# Infared led driver circuit needed?

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

1. ### curry87 Thread Starter Member

May 30, 2010
101
0
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 Moderator

Mar 24, 2008
20,772
2,540
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
101
0
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
770
90
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 Expert

Jul 17, 2007
22,183
1,728
430 Ohms is a standard E24 value of resistance.
Decade table of standard resistance values: http://www.logwell.com/tech/components/resistor_values.html

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.

Apr 7, 2010
770
90