Simple Light Sensor

AnalogKid

Joined Aug 1, 2013
10,987
The Radio Shack phototransistor and a light dependent resistor are two very different things. While both might respond to the same light level, the LDR has a much higher internal impedance. This limits the current through it to a value so low that the LED will not light very brightly, if at all. This is the reason the circuit in post #6 won't work. As noted above, what you want is a comparator to turn the sensor's variable output into a simple on/off signal.

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ErnieM

Joined Apr 24, 2011
8,377
cornishlad: I think the circuit in the link is incorrect and the comments pointing out the open collector and emitter pins are spot on.

It's possible the guy who wrote that page used a different chip but I'm not going to speculate any further on that.
 
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