38KHz or 40KHz signal generator

Posted on Jun 24, 2012

The small 8-pin PIC12C508 is pre-programmed to generate our 38KHz carrier frequency by simply pulsing I/O-pin GP1 (shown above). The PIC will generate either 38KHz or 40KHz, depending on the state of GP3 when power is first applied. If you connect GP3 to ground, then apply power to the circuit, the frequency will be 40KHz. This was implemented for people using the older 40KHz style infrared detector modules. The default is 38KHz with pin #4 (GP3) left floating or (not connected to anything), Pin #7 (GP0) is to inhibit (stop) the carrier output. When GP0 is connected to ground, the PIC will halt generation of the carrier frequency.

38KHz or 40KHz signal generator
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This is an optional control feature for power saving, and in most cases, un-necessary, but it's there if you need to use it. Ground pin #7 to inhibit the carrier frequency. Release this pin from ground to start the carrier frequency again. Connect GP1 (pin #6) of the PIC12C508 to pin #1 of the two-input NAND schmitt trigger as shown in figure 2. The data signal pin can be any I/O-pin from the BASIC Stamp or other microcontroller. By referring to the NAND truth table below, and the schematic above, you can see how the PNP transistor is held OFF by the logic "1" output of the NAND on pin #3 until both inputs #1 & #2 are logic "1". The 38KHz carrier generated by the tiny 8-pin PIC pulses from logic "1" to logic "0" at a rate of approximately 38,000 cycles per second, hence 38KHz. The serial data signal is 1200 or 2400 bps (bits per second), and considerably slower than the 38KHz carrier frequency (as shown below). You'll need a power supply that can deliver 5-volts @500mA for the circuit configuration shown in Figure 2. The Lumex OED-EL-8L, High-Power 180mW, 940nm Infrared LEDs will drop approximately 1.6V each. To figure total (peak pulsed current): 5V - ( 1.6V + 1.6V ) / 5.1-ohms = 352mA peak pulsed current.

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