Universal Power Supply Circuit Using LM371

This is a design circuit for a power supply, but this design is universal power supply. This circuit is using LM371 for control operation. This is the figure of the circuit.


The Universal Power Supply output voltage can be set to anywhere in the range 1.5V to 30V by selecting two resistances. By using a potentiometer, R2, as one of the resistors you can dial up the output voltage wanted. Either AC or DC input can be supplied to the PCB via a socket or terminal block. Connection can be either way around. This is because we have provided a bridge rectifier on board. The input DC voltage to the regulator must be at least 2.5V above the required output voltage. An off/on switch is provided. The LM317 will provide slightly higher output voltages than 30 volts. To be safe for continuous operation the maximal input DC voltage to the regulator should not be over 33V. With a 2.5V to 3.0V drop across the regulator this will give a regulated output of 30V. You can draw up to 1.5A from the LM317.

When external capacitors are used with any IC regulator it is good practice to add protection diodes to prevent the capacitors discharging back into the regulator in the event of abnormal operating conditions, like a sudden short circuit on the input or the output, or a back emf from an inductive load. That is the function of D1 and D2.

Electronic Doorbell Circuit Using Counter

This is a design for a door alarm circuit, but in this design using electronic system. This circuit uses a synthesized sound chip from Holtek, the HT-2811. This circuit reproduces the sound of a "ding-dong" chiming doorbell. Additionally, the circuit includes a CMOS 4026 counter display driver IC to count your visitors. This is the figure of the circuit.


The operating voltage must remain within 2.4 to 3.3 VDC and standby current is minimal. The reset switch zeroes the count, and the 7 segment display is a common cathode type. To save power consumption the display can be enabled or disabled with a switch as shown in the above diagram. The count will still be held in memory.

The envelope of the chime is set by the 220k, 330k, 3u3 and 4u7 resistors and capacitors. These values are the manufactures default values, but may be adjusted to alter the length and delay of the chime. The combination of the 2k2, 22k and 47u resistor capacitor network has a double function. It provides a de-bouncing circuit for the bell press and at the same time has a sufficiently long time constant. This ensures that anyone rapidly pressing the doorbell switch, only advances the count once. The 47u capacitor may be increased in size, if needed.

Pin IC CMOS can look for the list below.
Pin 1 is the clock input
Pin 2 is the clock enable
Pin 3 is display enable
Pin 4 enables the carry output
Pin 5 is the carry output
Pin 6 is display segment f Pin 7 is display segment g
Pin 8 is 0 V.
Pin 9 is display segment d Pin 10 is display segment a
Pin 11 is display segment e Pin 12 is display segment b
Pin 13 is display segment c Pin 14 is the2 output
Pin 15 is reset
Pin 16 is +Vcc

Light Sensor Circuit Using Op Amp

This is a one of the light sensor. This circuit is a dark sensor that is based on op – amp 741 IC as main control. This circuit is a simple design for sensor the light at the night. This is the figure of the circuit.


For sensor the light is using LDR. Operation of the circuit is under normal conditions the resistance of the LDR is high, keeping pin 2 low. When light falls onto the LDR the resistance drops to a couple hundred ohms and triggers pin 2 high which biases the base of Q1 via pin 6 and R4 and in turn activates the relay. Trimmer pot P1 and the two 470 ohm resistors, R2 and R3, are a voltage divider to adjust for sensitivity.

If you want the action reversed (make it a dark sensor), change the positions of the LDR and R1. If the relay 'chatters', add a bit of hysteresis by adding a 100K to 1Meg-ohm resistor (R6) over pins 6 and 2 of the 741 op-amp, but in most cases 100K to 330K will do the job. The LDR is a regular, general purpose type. D1 serves as a spark-arrestor when the relay contacts open. This circuit power supply is using 12 V DC.

Touch Switch Circuit Using 555 IC

This is a circuit for touch switch circuit. This circuit is almost same with touch door alarm. This circuit uses a 555 timer as the bases of the touch switch circuit. This is the figure of the circuit.


The operation of this circuit is begin, when the plate is touched the 555 timer is triggered and the output on pin 3 goes high turning on the LED and the buzzer for a certain period of time. The time that the LED and the buzzer is on is based on the values of the capacitor and resistor connected to pin 6 & 7. The 10 M resistor is on pin 2 causes the circuit to be very sensitive to the touch.

Touch Door Alarm Circuit Using 555 Timer IC

This is a design simple alarm that can be used to provide a audible alarm when someone touches the door knob or handle of your room. The door knob or handle must be made of metal for the circuit to work. The main chip in the circuit is a 555 timer which will be triggered if a hand comes close to or touches the door knob.


The circuit attaches to the door knob at the end of the 1 Mega ohm resistor. This is operation of the circuit. Once the timer is triggered the LED will light and the UJT will output a tone to the speaker. The timers will time out in 5 seconds. The sensitivity of the trigger can be changed by changing the 1 Mega ohm resistor to another value. The 5 second time out can be adjusted by changing the value of the resistor connected between pin 8 and pin 7. The output tone can be changed by changing the RC values on the base of the UJT.

Door Knock Alarm Circuit Using TLV3702

In the some building is need to using good security. For some example is using this design circuit. The door knock alarm is a simple design for the simple security for the house. This circuit is based by sensor piezoelectric wafer for detected the alarm. This is the figure of the circuit.


Operation of the circuit is the resistor R5 determines the knock sound sensitivity. The value shown should work in most cases. The box could be hard mounted to the door or suspended at about the middle of the door, by a string from the top of the door in such a way that the box rests against the door. It is suggested that the circuit be housed in a plastic box with a 9v battery holder.

DC to AC Inverter Using 555 IC

This is a design for AC inverter circuit. This circuit is produces an AC output at line frequency and voltage. The circuit is using 555 IC as main control. This IC is configured a low frequency oscillator, tunable over the frequency range of 50 – 60 Hz by potentiometer R4. This is the figure of the circuit.


The principle work of the circuit is the IC feeds its output that amplified by Q1 and Q2 to input of the transformer T1. A reverse is connected filament transformer with necessary step-up turns ratio. A capacitor C4 and coil L1 filter the input to T1, assuring that it is effectively a sine wave. Adjust the value of T1 to your voltage. Replacement types for Q1 are: TIP41B, TIP41C, NTE196, ECG196, etc. Replacement types for Q2 are: TIP42B, TIP42C, NTE197, ECG197, etc. The input voltage of the circuit is anywhere from +5V to +15Volt DC.

Birdie Doorbell Circuit Using Transistor

This is a design circuit for a doorbell. This circuit can produces a sound like bird sound. This circuit controlled by transistor NPN. This is the figure of the circuit.


The operation of the circuit is beginning when P1 is of experimental value. Start with 220 Ohms or so and modify to suit your needs. The transistor is a general purpose kind and is not critical, almost any PNP type will work. L1 is a bell-transformer which is usually already present in the house. If you wish, you could use a battery instead of the bell transformer. Just hookup a 9-volt battery (or wall adapter)to points 'A' and 'B' (A=+) the diode (D1) is to protect the circuit from accidental polarity reversal and is optional, but required as a rectifier for use with the bell transformer.

The transistor T1 is a General Purpose PNP transistor and probably anything will work. L2 comes out of an old am transistor radio. They look like miniature transformers and are usually colored red or green. You have to fiddle with different transformers as the sound can vary depending on the value. The loudspeaker is a 8 Ohm type and must be larger than 200milli-Watt. I used a 2Watt type, but anything over 0.2W will do.