Simple Light Activated Buzzer Circuit

This is a circuit for a simple buzzer circuit that is activated by light, i.e., the buzzer is on when there's ample light and off when it is dark. A general-purpose operational amplifier, the 741, is used as a comparator that determines whether there's enough light to turn on a self-oscillating piezoelectric buzzer. Its non-inverting input is connected to a photo resistor, a component whose resistance decreases as more light shines on it. Its inverting input, on the other hand, is connected to an almost fixed voltage, i.e., a proportion of the supply voltage as set by trimmer resistor R2. This is the figure of the circuit;

If there isn't enough light shining on the photo resistor, the buzzer is quiet. As more light shines on the photo resistor, its resistance decreases and causes the voltage across R1 to increase. At a certain level of lighting, the voltage across R1, which is also the voltage at the non-inverting input of the 741, becomes larger than the voltage at the inverting input. At this point, the 741 is triggered to output a 'high' level, turning on Q1. Q1 then activates the self-oscillating piezoelectric buzzer.

Phone Incoming Call Flasher Circuit

Here’s a design circuit of phone message flasher, an alternative way to get alarmed when there is an incoming call. This is the figure of the circuit;

When the phone rings, high line voltage will be detected by A differential amplifier with hysteresis (Q2, Q3, and Q1). This action will turn on multi vibrator Q5/Q6, Q4. The LED will flashed via Q7. When the phone line voltage drops to less then 9V, the Q2 and Q1 will be turned off, which indicates an off hook condition.

Low Cost Precision Light Control / Dimmer Circuit

Using a the controlled-half-plus-fixed-half-wave phase control method, this circuit can regulate an 860 a watt lamp load from half to full power. This circuit control the light output of the lamp from 30% to 100% because Half power applied to an incandescent lamp gives 30% of full light output. This is the figure of the circuit;