ARDUINO OSCILLOSCOPE FOR PC

Low frequency oscilloscope, bandwidth is DC to 1000Hz. Max sampling rate is 30000 samples per second. Arduino Uno or Nano can be used. The input is to pin A0 of the Arduino which connected to PC via USB.
The PC app is in VB6, code and exe file are included.
The Arduino software is mostly in C code in purpose to speed up the ADC on expense of conversion accuracy. For lower frequencies a delay for each sample gives slower sweep. Input is DC or AC not acceding 5V.
The Analogue Comparator of the Arduino (pins 6, 7) is used to detect zero crossing of input signal, it triggers start of sampling the trace when signal is rising.
The screen is constructed from 62 sampling. The ATMEGA ADC reads the input at resolution of 10 bits and the MSB 8 bits are used and the dote position is calculated.
Resistors bias the input to a voltage of middle of the screen, use them if your input is AC.

ARDUINO OLED OSCILLOSCOPE

OLED low frequency oscilloscope, bandwidth is DC to 1000Hz. Max sampling rate is 16000 samples per second. An oscilloscope with these specifications has limited use, but it is a good exercise in using OLED display.
Arduino Uno or Nano can be used. Oled is 0.96" I2C SSD1306 driver.
The screen is constructed from 64 sampling. The ATMEGA ADC reads the input at resolution of 10 bits. The MSB 8 bits are used and the dote position is calculated.
Switch is on for input frequency below 100Hz.
Resistors bias the input to a voltage of middle of the screen, use them if your input is AC.

LCD TFT OSCILLOSCOPE

Updated 19.7.2020.
LCD TFT is digital display at its best, it displays versatile clear picture. The picture is built by entering the address and the color of a pixel or rectangle, so it's simple to use too.
LCD TFT low frequency oscilloscope, bandwidth is DC to 1000Hz. Max sampling rate is 16000 samples per second. An oscilloscope with these specifications has limited use, but it is a good exercise in using LCD TFT.
The circuit is powered by 5V. The power to PIC16F690 is 3.3V. you can use the 3.3V supply from the TFT Module or add an external 3.3V regulator.
The screen is constructed from 80 sampling. The ADC reads the input at resolution of 10 bits. The MSB 8 bits are stored in the pic RAM and then transmitted to the LCD RAM by 4 wires SPI.
The time selector has 100ms per screen, 10ms and the third position is about 5ms. The trigger is done by software and can be switched off.
LCD TFT modules vary depending on the drive IC. The module in the project has the driver ST7735. I bought this module directly from China because the price was more sensible.
Code was written on MPLAB and compiled with the free HI TECH C compiler V9.80
Software includes PIC code . You are free to use the circuit diagram and the software with no limitations.

Circuit Description
See also Technical Tips

PIC16F690
Pin 8 is the analogue input. Input voltage range is 0 to 3.3V absolute max, voltage outside this range can latch the input or even permanently damage the IC.
The clock is 8MHz by the internal osc.
3.3V to the pic can be connected to the 3.3V regulator on the TFT module, or external regulated 3.3V.
The module in the project has the driver ic ST7735.

Circuit Description
See also Technical Tips

PIC16F876A
Pin 2 is the analogue input. Input voltage range is 0 to 5V absolute max, voltage outside this range can latch the input or even permanently damage the IC.
The clock is 8MHz by the crystal osc.
The module in the project has the driver ic ST7735 .