need help with circuit

Discussion in 'General Electronics Chat' started by 3dOptics, Jun 1, 2009.

  1. 3dOptics

    Thread Starter Member

    Mar 15, 2009
    I am having problems with the following circuit I built. I am getting +5v from my computers molex 4 pin power supply connector red wire. I have Pin 3 on the circuit hooked up to pin 3 on my computers serial port. I have Pin 5 hooked up to both molex power supply ground black wire and to pin 5 on the serial port.

    I am using the transistor here:

    The Infrared led here:

    I am using a program called WinLirc that sends a signal to pin 3 on the serial port which results in the led blinking for half a second every time I click a button. I have a webcam that allows me to see the infrared led flash. I click the button, led will flash, wait a second, repeat. The problem is every once in a while the led does not flash, it happens randomly. I am certain the problem is not the software or the serial port, it has to be something that is wrong with the circuit.

    I measured voltage from the power supply red wire to the black ground wire it is +5.18v. I removed the led from the circuit and measured the voltage from the collector pin on the transistor to the pin on the 220 ohm resistor and it measures 7.08v. Every time I click the button on the program the voltage on my meter goes up about .20v and then drops back to 7.00v. I put the led in the circuit and connected the meter to each of the led's pins. The voltage reading is around 0.68v. When I click the button the voltage between led pins sames about the same.

    Before I put the 3904 transistor in the circuit I tested the Hfe with my multimeter and it measured 160. Later, I took it out of the circuit and tested it again and it measured 74. Why is the Hfe dropping? Any idea why the circuit is not working correctly.

    Last edited: Jun 1, 2009
  2. bertus


    Apr 5, 2008

    I think the voltage on the rs232 port is the problem.


    The picture comes from this page:

    The rs232 signal will go negative and positive (upto 25 Volts, 12 Volts in PC most times).
    You will have to protect the transistor for the high and negative voltage.

  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    yup, the rs232 signal is the problem, you have to clamp the negative voltage with proper zener diodes and use a resistor in series with the base to limit base current to a safe value, without base resistor the high voltage is killing the tr.

  4. 3dOptics

    Thread Starter Member

    Mar 15, 2009
    I replaced the infrared led with a different led that uses less voltage and now it flashes every time. So I guess the problem was that the led was not getting enough voltage or there was something wrong with the infrared led.

    What value resistor and zener diode would you recommend?