For design goals, I wanted to: 1) have no
electrical connection to the dryer; 2) use an
inexpensive PIC microcontroller; and 3) minimize
wiring and build time, including debug time. To meet
goal 1, I decided that a current sense transformer
would meet the “no contact” criteria. For goal 2, I
decided to leverage the PIC16F687 due to its real
time clock-able timer, sufficient pins to keep the
programming and project pins mostly separate, and
low cost. Goal 3 is met by use of a solder
breadboard, the useful serial output of the
PIC16F687 for debugging, and other test features.
Figure 1 shows the main Dryer Minder printed
circuit board (PCB), ready to be mounted externally
from the dryer.
CAUTION: While the Minder has no electrical
connection to the dryer, it does require opening the
control panel which has hazardous voltages. Be sure
that the dryer is unplugged when installing the
portions of the project which reside in the dryer. For
my recent model GE dryer, removing the back panel
screws, tilting the control panel forward, and sliding
the panel to the side was sufficient. You Tube has
many instructional videos on how to open common
dryers, so finding out how to open the panel was trivial.
The “Aha!” moment of the project was realizing that I
did not need to sense the full dryer current, but instead
could sense motor current and use that to enable the
Minder. An economical current sense transformer slipped
over a motor power wire makes sensing the dryer run
The next task was to interface the current transformer
to the PIC. At typical motor current, the current
transformer provides about two volts of signal. A simple
voltage doubler circuit boosts this voltage, turning on a
transistor which acts as a switch. This is wired on a “sense
board” which is mounted on the dryer back panel in the
The main Dryer Minder board contains the control
PIC. To reduce power during standby, timer 1 of the PIC is
configured to run as a clock using an interrupt, where the
PIC is asleep most of the time and only briefly wakes up
each second to check for input. This had more reliable
results than using the interrupt on change abilities of the
PIC. While this does incur some standby power, this
allows the project to expand to other sensors. For
example, a future version may also sense from a humidity
sensor in the laundry room which will be helpful when
drying clothes with fan-assisted air drying.
Making the system easy to test is made possible by
four features: 1) the inclusion of a Heartbeat LED that is
on when the processor is not in sleep mode; 2) a Run LED
which is on when the Run condition is sensed; 3) a test
mode that will turn the beeper on in 20 seconds instead
of the usual 20 minutes; and 4) output of the Run count
(in seconds) to the PIC UART serial output for viewing on
a serial LCD display.
Power for the Dryer Minder is from an inexpensive
five volt cell phone charger I found at a local Goodwill
Construction starts with the current sense transformer.
Two wires approximately 12” in length are connected to
the outermost pins of the transformer, tightly twisted
together for noise immunity, and press-fit into a two-pin
connector (TE Connectivity); refer to Figure 2.
Next, wire the sense board. This is the circuit that
takes the voltage from the current sense transformer and
produces the switch output. The circuit diagram is shown
in Figure 3.
April 2014 29
; FIGURE 2.
; FIGURE 3.