Transistor doesn't seem to be functioning right

Discussion in 'The Projects Forum' started by jordanwb, Jul 16, 2009.

  1. jordanwb

    Thread Starter Active Member

    Jun 22, 2008
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    I'm trying to make a circuit that operates like a car's signal light system using a 555 timer. I have the 555 working properly as I'd like. The signal coming out is ~4V, then goes through a SPST switch, which goes to the base leg of the transistor (NPN). Connected to the collector leg is a 9V DC source. The emitter goes to a 100Ω resistor, finally an LED, then to ground. All this works, the light flashes on and off as desired. However the voltage coming from the emitter doesn't seem to be right. I measured it with my voltmeter (bypassing the 555, constant 4.5V to the switch) and it's saying ~4V. Is 4 volts correct?

    I'll try and make a schematic if I can KTechlab working again.
     
  2. ELECTRONERD

    Senior Member

    May 26, 2009
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    Well then, we'll have to wait until the schematic comes "hot from the press."
     
  3. jordanwb

    Thread Starter Active Member

    Jun 22, 2008
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    All right I'll try and get ktechlab working. The capacitor is supposed to be 100 μf but when I try to change it ktechlab crashes.
     
    Last edited: Jul 16, 2009
  4. Audioguru

    New Member

    Dec 20, 2007
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    With a 5V supply, the output high voltage of an ordinary 555 (a Cmos one is different) is 3.8V. The output from the transistor's emitter is about 3.1V. The 100 ohm current-limiting resistor has about 1V across it if the LED is a 2.0V red one. Then the LED current is 1V/100 ohms= 10mA.

    If you apply 4.5V to the transistor's base then its emitter will be about 3.8V.
    The base-emitter voltage of a 2N3904 transistor is 0.72V at room temperature when its collector current is 10mA.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    You're using that NPN transistor as an emitter follower rather than a switching transistor.

    1) Move the 100 Ohm resistor and LED to between the collector and +9v.
    2) Add a resistor between the 555 timer output (pin 3) and the transistor's base; 220 Ohms to 1k Ohms will work fine.
    3) Connect the emitter of the transistor to ground.

    100 Ohms probably isn't enough to limit current through the LED.
    Calculate it as:
    Resistance = (Vsupply - VfLED) / DesiredCurrent

    Let's say you had a red LED that had a Vf (forward voltage) of 2v with 20mA current.
    Resistance = (9v - 2v) / 20mA
    Resistance = 7v / .02A = 350 Ohms. 360 Ohms is the closest standard value that is >= 350 Ohms.
     
  6. jordanwb

    Thread Starter Active Member

    Jun 22, 2008
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    So like this:
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, like that. But keep in mind that your 100 Ohm resistor is likely too low of a value to limit the current through your LED. You would be better off to start with a 360 or 390 Ohm resistor just to see if it works.
     
  8. millwood

    Guest

    a simpler approach would be to power the 555 from the 9v source and hook the led to 555's output directly (through a diode). that way, you save a transistor and a resistor.
     
  9. jordanwb

    Thread Starter Active Member

    Jun 22, 2008
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    Well I plan to use an incandescent bulb instead of a LED. What is the purpose of the resistor between the 555 output and the base of the transistor?
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    The resistor limits the current from the output of the 555 to the base of the transistor so that both do not blow up.

    The max allowed output current from an ordinary 555 is 200mA when its supply voltage is high or 100mA when its supply is low.

    But the base of the transistor needs only 1mA or 2mA.
     
  11. jordanwb

    Thread Starter Active Member

    Jun 22, 2008
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    I see. All right thanks.
     
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