switch an LED on from a TSSP58038

Using Arduino Sensors
1

Hi -

I would like some help with transistors and switching on an LED from a TSSP58038.

this is the transistor im using
Multicomp TO-92

and the IR Receiver

58038 receiver

Ive got the caps and the resistor on the receiver as per the spec sheet. When the receiver is not receiving a 38mhz signal - the out Pin is +0.3 v at 5ma if ive read the spec sheet correctly.
I want the Output from the receiver to connect to the base of the transistor - in order to switch on 5v to light an led.
Would just like to confirm that the transistor is fully saturated ....

so load in the collector is 5v 20ma.
gain from the datasheet is 100. So 0.2 mA for the base.
Because the output from the sensor is 0.3v at 5ma - it isnt hitting the Collector Emitter Saturation Voltage (0.7v max)....

My questions are:

  1. do i need a resistor on the base at all?
  2. how do i work out the saturation? normally would it be eg 5v-0.7v = 4.3v. 4.3 /0.001 = 4.3k resistor???
    my 0.3v - 0.7v = -0.4v...... and yet the led still lights up with no resistor on the base (i have a resistor on the led connected to 5v) - what are the implications?

thanks for any help

2

IR-receiver gets 5V. Internal output resistance 33k. If output is connected directly to base, U'll get a base current approx. 4/33k=0.12mA
Hfe>100 => 12mA LED current should be just fine => Series R=300Ohm+ for a RED led.

The voltage u measure isn't the same as a scope will show u

3

No real need for transistor....unless you need a really bright LED!

...you can put the LED+resistor (in series) on the output of the IR Receiver.

This IR receiver can drive up to 5mA on the output.

Connect other end of LED & resistor to GND or Vcc depending on when you need to LED to illuminate (idle or active IR NB: active Low only, see later posts)

4

to "AnalysIR". Take a look at the output circuit of tssop.. (I see 33k)
If LED on when resting =>> 5mA max =>> dim LED =>> use a transistor

5

to "AnalysIR". Take a look at the output circuit of tssop.. (I see 33k)

Yes, probably just when active low on the output, then. I will update my previous post.

6

liddlec7:
My questions are:

  1. do i need a resistor on the base at all?
  2. how do i work out the saturation? normally would it be eg 5v-0.7v = 4.3v. 4.3 /0.001 = 4.3k resistor???
    my 0.3v - 0.7v = -0.4v...... and yet the led still lights up with no resistor on the base (i have a resistor on the led connected to 5v) - what are the implications?

  1. Yes, when the 33k resistor in the sensor is too high.

  2. As long as you don't process high frequencies, driving the transistor into deep saturation is not a problem. When the output of the sensor can sink 5mA, any resistor down to 1k can be inserted between base and +5V, sufficient to drive a collector load up to 500mA (limited by the allowed Ic and power of the transistor).

Please note that the transistor is ON while the sensor detects nothing. Eventually you'll need another transistor as an inverter. You were better off with an pnp transistor and a 1k (min.) resistor between sensor output and base, load connected between collector and Gnd.

When the LED is sufficiently bright at 5mA, you can omit the transistor and connect the LED with a current limiting resistor (>=(5V-Vled)/5mA) between +5V and the sensor output.