Driving Infrared leds ?

[flak88]

If you use a pic to generate a frequency in the khz range on a Infrared led which is driven by a fet how do you calculate the input frequency on the gate required to compensate for the turn on delay present on the fet to output x frequency ?

For example if i wanted to have a modulate a infrared led at 38khz how would i calculate the fet gate input frequency required to achieve the 38khz output ?

 

[Diver300]

The frequency won't change. Even if there is a delay, that won't alter the frequency.

If the turn on delay is more than about 10 µs, then you won't get the LED turned on before it's time to turn it off. Delays beyond that make it more and more difficult to get the LED light to modulate but the frequency won't change whatever.

 

[crutschow]

If you drive the FET from a low impedance source to rapidly charge and discharge the FET gate capacitance, then it should readily switch the LED at 38kHz. Depending upon the FET gate capacitance, the PIC output may not have enough drive to do this fast enough. If so then you would need to add a driver for the FET gate.

 

[gilmanli]

Post the circuitry such that we can help you simulation the circuitry.

Best regards,
Gilman Li.

 

[Gayan Soyza]

Use a BJT instead of a FET ex:2N2222A