All-analog automatic night light circuit

Posted on Feb 7, 2014

Some ten days ago I installed an all-linear automatic night light circuit to control the lights inside the living room. Circuit consists of three op amps-two used as voltage followers and a third as a comparator. As the schematic shows, all user adjustable controls are at the comparator-with one potentiometer to set the trip point and the other

All-analog automatic night light circuit
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pot to adjust the width of hysteresis band. The sensor circuit is located several meters away at a location where it can have a view of the outdoors without being blinded or fooled by indoor lights at night. The high impedance sensor and low pass filter must have a unity gain buffer (voltage follower) within millimeters/centimeters. You can`t have it on the main control circuit board meters away. Trust me. I`ve tested this several years ago with and without a buffer and a (unshielded twisted pair) cable. The signal will be degraded and probably unusable without the follower. Notice the low pass filter includes a diode. It performs a very important and essential function. See my other automatic night light circuits for the explanation. Driving large capacitive loads can cause stability problems for voltage feedback op amps. As the load capacitance increases, the feedback loop`s phase margin decreases and the closed-loop bandwidth is reduced. This produces gain peaking in the frequency response, with overshoot and ringing in the step response. A unity-gain buffer (G = +1) is the most sensitive to capacitive loads, though all gains show the same general behavior. When driving large capacitive loads with these op amps (e. g. , > 100 pF when G = +1), a small series resistor at the output improves the feedback loop`s phase margin (stability) by making the output load resistive at higher frequencies. (DS21810F, 2008, p. 14) Well,...

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