1
\$\begingroup\$

I have created a prototype photo-detector circuit to use in spectroscopy. The micro-controller code take 120 A/D readings and then returns average.

I have noticed that when I use visible light (normal 12v DC lamp), all 120 samples are same (some deviates by +1 and -1). However, using a Deuterium lamp, the samples are sort of rising and falling.

This is the snapshot of the values (Blue when Gain = 0 , Green when Gain = 4) from DC source lamp at 12V DC DC Source Reading

This is the snapshot of values from Deuterium lamp (many samples to see the period)

enter image description here

I have noticed that when my Photo-detector circuit is exposed to normal AC light (India standard:230 volt, 50 Hz), then also I see some rising and falling pattern.

*I don't have access or understanding of Deuterium lamp supply.

*I have built a Monochromator to identify and seek to wavelength of my choice which detects the peaks correctly because of averaging.

\$\endgroup\$
1
\$\begingroup\$

Deuteurium lamps (I have one kicking around here somewhere) are discharge lamps, and as such the brightness is going to be dependent on the arc stability and the supply current.

You may well be seeing variations or aliasing of variations (probably at 100Hz full wave rectified mains-frequency) ripple on the deuterium lamp supply. Aliasing is related to your microcontroller sampling frequency beating with the ripple on the lamp.

I suggest connecting your photodetector to an oscilloscope rather than the micro ADC to observe the magnitude of any ripple directly. Any oscilloscope will be able to display 100Hz waveforms accurately. Use AC coupling if necessary, but it may be bad enough you won't have to. If you don't have access to an oscilloscope, try to get ahold of one first (it will give you lot of information) and if not, try to take samples at a known rate and compare with mains frequency.

\$\endgroup\$
  • \$\begingroup\$ I will use oscilloscope over the weekend. Thanks for the information. However, I am a bit confused. The D2 lamp and Visible lamp use same power supply and both were ON during my samples. There is a quartz mirror to focus either of these on the photo detector. So if the wave in the samples is due to power supply, should'nt it affect both samples? What can be done to correct it (or do I need to correct it in the first place)? \$\endgroup\$ – Anuj Purohit Mar 13 '15 at 4:55
  • \$\begingroup\$ If there is not a proper circuit to supply the Deut lamp with smooth current (filter + current regulator) then it will act like an old-fashioned fluorescent lamp and flicker. Incandescent lamps have a hot filament and the lamp filament doesn't cool down that much during the cycles so they act as a low-pass filter. You may still see a bit of ripple on a 'scope- especially with a relatively low power high voltage bulb. \$\endgroup\$ – Spehro Pefhany Mar 13 '15 at 4:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.