Nuts & Volts - September 2005

Project

by Michael Simpson

Pool Timer

This Month’s Projects

Pool Timer . . . . . . . . 32

Cheap RC Car . . . . 37

Instant Replay . . . . . 42

The Fuzzball Rating System

To find out the level of difficulty for each of these projects, turn to Fuzzball for the answers.

The scale is from 1-4, with four Fuzzballs being the more difficult or advanced projects. Just look for the Fuzzballs in the opening header.

You’ll also find information included in each article on any special tools or skills you’ll need to complete the project.

Let the soldering begin!

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.

The Control

Figure 1. The old pool timer.

Figure 2. Duty cycle positions.

F o r

E l e c t r o n i c s

NUTS & VOLTS E v e r y t h i n g

32

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.