I Used a .1 uf Cap but with Various Resistance Values and got Correct Frequencies on my Frequency Counter.
If you use a .01 Cap and a 5.8K resistor, you will come Close to your 19Khz.
Remember that Just because a Cap is Marked .01, Does Not mean it is that Exact Value.
Possibly use a 5K6 resistor in series with a 500 Ohm trimpot to Get the Exact Frequency of 19Khz.
Note the Supply Voltage MUST be Regulated at 5 Volts.
Or the Frequency will Change with Supply voltage changes.
Thanks for that. I use L7805 to stabilise the voltage at 5V. A 9V battery is used to power that.
The input signal is coming from a USB soundcard.
Here are the other components I used:
C3 (PIN1) = 0.02μF
C2 (PIN2) = 0.007μF
C4 (PIN3 - input signal) = 0.01μF
PIN4 - Supply Voltage (from L7805) = 5.07V
R1 (PIN5) = 5K8 (to be precise, multimeter shows 5831Ω)
C1 (PIN6) = 0.01μF (multimeter: 97.6 nF here)
PIN7 is GND
PIN8 - LED for testing. There is a 47Ω resistor between the cathode of the LED and the cathode of the 9V battery.
The circuit responds mainly to frequencies between about 16kHz and 19.5kHz, but also to those around 6-8kHz. Also, it's not very stable - a small change in the input signal volume (coming from the soundcard) affects the bandwidth greatly, but the F0 seems to fluctuate as well. I feel there may be a flaw in the circuit somewhere.
Here's what it actually looks like: