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Where are the daylight SMD photodiodes (or does everyone use an LED?)

ACharnley

Member
I've a circuit using a hole-through LDR. it works fine but for assembly reasons I've prefer to switch it to a photodiode. There'll be a pull-up resistor and it'll go straight into an ADC which I can control the timing of to give suitability for 10k or 100k impedence. 100k should avoid needing a phototransistor (and avoid the non-linearity).

So in my quest to find a common, reasonably priced, daylight based SMD photodiode I've ran out of luck. Any suggestions?

If I go with using an LED, as long as the LED isn't a paralleled device internally a larger footprint would be more sensitive, correct? Say a 1206 over a 0603.
 

schmitt trigger

Well-Known Member
There are trade offs depending whether you use the photodiode in photovoltaic or photocurrent mode.

Suggest that you become acquainted with both and decide which better suits you.
 

ACharnley

Member
I've been playing around with high output red led's which I read are the most sensitive to daylight. I've found they work well in good light but in dim to dark conditions there's very little voltage produced, in the 10mV's region. It could be the LED's as I have yet to test some others but therein lies some additional risk. Perhaps sticking to the LDR is best.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Apparently a LED should also work in photoconductive mode, as long as the bias voltage is less than it's reverse rating.
I've never tried it, but it may give better sensitivity at low levels, as you can use as high a load resistor as you need?

If you need to sense light levels at high illumination as well, you could add a diode in series with a lower value resistor across the fixed high value load, to give a two-slope effect.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I've been playing around with high output red led's which I read are the most sensitive to daylight. I've found they work well in good light but in dim to dark conditions there's very little voltage produced, in the 10mV's region. It could be the LED's as I have yet to test some others but therein lies some additional risk. Perhaps sticking to the LDR is best.
If it ain't broke, then don't fix it :D

LDR's are better than photodiodes for lot's of applications - making a large complicated photodiode circuit to try and replicate the performance of an LDR (and doing it fairly poorly) seems a silly idea. Likewise, trying to replace a photodiode in a photodiode circuit with an LDR is just as silly - both have advantages and disadvantages, use the correct one.
 

ACharnley

Member
rjenkinsgb -> I've found the opposite, sensitively in low light is very poor. In good light it is useable.

Nigel -> I'd save on manually soldering them into a smd PCB, that's it.
 

ACharnley

Member
No, that's directly to a multimeter. No bias resistor. :)

When you say bias resistor do you mean a voltage divider with the LED (reversed) in parallel to the bottom resistor?
 

Visitor

Well-Known Member
Might want to take a look at this story about using LEDs as light sensors. It seems LEDs are too directional to make decent ambient light sensors.

 

ACharnley

Member
The red SMD 1206 led I'm using doesn't have a diffuser on it so should be wide enough angle.

In good light I'm getting a read of ~0.7v.

In dim light I'm getting a read of ~0.04v

In darkness it's ~0v.

A bias resistor to a reference of say 3v would be blocked by the LED so would always read 3v to the ADC. I assume when rjenkinsgb mentioned it he meant a second resistor to gnd, so nominally the ADC sees say 1.5v...?
 

Visitor

Well-Known Member
The article linked in the Hack-A-Day article I posted above does provide detailed discussion of the best ways to use an LED as a light sensor, with circuit diagrams.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
No, that's directly to a multimeter. No bias resistor.
Connect the LED and resistor in series; LED from 5V to resistor, resistor to ground.

With the LED reversed, so it works as a photoconductive sensor.

Then see what voltage you get across the resistor, using the highest resistor value compatible with your input circuit.
It may work well - or it may be terrible - but the only way to be sure is try it!
 

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