Introduction

This circuit was built as an experiment to replace a non-functional controller board on a pellet stove. A "pellet stove" is a wood-burning stove that burns small pellets of compressed wood that are periodically feed in from a hopper by a screw auger. This circuit controls the time and duty cycle of a relay that in turn controls the motor that drives the auger.

Features

The auger motor cycles periodically at a duty cycle that is determined by the two pushbutton switches. A bank of eight LED's shows the setting. An over-temperature thermostat will shut the unit down.

Specifications

• The circuit drives a 5V relay coil, 15mA max.
• The motor relay cycles on from 9 to 72 seconds, then two minutes off.
• Overtemp input shuts motor off until reset.

Operation

There are eight "motor on" time settings in nine-second increments. Depending on how many times the "hotter" or "colder" pushbuttons is pressed, the motor can be on from a minimum of nine seconds to a maximum of 72 seconds. "Off" time is always two minutes.

When powered up, the first ("MIN") LED is always on and the motor "on" time is set to nine seconds. Pressing the "hotter' pushbutton will sucessively turn on additional LED's in the the bamk of eight LED's to indicate how many nine-second time periods are being used as the "on" time. The longer the "on" time, the more pellets are feed into the stove, and the hotter it burns.

If the overheat input (overheat switch #1) is grounded by an external thermostat (whose contacts close on high temperature), the motor relay goes off permanently and the eight LED's flash at a one second rate. When the overheat condition is removed, normal operation may be restored by pressing either button, at which time the heat setting starts back at "1" (one LED on, motor time is nine seconds.)

NOTE: It is necessary to equip the pellet stove with a second "overtemp" thermostat (overheat switch #2), strategically located, connected in series with the auger motor, that will open on a high temperature condition.

Technical Reference

HARDWARE

• The motor times in this experimental device are based on observations of a particular stove made by Vermont Castings. Your stove characteristics may vary.
• If the relay coil current is 15mA or less, you might be able to omit the 2N3904 transistor driver and drive the relay directly from pin #2 on the PIC. If the relay is on the insensitive side, you may need to use a darlington transistor or circuit.

SOFTWARE

• This is simply a "large loop" architecture in which the timing loops are nested to allow the pushbutton switches to be checked fairly often (every 10mS.) No interrupts were necessary for buttons or timing.
• For experimentation, the watchdog could be used as an additional measure of safety.

CORRECTIONS OR UPDATES

• v1.0 - first release