Instant voltage transients are common in the power lines especially in places where machinery is operating. Inductive loads and high power transformers creates transients and spikes in the mains lines. Such voltage fluctuations are dangerous to SMPS based power supply and Microcontrollers. Here is a simple circuit that constantly monitors the power lines and beeps with each voltage fluctuation.
The circuit intimates you whether there is any risk of voltage drops and hikes in the power supply connected to the Microcontrollers and SMPS based electronic devices. Even a small voltage spike as small as 10 volts in the mains can cause operating errors and data loss in such devices. SMPS power supply is most vulnerable to such voltage changes in the mains. The circuit is based on the working of the popular timer IC 555. Here it is used as a short duration monostable with timing elements R1 and C2. Power to the circuit is derived from a 0-12 volt step-down transformer with a bridge (D1-D4) for rectification and C1 for smoothing.
Any change in the voltage level in the primary of the transformer will reflect as a corresponding change in its secondary. The trigger pin 2 of IC1 is connected to a potential divider R2 and VR. The trigger threshold voltage of IC1 is set by VR so as to keep the voltage level at pin 2 slightly higher than the triggering threshold so that output of IC1 will be low, if the mains voltage is normal. If the voltage drops, the voltage at pin 2 of IC will be reduced and causes its triggering. Buzzer beeps indicating the transients. If the beeps are frequent, the electronic devices should be unplugged to save them.