# Create a circuit that can flash IR LEDs at 38.5kHz and be turned on or off by microprocessor

Several other questions are posted about methods for flashing an LED, but one additional requirement I have is the ability to turn the circuit on or off with a micro controller (Netduino, 3.3v). This schematic is very close.

What changes would be necessary to achieve 38.5kHz? Also, how could I make an additional adjustment to start or stop the circuit via a pin on the Netduino?

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It sounds like you have an IR reciever that is tuned to 38.5kHz and you are wanting to transmit digital data to it? – Kellenjb Nov 19 '10 at 18:48

You can tweak the frequency with the 5k potmeter. I think you can get a pretty good range with that. If it's not enough, you could probably change the other resistor to something or a little larger value so the frequency drops.

To make it possible to switch the LEDs on and off I would probably a transistor (or a MOSFET) after the two leds. You could then connect the base or gate to your netduino with a 1k resistor or so. It doesn't start/stop the circuit, but it does start/stop the LEDs which I think is the result you want to go after.

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If you're using Netduino, maybe you can drop the 555 circuit and use software temporization. Here (http://www.netduino.com/projects/) we have two tutorials on how to blink the led and also how to turn it on and off using a button.

You will just need to attach the infrared led to an I/O port and map it in the software.

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 Thanks - I've tried to do this with the PWM pins but had trouble getting the frequency correct. – James Cadd Nov 19 '10 at 22:58 @JamesCadd, I have helped people do this, there was debug time in their software, but it was well worth saving the hardware in every case I have helped. – Kortuk♦ Nov 20 '10 at 16:23

Since you have a microcontroller around for switching the signal on and off it makes sense to let it do the dirty work as well. I'm not acquainted with Netduino, but I see that it's an ARM7 running at 48MHz, which makes it more than powerful enough for this. Set a timer to a 13$\mu$s period, and let it toggle an I/O pin on each timeout. The signal's period is then 26$\mu$s, giving a frequency of 38.5kHz with only 0.1% error. Duty cycle will be 50%.

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