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A new PIC-based Flexitimer Mk.4

Here's a new and enhanced version of a very popular project: an easily-programmed low-cost electronic timer module. It's compact, easy to build and offers a choice of either a single on period or continuous on/off cycling with independently programmable periods.


Main Features & Specifications

  • Operates from nominal 12V DC, with low current drain: <50mA when relay is on, <5mA when relay is off. Relay status indicated via a red LED.

  • Outputs via the contacts of a DPDT relay (ie, 2x normally closed, 2x normally open), with 5A contact rating.

  • Jumper link selection for either a single ON timing period or continuous ON/OFF cycling.

  • Relay ON and OFF times separately programmed via jumper links for any of 54 different time periods; ie, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 40, 50, 60, 70, 80, 90 seconds, minutes or hours.

  • Timing accuracy is ±1% at all settings.

  • Timer may be restarted at any time by pressing a reset pushbutton.

  • Module fits inside a standard UB3 utility box.

  • In the March 1991 issue of "Electronics Australia", Rob Evans presented the design for a "cheap and cheerful" electronic timer module called the "Flexitimer". It could be programmed using a set of wire links and also by changing the value of the timing oscillator components, over a range from a few seconds to approximately one day. It could also be set for either one-shot or continuous on-off cycling, although the on and off times were always the same.

    This simple, low-cost circuit offered a great deal of timing flexibility and as a result, it became extremely popular. An updated design was published a few years later and the parts retailers sold kits of both this and the original version for many years.

    It was partly because of the popularity of the original Flexitimer that we
    subsequently developed the Programmable Flexitimer, described in the August 2005 issue of SILICON CHIP. It was based on a PIC16F84A microcontroller and was programmed rather like a microwave oven, using a set of pushbuttons and a small LCD screen.

    It worked well but in many ways it was "overkill". It was rather expensive and as a result, it hasn’t been anywhere near as popular as the original Flexitimer.

    Despite that, it’s clear that many people still want a timer module that’s low in cost and just as easy to program as the original Flexitimer but which offers even more flexibility. For example, many people want independently programmable on and off times, as well as a considerably wider range of programmable times for each.

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