Project 1

Digital Stopwatch

Introduction :A Digital Stop Watch built around timer IC NE555 and 4-digitcounter IC MM74C926 with multiplexed 7segment display. MM74C926 consists of a 4-digit counter, an internal output latch, npn output sourcing drivers for common cathode,7 segment display and internal multiplexing circuitry with four multiplexing outputs.The counter advances on the negative edge of the clock. The clock is generated by the timer NE555. The circuit works of a 5 volt power supply. It can be easily assembled on a general-purpose PCB. Enclose the circuit in the wooden box with provisions for 7-segment displays, time varying switch S1, start/stop switch S2, and reset switch S3.

Components used:

IC MM74C926

IC NE555N

COMMON CATHODE 7-SEGMENT DISPLAY

PRESET POT 20k

RESISTORS 10k, 4.7k, 1k, 3.9k, 200k, 330Ω

TRANSISTORS BC547, BC557

CAPACITORS 3.3µF 10V, 0.01µF

Theory:

General Description Of MM74C926:The MM74C925 and MM74C926 CMOS counters consist of a 4-digit counter, an internal output latch, NPN output sourcing drivers for a 7-segment display, and an internal multiplexing circuitry with four multiplexing outputs. The multiplexing circuit has its own free-running oscillator, and requires no external clock. The counters advance on negative edge of clock. A HIGH signal on the Reset input will reset the counter to zero, and reset the carry-out LOW. A LOW signal on the Latch Enable input will latch the number in the counters into the internal output latches. A HIGH signal on Display Select input will select the number in the counter to be displayed; a LOW level signal on the Display Select will select the number in the output latch to be displayed. The MM74C925 is a 4-decade counter and has Latch Enable, Clock and Reset inputs. The MM74C926 is like the MM74C925 except that it has a display select and a carry-out used for cascading counters. The carry-out signal goes HIGH at 6000, goes back LOW at 0000.

PIN DIAGRAM OF MM74C926:Functional Description

Reset — Asynchronous, active high

Display Select — High, displays output of counter Low, displays output of latch

Latch Enable — High, flow through condition Low, latch condition

Clock —Negative edge sensitive

NE555 TimerGeneral Description:

The LM555 is a highly stable device for generating accurate time delays or oscillation. Additional terminals are provided for triggering or resetting if desired. In the

time delay mode of operation, the time is precisely controlled by one externalresistor and capacitor. For astable operation as an oscillator, the free running frequency and duty cycle are accurately controlled with two external resistors and one capacitor. The circuit may be triggered and reset on falling waveforms, and the output circuit can source or sink up to 200mA or drive TTL circuits.

Features:

Direct replacement for SE555/NE555 Timing from microseconds through hours Operates in both astable and monostable modes Adjustable duty cycle Output can source or sink 200 mA Output and supply TTL compatible Temperature stability better than 0.005% per °C Normally on and normally off output Available in 8-pin MSOP package

Pin Diagram:

ASTABLE OPERATION

If the circuit is connected as shown in Figure pins 2 and 6connected it will trigger itself and free run as a multivibrator. The external capacitor charges through RA + RB and discharges through RB. Thus the duty cycle may be precisely set by the ratio of these two resistors. In this mode of operation, the capacitor charges and discharges between 1/3 VCC and 2/3 VCC. As in the triggered mode, the charge and discharge times, and therefore the frequency are independent of the supply voltage.

Astable Mode

Waveforms generated in this mode ofoperation.

Common Cathode 7-Segment Display :A seven-segment display may have 7, 8, or 9 leads on the chip. Usually leads 8 and 9 are decimal points. The figure below is a typical component and pin layout for a seven segment display.

The light emitting diodes in a seven-segment display are arranged in the figure below :

DIODE PLACEMENT IN A 7-SEGMENT DISPLAY, NO DECIMAL .

The wiring for the common cathode is shown below:

COMMON CATHODE INTERNAL WIRING

PIN DIAGRAM OF TRANSISTORS:BC547:

FEATURES• Low current (max. 100 mA)• Low voltage (max. 65 V).

APPLICATIONS• General purpose switching and amplification.

