The simple LED oscilloscope circuit can be used for analyzing low frequency waveform through a 10 x 10 LED matrix display.
This process is accomplished by the IC 4017 outputs.Since the IC 4017 outputs generate a shifting logic high, and the IC LM3915 outputs generate a shifting logic low, all the anodes of the LEDs are joined with the IC 4017 outputs, and all the cathodes of the LEDs are hooked up with the IC LM3915 outputs.
This allows the IC 4017 outputs to work like a time base generator, while the outputs of the IC LM3915 works like a signal modulator.
The IC 4017 can be also imagined like a carrier signal generator as we have in a AM/FM transmitter waveform.
Now, as discussed earlier, the outputs of the IC 4017 will shift sequentially only in response to an oscillating pulse applied across its pin#14.
This is achieved by the IC 555 circuit which is configured as a astable multivibrator.
The oscillating pulse generated at pin#3 of the IC 555 is applied to pin#14 of the IC 4017, which causes the outputs of the IC 4017 to produce a sequentially running high logic from its pin#3 towards pin#11.
How Waveform Pattern is Generated on the LED Matrix
Now let's analyze what happens when an external waveform signal is applied across pin#5 of the IC LM3915.
The LEDs on the Y-axis are controlled by the LM3915 outputs, which indicate the amplitude of the waveform.
The LEDs on the X-axis are controlled by the IC 4017 outputs, which indicate the time base frequency of the oscilloscope.
The IC LM3915 detects the amplitude of the waveform, and generates a corespondingly shifting up and down low logic across the connected LEDs.Since the 4017 also supplies a shifting logic with some frequency, the up/down LED illumination on the Y-axis from the LM3915 outputs is swept across the X-axis, so that it corresponds to the time period of the waveform.
This allows a sweeping waveform pattern to be simulated on the 10 x 10 LED matrix.
The speed at which the outputs of the IC 4017 shift determines the direction of the waveform.
If the speed is less than the time period of the waveform, the LED waveform appears to move from right to left, and when the speed is higher than the waveform time period, the LED waveform appears to move from left to right.
This speed determines the time base frequency of the IC 4017 which is generated by the IC 555 astable, and can be adjusted with the help of the VR1 variable resistor.
Oscilloscope Adjustments
All oscilloscopes have three fundamental adjustments through potentiometers, the time base frequency adjustment, amplitude or the voltage scale adjustment, and Y-axis position adjustment.
These adjustments allow the waveform pattern on the screen to be correctly optimized for facilitating proper analysis of the waveform.
The discussed LED oscilloscope circuit also includes these three basic adjustment features.
The pot VR1 allows the adjustment of the time base frequency, across the IC 4017 outputs.
The pot VR2 adjusts the voltage level of the waveform on pin#5 of the LM3915, and thus helps to adjust the amplitude voltage scale of the waveform on the LED display.
VR3 helps to adjust the Y-axis shift on the 10 x 10 LED matrix display.
The switch SW1 can be pressed to disable the IC 4017 time base momentarily, preventing the horizontal sweeping of the waveform.