The LED is sometimes there to act as a light "bias", so the circuit can be made more sensitive to ambient light. Light from the LED can be adjusted so that the usual light input causes the CdS cell to adjust its resistance such that (in series with a resistor) it produces some median voltage, like +2.5 volts. Then even small changes in ambient light can cause the circuit to produce a CHANGE of output signal that can trigger the desired activity. This CHANGE is generally measured by having a capacitor from the output coupled to a resistor tied to ground. Any AC change will impress itself on the ground referenced resistor. This signal is monitored, and when it exceeds a certain value a trigger signal is produced. A cheaper way to achieve the same end is to make the CdS series resistance variable. Cheaper but not better. To save a few pennies some of these "extra" features are left out of the final product, but the places for these components are still there on the pc boards. Fr. Tom McGahee ----- Original Message ----- From: David Lions To: Sent: Monday, August 20, 2001 9:58 PM Subject: Re: [OT] Crocking Frog, People sensor > I just pulled one apart and it uses a CdS. On the (small) circuit board > there is silkscreening for "CdS" and "LED", but there is no LED fitted. > > -- > http://www.piclist.com hint: PICList Posts must start with ONE topic: > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads > > -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics