currentwith IR light: a very small noticeable current; without IR light: no noticeable current

Voltagewith 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?

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

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.

I appreciate the feedback.  Yes, I misdrew the diagram -- I've fixed it now.

We'll try what you mentioned and see what happens.

I'm always a bit confused about the real difference between photodiodes and phototransistors. They are very similar, and it probably makes no difference for your application.

A review of the subject on wikipedia shows me that phototransistor have better responsivity to light, which means for the same amount of light they will have more gain. Howver, they are no better at detecting low light levels than photodiodes. Also, phototransistors have longer response times.

Still not sure what we have, but I don't guess it matters -- we know how it behaves.  I did find a very useful resource on photo detectors in general.

We did some of the suggested tests, and PD1 (if that's what it is) likes the IR emitter far and above anything else.  We also did a test where we put them parallel to each other and detect the reflected IR light.  The indicator LED changes brightness in response changes in distance, and with the multimeter we can see the range of the 'output' of PD1, so I think we've got the basics of object-detection.  We added a movie showing this new setup.  The emitter appears to be white but that's just the digital camera picking up the IR light.

Next step is to try pulsing the emitter and detector with a timer.  This is fun!

You are on the verge of discovering a device known to LMR as "AmandaLDR". Search that if you don't mind spoilers!

I think he is closer to OddBot's IR obstacle avoidance detector.


Nice work.

me := off_hook