🚀 go-pugleaf

Adam Seychell wrote:
> 
> I am trying build a photometer that will measure absorption of light
> from an high intensity red LED through a substance. Knowing that LEDs
> and photodiodes are quite temperature sensitive devices (in terms of
> luminescence vs temperature for fixed current) I designed a
> compensating photodiode circuit so the LED puts out constant
> brightness. So the arrangement is a LED light source that shines some
> of its light off a mirror to a compensating photodiode and some of its
> light to another photodiode which does actual measurements. My circuit
> works well and everything is mechanically stable, but the LED still
> remains temperature sensitive. The only conclusion I can come up with
> is that the distortion pattern emitted from the LED is temperature
> dependent. Intensity change alone is canceled out with the
> compensating photodiode so it must be some other effect. Does anyone
> have information about this effect with LEDs ?
> 
> So I'm wondering if my idea of using a LED for a photometer is such a
> good idea after all. I don't really want to do anything fancy like
> calibrated microprocessor temperature compensation and ADCs, including
> pulse sampling techniques to avoid LED self heating effects.

Run your photo diode in short circuited mode, into the inverting input
of an opamp, so that you amplify its current, without allowing any
voltage to pass through its leakage resistance.  This also gets rid of
the temperature effects on forward voltage.
-- 
John Popelish