Hi all,

As a simple reference followup on the optical sensing subject; one can see dialog
and a circuit to Chris's Chapmans "Differential Photo Detector", on John Lahr's extensive
subjects and excellent web site:

http://www.jclahr.com/science/psn/chapman/photo_detect/index.html

Meredith Lamb
Hi George, 

      To measure seismometer movements, you usually allow over +/-0.5mm total movement (or your seismometer becomes very difficult to set up) and you require to measure the arm position to maybe 20 nano metres for 'amateur' use. To do this you have to reduce / compensate / design for low drifts and minimum noise, both with time and with temperature - you are considering 1 part in 25,000. 
      You need two large area photo diodes (~7sq mm - VTD34?) connected to the inverting input of two low noise opamps TLC2201? with a suitable value of feedback resistors. You then subtract the opamp outputs with a differential opamp eg INA118 and apply bandwidth filtering. This reduces the effects of temperature on the photo sensitivity and on the leakage currents. Noise considerations in photodiodes require you to use a photo current of the order of 0.5 mA, which implies a high intensity light source. The photodiodes should be fixed to a common heatsink to minimise temperature variations. 
      It is possible to use light from one the high power, metal cased IR LEDs, but the photo output of a LED shows an exponential decrease as the temperature increases. This makes getting high stability and low noise a little bit difficult. Laser diodes tend to be very noisy. (You can buy laser diodes which have an internal photo diode to 'stabilise' the output.) An easier approach is to use a tungsten filament bulb in a feedback bridge circuit, which stabilises the hot filament resistance. You reduce the voltage on the bulb to < 0.8 x that rated. This gives an essentially infinite filament life. The dimmer filament doesn't effect the sensitivity as much as you would expect, since the sensitivity of Si photo cells increases in the near Infra Red. GaAs photocells may also be used with superbright visible orange LEDs. 'Ordinary' LEDs tend to be quite to very noisy. the superbright ones tend to be quieter. 
       You need the dimensions of the photocells to be large compared with the wavelength of light, to minimise interference fringe effects. The minimum conduction noise in a photodiode is proportional to the square root of the photocurrent, so increasing the photocurrent will give a lower overall noise. With a significant amount of heat being shone on the photocells + optical shutter, the detector needs to be near the top of the seismometer case, maybe in a semi isolated light box, to minimise heating effects and air currents. The bulb is best mounted outside the main seismometer case, to give adequate cooling by direct contact with the housing. You can make quite good windows using Microscope slides / cover slips. 
      There was a note on PSN some years ago which said that you couldn't use optical detectors, but this was a misunderstanding. If you try to use interference fringe methods, your resolution will be limited, commonly to a fraction of a micron.     

      Regards, 




Hi all,
 
As a simple reference followup on the optical sensing subject; one can see dialog
and a circuit to Chris's Chapmans "Differential Photo Detector", on John Lahr's extensive
subjects and excellent web site:
 
http://www.jclahr.com/science/psn/chapman/photo_detect/index.html
 
Meredith Lamb
Hi George, 

      To measure seismometer movements, you usually allow over +/-0.5mm total movement (or your seismometer becomes very difficult to set up) and you require to measure the arm position to maybe 20 nano metres for 'amateur' use. To do this you have to reduce / compensate / design for low drifts and minimum noise, both with time and with temperature - you are considering 1 part in 25,000. 
      You need two large area photo diodes (~7sq mm - VTD34?) connected to the inverting input of two low noise opamps TLC2201? with a suitable value of feedback resistors. You then subtract the opamp outputs with a differential opamp eg INA118 and apply bandwidth filtering. This reduces the effects of temperature on the photo sensitivity and on the leakage currents. Noise 
considerations in photodiodes require you to use a photo current of the order of 0.5 mA, which implies a high intensity light source. The photodiodes should be fixed to a common heatsink to minimise temperature variations. 
      It is possible to use light from one the high power, metal cased IR LEDs, but the photo output of a LED shows an exponential decrease as the temperature increases. This makes getting high stability and low noise a little bit difficult. Laser diodes tend to be very noisy. (You can buy laser diodes which have an internal photo diode to 'stabilise' the output.) An easier approach is to use a tungsten filament bulb in a feedback bridge circuit, which stabilises the hot filament resistance. You reduce the voltage on the bulb to < 0.8 x that rated. This gives an essentially infinite filament life. The dimmer filament doesn't effect the sensitivity as much as you would expect, since the sensitivity of Si photo cells increases in the near Infra Red. GaAs 
photocells may also be used with superbright visible orange LEDs. 'Ordinary' LEDs tend to be quite to very noisy. the superbright ones tend to be quieter. 
       You need the dimensions of the photocells to be large compared with the wavelength of light, to minimise interference fringe effects. The minimum conduction noise in a photodiode is proportional to the square root of the photocurrent, so increasing the photocurrent will give a lower overall noise. With a significant amount of heat being shone on the photocells + optical shutter, the detector needs to be near the top of the seismometer case, maybe in a semi isolated light box, to minimise heating effects and air currents. The bulb is best mounted outside the main seismometer case, to give adequate cooling by direct contact with the housing. You can make quite good windows using Microscope slides / cover slips. 
      There was a note on PSN some years ago which said that you coul