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Timer circuit

Discussion in 'Electronic Repair' started by Martik, Jan 21, 2005.

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  1. Martik

    Martik Guest

    I want to build a timer, limited to 5 hrs, for my NiCad charger which runs
    on 120VAC. I understand that a 555 IC and a properly sized capacitor and
    resistor will provide the timing. How do I connect this to AC? Relay? Anyone
    have a schematic? Maybe it would be easier to build a decent charger that
    won't damage the batteries?
  2. Five hours is much too long for a standard 555 IC. You'd better buy a time
    switch. The simple mechanical ones can be set by the quarter. Switch it on
    by hand and place an off stop only at the time you want to swich off.

    petrus bitbyter
  3. Why not just use an appliance timer? Not the most high tech, but quite
    effective. See Red Scholefield's "R/C Battery Clinic" for lots of great info on
    batteries and charging.

    Alan Harriman
  4. Martik

    Martik Guest

    Problem is I have to keep changing the stop time, I've seen timers that have
    fixed times like 1 2 3 hours but they are $25
  5. Kevin R

    Kevin R Guest

    buy one a 24 hour normal plug in timer with pins you move, take out the on
    pins and only put a off pin at the required time
  6. NSM

    NSM Guest

    I got one that has little pins to select on and off to the half hour. It was
    free after rebate.

  7. James Sweet

    James Sweet Guest

    Wiring a 555 timer circuit to a relay is trivial, just use a small
    transistor like a 2N2222 and connect the output of the 555 through a
    resistor to the base, ground the emitter, connect the collector to one side
    of the relay coil and connect the other side of the coil to the power supply
    for the timer. A diode backwards across the coil to absorb spikes is a good
    idea too. You can also get solid state relays that you should be able to
    drive directly from the output pin of a 555.
  8. Ross Herbert

    Ross Herbert Guest

    As petrus has said, 5 hours is far too long for a 555 to handle. In
    order to get LONG delays you need to use some sort of a counter
    circuit. Have a look at 4th last circuit near
  9. Martik

    Martik Guest

    Thanks, I'll check it out
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