meter

The ICL7136 is the IC of the CMOS that the function to measure the voltage of the input correctly, the function to control the LCD(Liquid Crystal Display) and so on are housed in 1 chip. It is possible to measure the voltage to ±200 mV or ±2 V. The ICL7136 is the type which uses [...]

This frequency meter is suitable for sinusoidal wave and square wave input signals. Minimum input potential must be 25mV(rms). Measurement range can be determined from 10 Hz to 100 kHz by using the S1 switch. Each switch position part is calibrated by using 20 kOhm potentiometer. Total current consumption of the circuit is about 10mA. [...]

The circuit Bargraph Voltage Indicator used IC op-amp two quad voltage comparators to illuminate a series of 4 LEDs indicating volume level.
The op-amp used in this particular circuit is the LM339. It is a popular IC and should be available from many parts stores.
The 1K resistors in the circuit are essential so that [...]