Automating dawn to dusk energy recovery ventilation

Light sensitive relay control for HRV

Recently, I installed a single room Energy Recovery Ventilator in the dining area of our house because I had become concerned about interior air quality in the winter when the windows are all sealed up with interior insulating storm windows.  One of its nifty features includes a light sensor that turns it down to the slowest fan speed when it is dark.  While it is 92% efficient, there is no good reason to ventilate a room when it is very cold at night and there is no one in the room.  I was impressed by this simple automation feature.

Back in 2008 I had installed a Heat Recovery Ventilator in my well sealed and super insulated workshop in order to get fresh air into my office in the winter when the windows are also closed up and sealed with my DIY interior storm windows.  I was running this HRV on a programmable digital timer and found that I needed to keep adjusting the settings so that it would turn on after sunrise and turn off around sunset.  It just occurred to me that I could build a device that would do this automatically based on the ambient light in my utility room that has a south facing window.  Being an electrical engineer who designs products for living, it was relatively simple for me to design a circuit for this application. The device is pictured above and plugs into an outlet just like a wall power adapter.  It contains a simple circuit that reads a photocell and activates an internal solid-state relay based on the light level set by an adjustable knob. 

Now my HRV automatically operates only from dawn to dusk, keeping the air in my office fresh while I am there and saving energy when it is not needed.

For the DIY inclined, here is a link to the schematic of the circuit I built.  I am considering making a small production run of these useful light triggered switches, so drop me a line if you have an interest and I will consider making a batch of them.  This is the opposite of the kind of device you would use to turn on a Christmas tree light at night and I am pretty sure there is nothing commercially available that turns on power when the room light gets brighter.  I built everything into an AC wall plug enclosure made by Polycase, but it could be built into a standard electrical box with an extension cord spliced into it.