DESCRIPTIONNPN transistor in a TO-92; SOT54 plastic package.PNP complements: BC556 and BC557.

PIN DESCRIPTION1 emitter2 base3 collector

BC557:

FEATURES

· Low current (max. 100 mA)· Low voltage (max. 65 V).

APPLICATIONS· General purpose switching and amplification.

DESCRIPTIONPNP transistor in a TO-92; SOT54 plastic package.NPN complements: BC546 and BC547.

PIN DESCRIPTION1 emitter2 base3 collector

WORKING:Circuit Diagram:

DESCRIPTION:

A Digital Stop Watch built around timer IC NE555 and

4-digitcounter IC MM74C926 with multiplexed 7segment display. MM74C926 consists of a 4-digit counter, an internal output latch, npn output sourcing drivers for common cathode,7 segment display and internal

multiplexing circuitry with four multiplexing outputs.The counter advances on the negative edge of the clock. The clock is generated by the timer NE555. The circuit works of a 5 volt power supply. It can be easily assembled on a general-purpose PCB. Enclose the circuit in the wooden box with provisions for 7-segment displays, time varying switch S1, start/stop switch S2, and reset switch S3.

Switching Time Waveforms

Testing:

First, reset the circuit by pressing S3 so that the display shows ‘0000’ . Now open the switch S2 for stop watch to start counting the time. If you want tostop the stop watch, close switch S2. Time varying switch S1 is used to select the different time periods at the output of astable multivibrator(NE555).

Project 2

Automatic Night Lamp

&

Morning Alarm

Introduction :This circuit automatically turns on a night lamp when bedroom light is switched off. The lamp remains ‘on’ until the light sensor senses daylight in the morning. A yellow LED is used as the night lamp. It gives bright and cool light in the room. When the sensor detects the daylight in the morning, a melodious morning alarm sounds.

The circuit utilises light-dependant resistors (LDRs) for sensing darkness and light in the room. The circuit is designed around the popular timer IC NE555 , which is configured as a monostable. NE555 is activated by a low pulse applied to its trigger pin 2. Once triggered, output pin 3 of NE555 goes high and remains in that position until until timer is triggered again at its pin 2. The musical tone of the alarm is generated by UM66 IC. The circuit can be easily assembled on a general purpose PCB. Enclose it in a good-quality plastic case with provisions for LDR and LED. Use a reflective holder for LED to get a spotlight effect for reading. Place LDRs away from the LED, preferably on the backside of the case, to avoid unnecessary illumination. The speaker should be small so as to make the gadget compact.

Components used:

IC NE555N

LIGHT DEPENDENT RESISTOR

MUSIC GENERATOR UM66

8Ω, 4.5W SPEAKER

RESISTORS 220Ω, 560Ω, 580Ω, 1k, 120k, 150k

LIGHT EMITTING DIODE

ZENER DIODE

TRANSISTOR BC548

CAPACITOR 0.01µF

Theory:

NE555 Timer :General Description:

The LM555 is a highly stable device for generating accurate time delays or oscillation. Additional terminals are provided for triggering or resetting if desired. In the time delay mode of operation, the time is precisely controlled by one externalresistor and capacitor. For astable operation as an oscillator,the free running frequency and duty cycle are accurately controlled with two external resistors and one capacitor. Thecircuit may be triggered and reset on falling waveforms, andthe output circuit can source or sink up to 200mA or drive TTL circuits.

Features:

Direct replacement for SE555/NE555 Timing from microseconds through hours Operates in both astable and monostable modes Adjustable duty cycle Output can source or sink 200 mA Output and supply TTL compatible Temperature stability better than 0.005% per °C Normally on and normally off output Available in 8-pin MSOP package

Pin Diagram:

MONOSTABLE OPERATIONIn this mode of operation, the timer functions as a one-shot. The external capacitor is initially held discharged by a transistor inside the timer. Upon application of a negative trigger pulse of less than 1/3 VCC to pin 2, the flip-flop is set which both releases the short circuit across the capacitor and drives the output high. The voltage across the capacitor then increases exponentially for a period of t = 1.1 RA C, at the end of which time the voltage equals 2/3 VCC. The comparator then resets the flip-flop which in turn discharges the capacitor and drives the output to its low state. Since the charge and the threshold level of the comparator are both directly

proportional to supply voltage, the timing interval is independent of supply.

