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Programmable Flexitimer

It's easy to build, easy to program and can be set to give to sequential time periods of up to 99 minutes and 59 seconds. What's more, you can program it for up to 99 cycles of the time periods.

By Jim Rowe

ELECTRONIC TIMERS have always been popular. In fact, there are so many applications for this kind of project that it’s only scratching the surface to mention the following: parking meter reminders, EPROM erasure timing, darkroom printer/enlarger timing, PC board etch timing, battery charge timing, industrial process timing, chess and other board game timing, debate timing and even kitchen timing.

Click for larger image
Fig.1: the circuit is based on a programmed PIC16F84A-04 microcontroller and this drives an LCD module, two relay driver circuits (one for Time A and one for Time B) and a piezo buzzer circuit. It's programmed using 10 pushbutton switches. Resistor RBL sets the current through the backlighting LEDs on the Altronics and DSE modules (see text).

If you want to make a timer with the widest range of applications, it needs to have at least two independently adjustable or "programmable" timing periods. Ideally, it also needs to be flexible in terms of the number of time periods and/or time period sequences (or cycles) that can be programmed.

Some applications need just a single time period, timed in "single shot" fashion, while others need a single sequence of one time period followed by a second (and probably different) time period. Still other applications may need a sequence of two time periods repeated many times, say for a total of 20 cycles.

Previous timer designs published in SILICON CHIP and other magazines have provided most of these features but at the cost of operating and programming complexity. This made them a bit daunting to use and limited their popularity as a result.

By contrast, this new timer is programmed in exactly the same intuitive fashion as a microwave oven. First, it has tens and units buttons to allow you to key in the exact number of minutes and seconds for the time period(s) you want and also the number of timing cycles you want. There are also buttons to start and stop the timer manually and to save its settings for the next time it’s used and so on.

We’ve dubbed this new unit the "Programmable Flexitimer", because it’s designed to provide the same order of flexibility as the Flexitimer projects described by Rob Evans in "Electronics Australia" back in the early 1990s. However, this new design is based around a programmed PIC microcontroller chip and, as a result, is much easier to "drive" than those earlier timers.

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