connecting to a 555

Discussion in 'The Projects Forum' started by lotusmoon, Jun 29, 2013.

  1. lotusmoon

    Thread Starter Active Member

    Jun 14, 2013
    I am new to electronics. I have built a 555 timer circuit to run an LED cluster but the output of the timer circuit is not enough to run the LED's so I am looking for an indirect way of connecting them. I have heard this can be done with a transistor, but am unsure how this works is there some material to look at for this?
  2. MrChips


    Oct 2, 2009
    #12 and lotusmoon like this.
  3. bertus


    Apr 5, 2008
    lotusmoon likes this.
  4. lotusmoon

    Thread Starter Active Member

    Jun 14, 2013
    Is there a big difference between a MOSFET and a NPN transistor as they have been both mentioned as a solution to my project enquiry
  5. wayneh


    Sep 9, 2010
    The biggest difference in your circuit is that a MOSFET requires almost no current on its gate pin to turn on, whereas the BJT (normal transistor) needs a base pin current up to about 10% of the current it is controlling. So if your bank of LEDs needs 5A, the base current of a BJT would need to be designed for 0.5A. That's more than the 555 can handle, so you'd need another transistor to help (a darlington configuration). So for switching big currents at low speed, think MOSFET.

    At very high frequency (>1? MHz), the capacitance of the MOSFET gate becomes an issue - you have to rapidly charge and discharge the gate capacitance and this can become a challenge relative to using a BJT. Also, a BJT can easily be configured to give a proportional or linear response to the base current. A MOSFET is far better at being either on or off, not in between.
    #12 and lotusmoon like this.
  6. lotusmoon

    Thread Starter Active Member

    Jun 14, 2013
    I have been looking to buy an npn for this circuit and am a bit confused by the spec.
    some of these I think i understand but have put them all in just in case

    "Maximum Collector Base Voltage 30 V" is this the maximum voltage for the base and collector, or the maximum basic voltage for the collector?

    "Maximum Collector Cut-off Current 0.0001mA" - is the minimum current for the collector to work?

    "Maximum Collector Emitter Saturation Voltage 1.5 V" - is this the minimum voltage for the collector and emitter to work?

    "Maximum Collector Emitter Voltage 30 V" - it does not work over 30 volts through Collector and Emitter

    Maximum Continuous Collector Current 0.5 A - must not run a over 0.5 A through the collector

    "Maximum Emitter Base Voltage 10 V" - maximum voltage through the emitter and base, or maximum base voltage through the emitter, or maximum basic voltage for the emitter?
  7. sheldons

    Well-Known Member

    Oct 26, 2011
    depending on how many leds in your cluster-if you want to increase the number within reason you can also use this op stage using a fet