AVR Freaks

The detector LED is not specific for the stated wavelength, that's just the max sensitivity wavelength. This is a typical response curve:

So, the IR detector can detect any visible light, in particular red at 70-80% of maximum efficiency. For this reason they are often protected by dark filters that let pass only IR light, so be sure to get unfiltered photodiodes if you decide to go that route.

Lee's light sensor would be a good approach.

Photocells are "old" cadmium devices, with a slow response time, (slow by todays digital signal standards...).

They are two lead devices.

These are basically light sensitive resistors, and are not polarized.

Photodiodes are improved photocells.

They have two or three leads and have a much faster response time.

They are polarized, (which lead is which matters).

Photo transistors are basically photo diodes, (transistors), with three leads, the third lead usually isn't needed and isn't used.

The third lead, (the base or gate of the transistor), can, sometimes, be used to pre-bias the operating point of the device, (generally not needed).

These look like small clear plastic cased transistor with three leads.

The basic concept is as shown in the photo.

It is analogous to using a thermistor to measure temperature.

You put the photodiode, (or photocell, or photo transistor), in series with another resistor.

You feed the "resistor divider" combination with Vcc, typically +5 V from your micro's Vcc.

You read the junction of the two resistors, and the light changes the resistance of the photocell / photo diode / photo transistor.

Change the resistance of one of the two resistors and the output voltage changes.

So, this is typically an analog output device, which you would feed to an ADC input.

The value of the fixed resistor in series takes some testing and experimenting.

The photo sensor might have a couple of Meg ohm resistance in the dark, and a few K ohms when lit up, for example.

You might just try the system with a 100 K ohm resistor and see what the output voltages are, in the dark, and in the light, (led on).

You can increase  or decrease the fixed resistor to move the output voltages up and down the scale.

Typical opto-isolators, similar to the above, are designed to output a high or a low output voltage, and can feed a digital input.

Your setup would best be read by the ADC, (without a lot of specific tweaking, otherwise).

You might, if you got lucky, be able to feed the signal into the Analog Comparator.

That, however, would take a lot of careful testing to account for the presence or absence of background lighting, and the value of the fixed resistor, and the delta voltage (with or without the LED being on), making the output voltage swing back and forth across the A.C.'s switching threshold.

Finally, the tricky part of this setup:

The background lighting in the room!

You will likely have to use black electrical tape to mask off the heater's display and shield your sensor from any room light.

Even the ambient light shining into the heater's display, and reflecting internally, can mess up your data readings...

The issue is that you will have a rather small delta voltage based upon the LED being on or off.

The sensor, however, will also detect the presence of sunlight through a window, and the other electrical lamps in the room.

Put an O'scope probe on the output signal and you can see the "flicker" with 2 x Mains frequency from the lights, which the human eye (generally) can't see.

You need to minimize that Mains signal and maximize the red LED's signal.

The entire output voltage will shift up and down as the background lighting in the room changes.

Hence the goal of masking off the background ambient light.

So, two components, analog wise, can't get much simpler than that.

But in fact the signal isn't clean, so some testing and tweaking will be needed.

Suddenly putting a thermistor on the heater itself, or a current monitor on the Mains power cord, or using Lee's suggested smart sensor, suddenly have merit.

Do you have a cheap O'scope, ($15 USD, Bangggod Electronics...), so "see" the output voltage of your sensor setup?

JC

Edit:

Note that the Photo Diode is "reverse biased" compared to hooking up a normal LED when one is using the LED in the normal manner, to make light.

This project was somewhat similar.

Two DS18B20 temp sensors taped (the red tape) to the hot water tank's exhaust gas flue so I could monitor exhaust gas flow.

The spare PCB was repurpose from another project.

The small LCD was care of MicroCarl.

This project required that I looked at the display, however.

It didn't have BT or WiFi connectivity...

JC