MICROCONTROLLER-BASED INDUSTRIAL TIMER

A microcontroller-based industrial timer can be programmed and used as a timer, counter and time totaliser. Here is a simple design based on 40-pin Atmel AT89S52 microcontroller that performs count-down operation up to 9999 minutes/second with four 7-segment displays showing the actual time left. The relay energises as you press the start switch and remains on till the countdown reaches ‘0000.’ Four tactile, push-to-on switches are used to start/stop, select either minutes or seconds, and set the initial value for countdown operation (using‘up’ and ‘down’ keys). Circuit diagram

Fig. 1 shows the circuit of the microcontroller-based industrial timer. The microcontroller is Atmel AT89S52 (IC1), which is a 40-pin deisvice with 8 kB of program flash memory, 256 bytes of RAM, 32 I/O lines, Watchdog timer, two data pointers, three 16-bit timer/counters, a sixvector two-level interrupt architecture, a full-duplex serial port, on-chip oscillator and clock circuitry. The powerdown mode saves the RAM contents but freezes the oscillator, disabling all other chip functions until the next interrupt or hardware reset is activated. Port P0 of microcontroller AT89S52 deis configured for segments of the 7- segment display. Port 0 is an 8-bit open-drain bidirectional I/O port. Port 0 is pulled up with 10-kilo-ohm resistor network RNW1. Port pins P0.0 through P0.6 are connected to pins of segments ‘a’ through ‘g’ via resistors R2 through R8, respectively. Port P0.7 is connected to decimal via resistor R9. Resistors R2 through R9 are used as current limiter for various segments of displays, respectively. Port 2 is used to control DIS1 through DIS4. Port 2 is an 8-bit bidirectional I/O port with internal pullups. When port-2 pin is low, the transistor conducts and provides supply to the common pin of 7-segment display. Port pins P2.5 through P2.2 control DIS1 through DIS4 with the help of transistors T1 through T4, respectively. The microcontroller drives the 7- segment displays in multiplex mode. This helps in reducing current consumption while maintaining the brightness of the display. For driving the displays, timer 2 inside the microcontroller is used. It enables display of each digit every two milliseconds. Semiconductor:
Part listed
IC1 - AT89S52 micrcontroller
IC2 - 7805 5V regulator
T1-T4 - BC557 pnp transistor
T5 - BC547 npn transistor
BR1 - 1A bridge rectifier
D1 - 1N4007 rectifier diode
DIS1-DIS4 - LTS542 common-anode display
Resistors (all ¼-watt, ±5% carbon):
R1, R14-R18 - 8.2-kilo-ohm
R2-R9 - 270-ohm
R10-R13 - 470-ohm
RNW1 - 10-kilo-ohm resistor network
Capacitors:
C1 - 10μF,16V electrolytic
C2, C3 - 33pF ceramic disk C4,
C6 - 0.1μF ceramic disk
C5 - 1000μF, 25V electrolytic
Miscellaneous: X1 - 230V AC primary to 6V, 350m
A secondary transformer XTAL - 20MHz crystal
RL1 - 6V, 1C/O relay S1-S5 -
Push-to-on switch S6 - On/off switch