The second kit in the Tron Club basic line involves a variety of oscillator circuits along with a servo motor, shift register, and a myriad of other components. This article documents a few changes that helped me when working through the kits.
This exercise attempts to use a comparator, electret microphone, and high pass filter to turn on an LED when a high pitch is detected. I had trouble getting reliable results with this circuit so I modified it so that it made more sense to me. The idea here is that the signal from the microphone is high pass filtered so that low frequency signals will be small, leaving high frequency signals larger and easier to pick up with the comparator.
In this circuit, the output of the microphone is passed through a high pass filter with a cut-off frequency of 160 Hz. The filter attenuates any signals below 160 Hz, leaving higher frequencies untouched. Ideally, this frequency would be set higher (> 1 kHz) but this kit doesn’t contain the correct component values to do so. Next, the filtered signal is amplified by a gain of 20 with a transistor amplifier. Capacitor C2 then removes any existing DC offset before the signal is added to the DC offset of Vdd/2 created by R11 and R12. The end result is an audio signal centered around 2.25V that typically peaks around 3V. The threshold to turn on the LED is set by the voltage on the positive input of the comparator. With the pin pulled up through a 100k, the voltage is just over 4V. To fine tune the threshold, experiment with resistor dividers or use a potentiometer.
There is an error in the schematic for this exercise. The motor’s PWM input is shown connected to the base of the transistor. In order for the circuit to function properly, the motor’s PWM input needs to be connected to the collector of the transistor as shown below.
I struggled to get the comparator oscillator circuits from Exercises 10, 11, and 22 to work reliably as shown. Through a bit of trial and error, I found that adding a pull-up resistor to the positive inputs of the comparators in these circuits caused the oscillators to work correctly. My final circuit for exercise 22 is shown below which can easily be translated to the other circuits.
Any questions, comments, or feedback can be sent to us with questions through the TechForum.