Telephone Controlled Night Light Telephone Controlled Night Light| DISCLAIMER: I\'ve collected these information from different sources. The author may revise this documentation from time to time without notice. THIS DOCUMENTATION IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. IN NO EVENT SHALL THE AUTOR BE LIABLE FOR INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES OF ANY KIND ARISING FROM ANY ERROR IN THIS DOCUMENTATION, INCLUDING WITHOUT LIMITATION ANY LOSS OR INTERRUPTION OF BUSINESS, PROFITS, USE, OR DATA. |
When the telephone rings, or when
the handset is lifted, the night light is turned on and remains on while the conversation
takes place. When the handset is replaced in the cradle, the light remains on for about 11
s. During standby conditions, the -28Vdc bias on the phone line maintains the output of U3
in a high state. When the ac ring signal is applied to the phone line, it is processed by
the ring detector U1, producing a negative output pulse at pin 2 for each ring. These
pulses trigger U2, causing its output to become high and the discharge transistor to turn
off. The high output of U2 activates optoisolator U4, which turns on the night light. Each
ring retriggers the timer and discharges C1, preventing it from reaching the 2/3 VDD
threshold level. Thus, the night light will remain on while the phone is ringing and for
about 11 s after the last ring. After 11 s, C1 will be charged to the U2 threshold level
(2/3 VDD) resulting in the U2 output returning to a low level and its discharge output
turning on, discharging C1. The lamp will turn off if the phone is not answered.
When the phone is answered, a 1-KW load is placed across the phone. This removes the
differential input to op amp U3, causing its output to become low, and capacitor C1 starts
discharging through R1. As long as the voltage across C1 remains low, timer U2 cannot
start its cycle and the lamp will remain on. When the phone is hung up, the low impedance
is removed from the phone line and the differential voltage across the line causes the U3
output to become high. This allows C1 to start charging, initiating the timing that will
turn off the night light.
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