Darkness Activated Buzzer Circuit

Posted by Micro System

on Monday, September 27, 2010

This is a circuit for a simple buzzer that is activated by darkness, i.e., the buzzer is off when there's light and on when it is dark.  A general-purpose operational amplifier, the 741, is used as a comparator that determines whether it is dark enough to turn on a self-oscillating piezoelectric buzzer. This is the figure of the circuit;

This circuit inverting input is connected to a photo resistor, a component whose resistance decreases as more light shines on it.  Its non-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. If there is ample light shining on the photo resistor, the buzzer is quiet. As less light shines on the photo resistor, its resistance increases and causes the voltage across R1 to decrease.  At a certain level of lighting, the voltage across R1, which is also the voltage at the inverting input of the 741, becomes smaller than the voltage at the non-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.

220VAC Christmas Flashing Light Circuit

Posted by Micro System

on Tuesday, August 17, 2010

This is a design circuit for line powered flasher, can be used in any application, and Christmas lamp is one of the example. This is the figure of the circuit;

The control for triggering the SCR is done by  Q1, Q2 and related resistors. The timing depends on R1,R2 & C1. To change flashing frequency don’t modify R1 and R2 values: set C1 value from 100 to 2200µF instead, this because the resistor sis already set for the proper current and voltage point for proper operation. Best performances can be achieved with C1=470 or 1000µF and R4=12K or 10K. A number of 10 to 20 lamp in series for Christmas trees (60mA @ 220V typical for a 20 lamp series-loop) is common, and this low current consumption only need a cheap SCR devices, e.g. C106D1 (400V 3.2A) or TICP106D (400V 2A), and the last, the most recommended is P0102D devices having TO92 case.

Random Blinking (Flashing) LED Circuit

Posted by Micro System

on Thursday, August 5, 2010

This is a circuit for flashing circuit. We can using this circuit to make the LEDs blink in a random pattern according to the slight differences in the three Schmitt Trigger oscillators. This is the figure of the circuit;


CD4511 is BCD to 7 segment Driver, so the pattern of the led is actually the come from seven segment representation. Arrange the LEDs to be far from seven segment pattern if you want to avoid people analyze your circuit. The randomness is based on the frequency difference because the 22uF and 47k resistors that determine the frequency have at least 5% tolerance.

LED Light Flasher Circuit

Posted by Micro System

on Sunday, July 25, 2010

This is a simple LED flasher project that uses a CMOS 74C04 Integrated Circuit to alternately ON and OFF two LEDs that are connected in parallel. The Hex inverter MM74C04 from Fairchild Semiconductor has a wide operating power supply voltage range from 3V to 15V DC. It has a typical low power consumption of 10nW/package and has high noise immunity. This is the figure of the circuit;


It is back to back compatible with the standard 74 logic family which is freely available in the market. All its inputs have diode clamps to VCC and GND which protect them from damage due to electrostatic discharge. The schematic above shows the simple configuration of the project. It uses two inverters U1A and U1B to form an oscillator configuration where the frequency of the oscillation is given by :

f = 1/[1.4RC]
= 1/[1.4(10 M Ohm)(0.1uF)]
= 0.7 Hz

The square wave frequency of 0.7 Hz is used to feed the input of U1D which is used as a buffer circuit. At the same time, the other inverter U1C gets its input from pins 2 and 3 of U1. With this configuration, when U1D output is high, U1C output will be low and vice versa. In this way when LED1 is ON, LED2 will be OFF and this will alternate at a frequency of 0.7Hz.