Help edit my Temperature sensor circuit

Discussion in 'The Projects Forum' started by paddyhughes086, May 15, 2013.

  1. paddyhughes086

    Thread Starter Active Member

    Jan 30, 2011
    48
    0
    Hi I work in a school and we do go in training but it is very basic Electronics. We were given a circuit that was a temperature sensor that could be built 2 ways with an end result of a led lighting up.
    The issue is we need to have the last circuit that’s highlighted in yellow on the image attached run a fan and not a led when the thermisitor heats up the power goes normally to the led on the circuit we supply 9v into the circuit but the power come out were the led is connected is around 2.3v as any hire would burn out the led we need to have a minimum of 4v if not more come out at this point could any one suggest a few alterations to are project please thanks
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    You misunderstand the LED circuit. The LED is a current controlled device, not voltage controlled. The 470 ohm resistor in series with the LED limits the current to a certain amount (generally 20 mA or less).

    In order to run a motor, you may need to change the value of the 470 ohm resistor or eliminate it entirely. This depends upon the voltage you are using to power the circuit (which must be within the fan's acceptable range) and the current required by the fan. Also, you must consider the current available through the transistor.

    So, state the voltage you are using to power the circuit, the voltage and current requirement of the fan, and the number of the transistor you are using, and you will get more help.

    Never mind about the transistor number; I see from the parts list it is a BC548...correct?
     
  3. paddyhughes086

    Thread Starter Active Member

    Jan 30, 2011
    48
    0
    Hello thanks for the input
    the voltage for the circuit is 9v and the voltage and current required for the motor is as follows and the transistor is a BC548
    The specs for the motor are as follows
    Specification (1:200 ratio model),
    Based on a regulated 4.5V input:
    Operating temperature range -20C~+50C
    Humidity range 20% ~70%
    No load speed 39 +/- 15% rpm
    No load current 0.11A Max.
    Output torque 3.6 kgf.cm
    Weight 30g
    thanks for your help
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    There are a couple of issues. First, if the motor is limited to a maximum voltage of 4.5V, then you will have to reduce the supply voltage to about that level. Secondly, the BC548 is only rated to 100 mA, so it should not be used to power a motor that draws 110 mA with no load.

    There are options to overcome this, but it will require significant circuit changes.

    Is this actually to be used, or is this an academic exercise?
     
  5. CoHPhasor

    New Member

    May 13, 2013
    13
    0
    I would look at triggering either a relay or a MOSFET transistor that can handle the required current.
    Either way it won't take too much current to trigger, you might just have to change out the resistor feeding the LED and be set.
    (As these devices use a very small amount of power as a 'trigger' or 'switch' to allow much larger current cicuits to be completed)
     
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