Microcontroller-based automatic night light circuit

While discussing an all-linear automatic night light circuit I mentioned that I was testing an MCU-based ANLC. Well, I've been tweaking the firmware since. The other day I installed the sensor outdoors and have connected the circuit to control 220VAC loads.

Last night the circuit got fooled when clouds moved in at around 2am and reflected sufficient city lights and streetlights that the MCU thought it was already the beginning of dawn and switched off the load. This is because the software algorithm is such that a 10-bit ADC change of just 0x00A from the average dark ambient light level is interpreted as dawn. Competent programmer that I am, the firmware also includes a conditional which turns the load back on again should the light level drop back to the average dark level. And some half hour or an hour later (not sure because I went back to sleep before waking up again some time later) it did actually did return to dark level conditions. A sonalert buzzer is temporarily hooked up such that it sounds when the load (perimeter security lights) is off and as far as I know (sleepy head that I am) the next time the load was switched off by the circuit was around 5am--true dawn.

Earlier that same evening at around midnight while remotely monitoring the voltage output of the sensor ("ldr" in the schematic) with a DMM I was surprised to see the numbers climbing rapidly. The output had already settled at ~20mV but then increased quickly within minutes and peaked at ~310mV. I had to look out the window just to make sure I wasn't witnessing a circuit glitch but that in fact ambient light level was shooting up. And true enough the sky had turned orange and bright--from the cloud cover. The circuit did not turn the load off because the firmware was using a different hysteresis value at that time. Only after the load has been on for six hours does it use the hysteresis value of ten (the 10-bit ADC value mentioned above). By 2am this six hours had already elapsed.

Because of the detection sensitivity used, the light sensor circuit is located on the roof to minimize being affected by artificial lights. The LDR faces directly up (zenith) and is housed in a translucent plastic box. The box is shielded on the sides to prevent streetlights and lights from nearby buildings and billboards from unduly affecting the sensor. The very high impedance output of the sensor requires a unity gain buffer. The LDR used has a resistance of >30Mohms in complete darkness.

On the main control board, a low pass filter comprised of R_LPF and C_LPF (cut off frequency = 1/(2πRC) = 1/(6.28*100K*0.1uF) = 16Hz) attenuates high frequency noise which could be caused by EMI particularly those from lightning. A unity gain buffer is used to provide a low impedance signal for the MCU. High value load resistor R_L is used because in the event that the cable to the sensor breaks, the input of the LPF and buffer will not be left floating. In such an event R_L grounds the input, fooling the MCU to think that it's nighttime and thus turning on the load. This serves to alert the user (in the daytime) of a malfunction.

PB is used to select the light level at which load will be turned on. The user waits until the ambient light level is at the point where s/he wants the load switched on. At that moment PB is pressed. The circuit stores the light level in EEPROM and uses this value until it is changed. Pressing and holding down PB for a second resets the value to its minimum (I arbitrarily set it ADC value 0x064)--meaning load will be turned on only when it's really dark. LY is the indicator light for PB. It flashes briefly when PB is pressed (falling edge detected) and it blinks on and off for 1.5s when the trip level has been reset to minimum. A debouncing routine is used to, well, debounce PB, detect falling/rising edges, and monitor how long PB has been depressed.

REL1 and REL2 are mini relays with 24VAC coil. They are switched using a MOC3021 optoisolator employing a snubber circuit. REL1 and REL2 are at different locations. Using low voltage to control the relays translates to safety and perhaps even less expensive wires.