by Michael Simpson
Pool Timer . . . . . . . . 32
Cheap RC Car . . . . 37
Instant Replay . . . . . 42
To find out the level
of difficulty for
each of these
projects, turn to
The scale is from
1-4, with four
the more difficult
projects. Just look
for the Fuzzballs in
the opening header.
You’ll also find
in each article on
any special tools
or skills you’ll
need to complete
Build Your Own Pool Timer and Let
it Do the “Dirty” Work!
About a year ago, when I created a
floating pool light and shared it with
readers, I received a number of
positive responses. Around the same time, I
also developed a pool timer. I’ve since refined
the project and developed a new prototype,
which I’ve outlined here.
Shown in Figure 1, the first timer had a
clock-like interface and various buttons for
controls. You set each hour you wanted
the pump to turn on. The yellow LED in the
middle was the AM/PM indicator, so you had
24-hour control over the pump.
Although the timer worked, it was a bit
too complicated for anyone else in the family
to use. I promised my wife that, next year, I
would build a simpler timer.
As I used both this timer and others, the
one thing that surfaced was that turning on
the pump was more a factor of duty cycle
than actual time of day. For instance, I found
that, if the pool needed extra cleaning
because of a storm or heavy use, I would set
the timer for “ON 2 hours and OFF 1 hour.”
Once the pool was cleaned to my satisfaction,
I would drop the timer back to “ON 1 hour
and OFF 2 hours.” What I really needed was a
timer control that would let me set a long-term
duty cycle for the pump over a three- or four-hour period.
Let’s take a look at a few requirements for
the new timer.
• Ability to control duty cycle
• Simple control system
• Self-contained (No AC adapters)
• Ability to override the timer
• Ability to reset the timer
My first prototype used two knobs. One
knob was used to control the on-time, while
the second knob was used to control the off-time. While this worked, it required fiddling
with two knobs to override the timer. I also felt
that I could make the interface simpler.
Eventually, I ended up with a single-knob
control system. The only indicator was a red
LED to denote the status of the control relay.
The actual position of the knob gave the only
feedback needed to control the timers.
Figure 1. The old pool timer.
Figure 2. Duty cycle positions.
NUTS & VOLTS
Because the knob
is the only control, the
timer is very easy to set.
By placing the knob
indicator in the middle
position, the pump is on
two hours and off two
hours. By moving the
knob in either direction,
I could change the
amount of on- and off-time in proportion to the
position of the knob.
Figure 2 shows
how the various positions affect the duty
cycle. Placing the knob
all the way to one side
or the other let me
override the timers