An extra op-amp (A3) is used to provide acknowledgment to the user that the CRO has switched off after some idle time. For this, A3 is used in inverting-buffer mode. When output of A2 is high, output of A3 will be low, and vice versa.
When the CRO is off, output of A3 will be high. Due to this, the buzzer gets activated and produces a beep sound whose loudness gradually decreases within a short time. This happens because C3 initially acts as a short circuit and charges to +12V and blocks further current passing through the buzzer. Increasing C3 value makes the buzzer beep longer. This acknowledgement is like a communication between the user and the circuit. LED3 also flashes when the beeping sound is produced.
Auto and manual modes are provided by mode switch S1. When S1 is open, the circuit is in auto mode. With S1 closed, the circuit is in manual mode and bypasses the relay.
Construction and testing
A single side PCB for the vibration activated smart CRO probe is shown in Fig. 4 and its component layout in Fig. 5. After assembling the circuit on the PCB, enclose it in a suitable box. Connect 230V AC input across CON1 and CON3. Connect the CRO across CON4.
Fix the vibration sensor on top of the CRO probe as shown in Fig. 1(c). Use a highly-flexible and lightweight wire (like an earphone wire) to connect the sensor. Connect AC mains supply’s live wire L and neutral N across CON3.
Potmeter VR1 is calibrated in terms of time. 10.45V is the maximum C2 charging voltage when sensor is subjected to vibration. During calibration mark the minimum (one minute) and maximum (25 minutes) delays. Proposed front-panel dial is shown in Fig. 6.
Provide proper insulation between AC and DC voltages. Ensure all test points have voltages as per the table before using the circuit. Main DC voltage at TP1 is unregulated.