Design Criteria and Features

The Henrietta Project includes standard environmental control outputs for controlling a furnace, a fan and an air conditioner.

  • Standard furnace controllers operate on 24Vac, so the outputs must be 24 Vac.
  • Standard home controllers use the following connection format, so this is what the Henrietta output connector on the thermostat interface provides:








Fan (switched 24Vac)




Heat (switched 24Vac)




24 Vac Common




Air Conditioner (switched 24Vac)




24Vac Power




24Vac Cooling Power


  • To ensure the 24 Vac is isolated from the digital logic the outputs are optically isolated and use screw terminals to connect to the house thermostat wiring.
  • To switch 24 Vac, opto-isolated triacs are used. These solid state devices do not wear out or make clicking noises like electro-mechanical relays.
  • To ensure the optical interface does not load the PSoC4, logic level FETs are used to drive the opto-isolation LEDs. These input signals are connected on a 5 pin 0.1" header which is the standard pitch used for all other modules in the system.
  • Outputs are indicated with visible LEDs, one on each of the 3 signals
  • To avoid using a separate power supply for the PSoC4 and sensors, the 24 Vac power is rectified on this interface card and passed on to a DC-DC converter which supplies low voltage power to the rest of the Henrietta system. Standard controllers use a similar technique. The high voltage DC is brought out to a 2 pin connector which also uses 0.1" pitch, but it has a keyed latch to provide a secure connection and prevent reverse polarity connection.

TOC1a.pngTOC = Thermostat Optical Controller


The opto interface card was designed using Eagle Cad and fabricated using iMall (ITEAD Studio). Cost was less than $15 for 10 cards, including shipping. Total cost of a single assembled circuit card is about $7. I did not have the ideal LED colours in stock, so I just used all red LEDs. However, the Pioneer Kit does display the correct colour LED(s) for each signal.

To see the interface in action, the video linked here shows an android device controlling the furnace via Bluetooth:


And just for fun here is a video using voice to control Henrietta:


The opto interface card works well with no modifications.


Next Steps

I expect my next update to cover the light sensor and LCD backlight operation.


For more info on the Henrietta Project, which is entered in the Smarter Life Challenge, check these logs: