Building an IR Modulator: Troubleshooting Low Voltage Output

[jbord39]

Hey all. I am building an IR modulator but seem to have a small kink in the design. Here is my schematic:

Here is a picture of the oscilloscope, connected between ground and the output of the TL555 timer and the output of the second transistor (the output pulse going into the LED's).

The problem is that the transistor looks like it IS switching at 38kHz, but the off voltage is only in the low mV range. How could I make it so that this is actually modulating on/off?

Thanks for any help,

John

 

[waht]

The transistor is hooked in a common collector configuration which doesn't have any voltage gain.

You could replace the second transistor with a PNP, or move the LEDs up across Vcc and collector while grounding the emitter.

 

[jbord39]

So instead of putting the LED's between the second transistors emitter and ground, I would connect it from +5V to the collector, and connect the emitter to ground?

Thanks, I just want to be sure. Also, do you think this would increase the maximum current (Should I be careful to not burn out my LED's when switching)?

John

 

[waht]

So instead of putting the LED's between the second transistors emitter and ground, I would connect it from +5V to the collector, and connect the emitter to ground?

yes

Also, do you think this would increase the maximum current (Should I be careful to not burn out my LED's when switching)?

Yeah it would. One may place a current limiting resistor between 5V and the LEDs.

 

[vk6kro]

You probably need a circuit like the one in this post:


https://www.physicsforums.com/showpost.php?p=2612341&postcount=35"

As a general rule, never put semiconductors directly across a source of power either on their own or in series. They are quite capable of destroying themselves if you do that.
You always need a resistor in series with them to limit the possible current.