Microcontroller-based automatic night light circuit


Posted on Feb 7, 2014

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 sufficie


Microcontroller-based automatic night light circuit
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nt 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...




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