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Need Thought on Slot Switch

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
Reply to
Razzel
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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 D A Q A R T A Data AcQuisition And Real-Time Analysis

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Home of DaqGen, the FREEWARE signal generator

Reply to
Bob Masta

Bob mentioned modulating the light beam. I'll add this: filter and regulate the output from the wall wart.

Ed

Reply to
ehsjr

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