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Subject: Digital circuit design problem

Date: 09/17/00 at 6:49 AM
Posted by: Shankar
E-mail: shanxmail@yahoo.com
Message Posted:

I'm doing a small project in college -- the circuit
uses an infra-red LED and a phototransistor to count
objects as they pass between the source and the sensor.

When light falls on the photoxtor, it saturates (using
a collector resistor Rc = 47K) and collector voltage
drops to 0V (there is no emitter resistor). When no
light falls on the xtor, it remains cutoff and collect-
or voltage is Vcc = +5V. The collector is connected to
the clock terminal of a decade counter (DM74LS90), so
whenever there is a collector voltage transition from
+5V to GND (due to blocking and then unblocking of light), the counter is supposed to count. However, this does not happen. I've checked the high and low
voltage levels (4.1V and 0.44V) and they're OK. The counter when operated separately works fine, but when interconnected, they don't.

My initial thought was that due to the high collector
resistance (47K), the counter was probably not getting
enough current drive, so I inserted an emitter-follower
between the photoxtor collector and the counter. It
still doesn't work properly.

Could someone please tell me why?

My email address is shanxmail@yahoo.com

Thanks in advance.

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