Need Thought on Slot Switch

[Razzel]

Built a slot switch to feed into the soundcard. It is intended to
calculate the period of a pendulum. 1) IT WORKS. but not well enough.
When the beam is blocked by a 1/4 in. wide barrier (in the 1 in slot) my
returned values are a couple in the low 120's, then jump to a run of 4-5
130's then back to 119 or 122 before jumping again to 130's. The
unblocked values are all over the place. 94, 61, 140, 210, 190, 88, 60,
60, 138, 218, ...
Nothing is hot. Its running off of a wall wart which is 9.12v unloaded.
I would expect if it is the wall wart problem then both sets of values
would vary similarly.
Both the LED (super bright) and the photoreceiver transistor (R/S
276-145) are fed from this source.
Any ideas why the values are out of whack???
I can show the circuit if needed but hopefully someone has a similar
experience. I think it must be CJD !
Raz

 

[Bob Masta]

Built a slot switch to feed into the soundcard. It is intended to
calculate the period of a pendulum. 1) IT WORKS. but not well enough.
When the beam is blocked by a 1/4 in. wide barrier (in the 1 in slot) my
returned values are a couple in the low 120's, then jump to a run of 4-5
130's then back to 119 or 122 before jumping again to 130's. The
unblocked values are all over the place. 94, 61, 140, 210, 190, 88, 60,
60, 138, 218, ...
Nothing is hot. Its running off of a wall wart which is 9.12v unloaded.
I would expect if it is the wall wart problem then both sets of values
would vary similarly.
Both the LED (super bright) and the photoreceiver transistor (R/S
276-145) are fed from this source.
Any ideas why the values are out of whack???
I can show the circuit if needed but hopefully someone has a similar
experience. I think it must be CJD !
Raz

I'm not sure I understand exactly what you are
doing, but if you are using a steady-on LED to
feed the sensor, then the sensor output will
be mostly DC with transients only on the edges.
Sound cards are AC-only devices, with blocking
capacitors cutting out everything below 20 Hz
or so (maybe as low as a few Hz on some cards
but definitely not DC). So the trick here is
to modulate the LED with a high frequency,
something in the 1 to 10 kHz range, say. Then
your sensor signal will be easily detected by the
sound card. The only hitch is that the time
resolution is limited by the square wave frequency.

Best regards,




Bob Masta
dqatechATdaqartaDOTcom

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