Infared led driver circuit needed?

Discussion in 'General Electronics Chat' started by curry87, Nov 24, 2010.

  1. curry87

    Thread Starter Member

    May 30, 2010
    Looking for a driver circuit to drive a infa led (tsus540)at 100ma 1.7vf using a input frequency of 38khz at 50% duty cycle from a pic as the pic cant supply this much current im stuck between using a fet or a trans?
  2. Wendy


    Mar 24, 2008
    Yes, you will have to use a transistor. The requirements are pretty trivial though, so what is the question? Are you needing help designing the circuit?
  3. curry87

    Thread Starter Member

    May 30, 2010
    Sort of yeah, not sure which i should use a fet or trans sinking or sourching the infa led all very confusing?

    Also how do I calculate the difference between an input frequency in to the base/gate and the output frequency does this change much between pnp,npn ?

    How does placing resistors between a pwm source and the device affect the voltage/frequency etc of the pwm signal ?

    How do i calculate the reqired base resistor if im inputing a pwm signal into it?
  4. Jaguarjoe

    Active Member

    Apr 7, 2010
    You can use a garden variety 2N3904 or 2N2222 el cheapo transistor.

    The output frequency will be identical to the input frequency. If its not then you have invented/discovered a new device previously unknown to mankind!

    Placing a resistor between the PIC and the transistor does nothing to the frquency. It also does nothing to the PIC's output voltage as long as you don't consume excessive current (more than 25ma).

    The base resistor is determined by the collector current and the PIC output voltage:

    Ic= 100ma
    Hfe or Beta= 10 (from the data sheet)
    Ib= 10ma (Ic/Hfe)
    Vbe= 0.7v (common silicon diode Vf)
    PIC output voltage= 5v

    Rbase= E/I= (5v-0.7v)/10ma= 4.3V/10ma= 430 ohms.

    Use a standard value like 470 ohms. This gives you a tad less than 100ma but is also a tad easier on the PIC. 390 ohms would give you a tad more current along with a tad more work for the PIC.
  5. SgtWookie


    Jul 17, 2007
    430 Ohms is a standard E24 value of resistance.
    Decade table of standard resistance values:

    You will also need a resistor in series with the LED to limit maximum current.

    Rlimit >= (Vcc - (Vf_LED + Vce_sat)) / Desired_LED_Current
    Vf_LED = 1.7v
    Vce_sat ~=1mV
    Desired_LED_Current = 100mA
    Plug your numbers into the formula, and calculate Rlimit; then find the closest standard value >= than what you calculated.

    Then calculate the power dissipation for Rlimit; double it for reliability, and select a suitably rated resistor.
  6. Jaguarjoe

    Active Member

    Apr 7, 2010