Single Channel Controller for Reptile Enclosures

PCB is approx 10cm x 10cm dual layer. It features an 18F4620 which runs the code for a single pulse proportional thermostat that uses a PID routine to keep the temperature to within 0.2c of a programmable set point. A DS1307 RTC allows the unit to have a programmable temperature drop at any time of day. The unit has high and low temperature alarms, and a programmable timer for lighting.

Step 1: How it works

The temperature is read by a DS18B20 and it's data is sent to the PIC.  The PID routine running on the PIC then compares the value with the set temperature that the user has programmed into the unit (default temp on power up is 28C).  The PID routine then works out how long to apply power to the heater in order to reach or maintain the programmed set point.  The code also reads the time from teh DS1307 and checks to see if any conditions are met for the lighting to come on or off, or if a programmed night time temperature drop is required.  If such a condition is met then the lighting is turned on or off, or the tempertaure is lowered to match that set by the night time drop.  The code also monitors the current temperature and compares it to the min and max thresholds set by the user.  If these are exceeded an alarm is sounded to alert the user.

Control of the 240v ac mains is via zero crossing SSR, and protection is via 2A quick blow fuses.  The unit features a normal 16 x 2 LCD to display the time, set point, current temperature and alarm status.