Monostable Mode

Waveforms generated in this mode ofoperation.

MUSIC GENERATOR UM66 :

General Description:

UM66 is a pleasing music generator IC which works on a supply voltage of 3V. the required 3V supply is given through a zener regulator. its out put is taken from the pin no1 and is given to a push pull amplifier to drive the low impedance lowd speker. A clss A amplifier before pushpull amplifier can be used to decrese the noise and improve out put. UM66 is a 3 pin IC pakage just looks like a BC 547 transistor.

Features: 62 Note ROM Memory Voltage rating: 1.3V to 3.3 V

Power on reset

Pin Diagram:

PIN DESCRIPTION:

Pin No Designation Description1 Out put Emitter2 +Vcc Base3 -Vcc Collector

Light Dependent Resistor :

A light dependent resistor or photo resistor is a resistor whose resistance decreases with increasing incident light intensity. It can also be referenced as a photoconductor. A photoresistor is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron conduct electricity, thereby lowering resistance.

Photoresistors come in many different types. Inexpensive cadmium sulfide cells can be found in many consumer items such as camera light meters, street lights, clock radios, alarms, and outdoor clocks.

PIN DIAGRAM OF TRANSISTORS:

BC548:

FEATURES• Low current (max. 100 mA)• Low voltage (max. 65 V).

APPLICATIONS• General purpose switching and amplification.

DESCRIPTIONNPN transistor in a TO-92; SOT54 plastic package.PNP complements: BC558.

PIN DESCRIPTION1 emitter2 base3 collector

WORKING:Circuit Diagram:

DESCRIPTION:

This circuit automatically turns on a night lamp when bedroom light is switched off. The lamp remains ‘on’ until the light sensor senses daylight in the morning. A super-bright white LED is used as the night lamp. It gives bright and cool light in the room. When the sensor detects the daylight in the morning, a melodious morning alarm sounds. The circuit is powered from a 6V DC supply. The circuit utilises light-dependant resistors (LDRs) for sensing darkness and light in the room. The resistance of LDR is very high in darkness, which reduces to minimum when LDR is fully illuminated. LDR1 detects darkness, while LDR2 detects light in the morning. The circuit is designed around the opular timer IC NE555 (IC2), which is configured as a monostable. IC2 is activated by a low pulse applied to its trigger pin 2. Once triggered, output pin 3 of IC2 goes high and remains in that position until IC2 is triggered again at its pin 2. When LDR1 is illuminated with ambient light in the room, its resistance remains low, which keeps trigger pin 2 of IC2 at a positive potential. As a result, output pin 3 of IC2 goes low and the white LED remains off. As the illumination of LDR1’s sensitive window reduces, the resistance of the device increases. In total darkness, the specified LDR has a resistance in excess of 280 kiloohms. When the resistance of LDR1 increases, a short pulse is applied to trigger pin 2 of IC2 via resistor R2 (150 kiloohms). This activates the monostable and its output goes high, causing the white LED to glow. Low-value capacitor C2 maintains the monostable for continuous operation, eliminating the timer effect. By increasing the value of

C2, the ‘on’ time of the white LED can be adjusted to a predetermined time. LDR2 and associated components generate the morning alarm at dawn. LDR2 detects the ambient light in the room at sunrise and its resistance gradually falls and transistor T1 starts conducting. When T1 conducts, melody-generator IC UM66 (IC3) gets supply voltage from the emitter of T1 and it starts producing the melody. The musical tone generated by IC3 is amplified by single-transistor amplifier T2. Resistor R7 limits the current to IC3 and zener diode ZD limits the voltage to a safer level of 3.3 volts.

The circuit can be easily assembled on a general-purpose PCB. Enclose it in a good-quality plastic case with provisions for LDR and LED. Use a reflective holder for white LED to get a spotlight effect for reading. Place LDRs away from the white LED, preferably on the backside of the case, to avoid unnecessary illumination. The speaker should be small so as to make the gadget compact.