Audible Logic Probe/Indicator Circuit

Logic indicator circuit / logic probe is used to identify logic level at any point of logic circuitry. The indicator can be visual (using LED, LCD, of 7 Segments) or auditory (using beeper or speaker). This circuit has a audible indicator, with the help of  a loudspeaker to produce indication tone.  Using the opamp,U1, as schmitt trigger, the Audio oscillator Q3-Q1-Q2 is isolated from TTL. Beside that, the op amp also acts as high input impedance inverter. The R8-R9 is used to set reference level at +16.6V which is a midway between low and high logic level. The Q3-Q2 will generate tone that indicate a high logic when the probe voltage is above +1.6V. If the probe voltage is below +1.6V then the  OP-amp output will saturate the Q1 and disables Q3-Q2 to cut off tone. Here is the schematic diagram of the circuit:

Earth Fault Indicator Circuit

This is design circuit that used to indicates the integrity of wiring connections. It shows all the mains connections Phase, Neutral and Earth connections are intact or not. The circuit is too small and can be housed in a three pin plug case. This is the figure of the circuit;

The circuit is directly connected to mains to monitor the status of the connections. Earth connection is a must in domestic wiring to bleed current to the earth if the metal body of a device is accidentally touched with the phase line. This circuit indicates

1. Red and Green LEDs ON Phase, Neutral and Earth OK
2. Red and Green LEDs OFF Phase or Neutral Break / Power failure
3. Red LED ON Phase and Neutral OK
4. Green LED OFF Earth line break

The circuit gets power supply through C1 and R3. AC Capacitor C1 reduces the high volt AC to a safer level through capacitive reactance. Resistor R3 limits the inrush current and R4 gives discharge path for the stored current in C1 when the circuit is unplugged. Zener diode ZD regulates the voltage to a safer level to protect T1 when it is off. Voltage across ZD will be a square wave by the working of C1 and the voltage level depends on the breakdown value of zener (9 volts). When a potential of 230 volt is present between the phase and neutral lines, T1 turns on during the negative half cycle of AC and Green LED lights indicating that Earth connection is intact. This is because the base of T1 will be biased by the potential difference between the phase line and earth. If the earth connection is not intact, T1 will not get base bias and it remains off. Red LED lights during the positive half cycle of AC due to the potential difference between the phase and neutral lines. Enclose the circuit in a 3 pin plug and connect points A, B and C to the phase, neutral and earth pins respectively. Plug it into the 3 pin socket to test the wiring.