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Light switch with Time-out

Discussion in 'Electronic Basics' started by Martin, May 27, 2005.

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  1. Martin

    Martin Guest

    My two year old son is fascinated with light (and Fan) switches.
    He will happilly switch one on and off for hours.
    (well, for longer than I am willing to hold him up to the switch

    He has a plastic house he plays in in the yard.
    I want to wire it with a light switch, 7.2 volt Nicad battery
    (because I have several batteries and a charger) and a
    6 cell flashlight bulb.
    That would be easy ... but then if he leaves the light on and goes to
    play elsewhere, the battery will die.

    So I want to build a circuit that will work as a normal light switch,
    but if the switch is not opperated for a minute or two will turn off
    not waste any power. I have developed something I think will work

    FET1 |
    S1 __ ||-+
    SPDT o--------------------------| \ ||<-
    SWITCH | D1 | )o-----||-+
    | o---->|---o----|__/ |
    V+ ---o | __ | | |
    __--o---o---| \ | ___ | NAND2 |
    GND ---o | )o--o--|___|--o |
    o---|__/ | |
    | R1 | .-.
    | NAND1 --- LAMP1 ( X )
    | --- C1 '-'
    | | |
    | | |

    V+ GND GND

    (created by AACircuit v1.28.6 beta 04/19/05

    As the switch is opperated, it produces a (slow) pulse train.
    NAND1 (1/4 of a CMOS schmidt trigger NAND) is wired as an inverter.
    R1 and C1 form an RC timer enableing NAND2 when high.
    D1 (small signal diode) makes the recovery of RC pretty much
    Fet1 is on when NAND 2 is low.

    So if S1 is low, NAND 2 is high (light off)
    NAND 1 is also high, charging C1 rapidly through D1

    if S1 is now changed to high, NAND2 goes low, turning on FET1.
    NAND1 also goes low, discharging C1 through R1.

    If S1 is switched to low again the cycle repeats.
    If S1 is not switched, R1 will discharge C1 to below the Schmidt
    threshold of NAND2, and NAND2 will go high, turning off the light.

    I am not very familiar with FETs, so my questions are about the FET

    I believe I will need an N-channel FET to turn it on when NAND2 is low.

    Also, If I get a logic level FET, I would be able to run the circuit
    anywhere from 5 to 12 or so volts as long as I use the correct

    If anyone has any better suggestions for implementing this, please feel
    free to suggest them as well.

    Martin Bakalorz
  2. Bob Monsen

    Bob Monsen Guest

    Use a P-MOSFET, and it'll turn on when the output of NAND2 goes low. The
    manufacturers publish the max voltage that you will need to get between
    the gate and source (for your p-mosfet, that is your supply).

    You can also use a darlington PNP bipolar transistor, with a 1k resistor
    from NAND output to base lead.
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