Building an IR Modulator: Troubleshooting Low Voltage Output

In summary, the conversation revolves around building an IR modulator and a schematic with a common collector configuration. The issue is that the transistor is only switching at a low mV range. The solution suggested is to replace the second transistor with a PNP or move the LEDs across Vcc and collector while grounding the emitter. Additionally, a current limiting resistor is recommended to prevent damage to the LEDs. It is also advised to never put semiconductors directly across a power source without a resistor as it can cause damage.
  • #1
jbord39
74
0
Hey all. I am building an IR modulator but seem to have a small kink in the design. Here is my schematic:

Toyschematic.png


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).

DS0004.jpg


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
 
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  • #2
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.
 
  • #3
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
 
  • #4
jbord39 said:
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.
 
  • #5
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.
 
Last edited by a moderator:

What is an IR modulator?

An IR modulator is a device that converts an electrical signal into an infrared signal, which can then be used to control electronic devices remotely.

Why is the voltage output of my IR modulator low?

There are several potential reasons for low voltage output in an IR modulator. Some common troubleshooting steps include checking the power supply, ensuring proper connections, and testing the components for any faults.

How can I fix low voltage output in my IR modulator?

Depending on the specific cause of the low voltage output, there are a few potential solutions. These may include replacing faulty components, adjusting the power supply, or repairing any damaged wiring.

Can I use a multimeter to troubleshoot my IR modulator?

Yes, a multimeter can be a useful tool for troubleshooting an IR modulator. It can help identify any faulty components or connections that may be causing the low voltage output.

Do I need any special tools to build an IR modulator?

The specific tools needed may vary depending on the design of the IR modulator, but in general, you may need a soldering iron, multimeter, wire strippers, and various electronic components. It is also important to have a basic understanding of electrical circuits and safety precautions when working with electronic devices.

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