IR sensor diode

Hello, i'm beginner in electronic device.
Based on the picture (1) below (from youtube), LED is on if there a small fire around 40cm.
In other simple video, people use IR sensor for an obstacle detector.
So, :

1. How IR sensor diode characteristic, detect a heat or obstacle?
2. IR sensor diode need a reciever? Because some people don't.
3. And I wonder what device on the picture (2)?

Thanks for answering.

Picture (1)


Picture (2)

Picture #2 is a photo-transistor, although more likely it is an IR receiver IC/ See https://www.digikey.com/product-det...-opto-division/TSOP34438/TSOP34438-ND/4074442

The "IR Sensor" is a bit wierd because the arrows show emission.

A LED can sense as well as source radiation. It's not a normal application.

The schematic confuses me.

An IR sensor diode could pick up flame emissions in the IR spectrum. It would be sensitive to room light too.

The sensor is being operated in reverse bias, so photoconductive mode. https://www.researchgate.net/post/W...hotconductive_photovoltaic_mode_in_photodiods

You picked a non-easy circuit to try to understand.

KeepItSimpleStupid said:

You picked a non-easy circuit to try to understand.

Click to expand...

It's also an incredibly bad design, as with many internet / youtube circuits.

If the first transistor turns on fully, the power source is shorted out and the LED cannot light.

KeepItSimpleStupid said:

Picture #2 is a photo-transistor, although more likely it is an IR receiver IC/ See https://www.digikey.com/product-det...-opto-division/TSOP34438/TSOP34438-ND/4074442

The "IR Sensor" is a bit wierd because the arrows show emission.

A LED can sense as well as source radiation. It's not a normal application.

The schematic confuses me.

An IR sensor diode could pick up flame emissions in the IR spectrum. It would be sensitive to room light too.

The sensor is being operated in reverse bias, so photoconductive mode. https://www.researchgate.net/post/W...hotconductive_photovoltaic_mode_in_photodiods

You picked a non-easy circuit to try to understand.

Click to expand...

Ok, please ignore my example picture. And please can you answer my question number 1 & 2?
(About how IR sensor work?)
Thanks.

rjenkinsgb said:

If the first transistor turns on fully

Click to expand...

The diode (IR sensor) is in reverse bias,

ArivNidunami said:

Ok, please ignore my example picture. And please can you answer my question number 1 & 2?
(About how IR sensor work?)
Thanks.

Click to expand...

The diode "sensor" that is connected to the base of a transistor has the cathode connected to a more positive voltage than the anode. This means that diode is reverse biased. Usually, very little current will flow (leakage current) when reversed biased. BUT, if light hits the diode, the diode will allow current to flow (up to about 0.5uA). Noteb5hat the photons hitting the diode must be at lest the energy of the pn-junction band gap.

you should use the symbol below for the photodiode, even if it is just an LED IR Emitter used as a photodiode.