J.E.Lawrie                                                     HINTS-CT

            Low-Voltage Indicator Circuit and LED positioning.
                        ( for description - see HINTS-21 )

                                                 Battery +ve
            _________________________________________________>
            |                      |
            |                      |
            |                      |
            >                      >
            <                      <
            > 220k                 > 4.7k
            <                      <
            |                      |
            |                      |                 +ve
            |___________b  c_______|__________________________
            |                                                |
            |        NPN   e                                 a
            >    transistor|                           LED      3mm Red
            <              |                                 k
            > 47k pot.     |        _________________________|
            <              |       |                 -ve
            |              |       |
            |              |       |
            >              |       |
            <              |       |          1/6 watt resistors
            > 22k          |       |
            <              |       |
            |              |       |
            |              |       |
            |______________|_______|_________________________>
                                                 Battery -ve

             OPERATION.
            The transistor conducts, shorting out the LED, until the
            voltage across AC is 4.4 volts and across BC (by prior
            adjustment of the pot.) is less than 0.6 volts when the
            LED lights.

                         Resistor values are not critical.

                           Max circuit current - <1.5 mA



            LED Positions :-


                Green                      Red
     ____________________________________________
                |                               |  @ represents LED.
       Slot  3  |  @   LED centres - 4 mm  @    |
                |                    down.      |  Positions are
     ___________|_______________________________|   important to avoid
                >  <                       >     <   difficulties in
                4 mm                        15 mm     fitting the LEDs.


Back to HINTS-INDEX, HINTS-21