AC conversion and continuity buzzer test -
U1A LF353 opamp turns on a buzzer when the voltage
at Test + falls very low indicating a short
between Test + and Test - or a resistor less than 5
ohms. The R9-R14 divider determines the
minimum voltage that can be at Test + for buzzer to
just turn on. The Opamp here is a comparator and R10
10 Meg gives a very small hysteresis so that there
is no oscillation at threshold levels, that is when
both inputs at same levels.The Zener is for
protection and R6 to limit current.
U2A and U1B opamps form a precision rectifier, note
that this is not a true RMS rectifier circuit for
that see some devices from Analog Devices. When you
need to rectify a 200mV AC signal you cannot use a
diode like 1N4148 as the diode turns on at 700mV so
this circuit is used.
To view circuit below, Click the Link of PNG or PDF
and view the Circuit, PNG can be Scrolled with Mouse
and continuity buzzer test - del20012
The AC signal measurement is best to do with a True
RMS convertor. This Type is able to quantify the
Heating power of a AC voltage, includes all
harmonics and Noise too.
This circuit above is just a precision rectifier
with filter. It assumes a sine wave is being
measured and is calibrated for such a measurement. A
Triangle, sawtooth or square will give errors.
The Continuity tester is a Voltage comparator that
helps electricians see if the Wire or Switch is a
Open Circuit or Shorted. The Audible sound helps
quick sorting or check, sometimes a Flashing LED
will also help