shadfc

Hey all,
I'm trying to make a simple IR gate which produces a TTL (LVTTL would be preferable, but TTL is ok) compatible output and a medium switching speed (75us or less).
I found that the Fairchild QSE156 does just that so I got a few of them to test along with the paired emitters QEE123. Everything looked simple enough, but I cant seem to get the detector to do anything. Im running about 40mA through the emitter about 1.5 inches from the detector. The detector is hooked up to a 5V (+/-10%) source.
Now the problem is that no matter what I do, the Vout on the detector shows 4.6V. It doesn't matter if the emitter is off, blocked, or shining right on the detector from 1cm -- it's always 4.6V. I would expect (and I want) to see the Vout ranging from 0 to anywhere above 2.4V.

Any help would be appreciated. The datasheets for the parts I am using are at:
Emitter: http://www.fairchildsemi.com/ds/QE%2FQEE123.pdf
Detector: http://www.fairchildsemi.com/ds/QS/QSE159.pdf

thanks!
Jay

crutschow

Seems like it should work. You've tried more than one I assume?

I assume VCC of the device is connected to +5V, GND is connected to circuit common, and you're looking at Vout. Can you post a schematic of your hookup?

mneary

The QSE156 is 'high' whenever it sees IR. If the output is always high, it may mean that stray light is activating it.

The data sheets don't show any figures for operation at 1.5 inches (40mm); most of the characteristics are given for 4mm. Try it at 4mm with ambient light shielded. If it that works then begin testing, and see what breaks it.

crutschow

Since the normal QSE156 "high" output should be about 3V with a 5V supply it would seem there's either a circuit connection problem or a bad IC if 4.6V is measured.

ravix

Does a weak pulldown resistor attached to the output help?