30 Minute Timer with Circuit Diagram Using 555 IC & 7555 IC

The medieval Latin word for clock is – ‘clogga’, which means ‘bell’. These are one of the oldest human inventions. We are measuring time from centuries using different methods. With the invent of technology many new fast and accurate methods are being invented to measure time. The invention of dry cell battery also helped in making clocks which can work on electrical power. Based on the time intervals they measure, clocks are named as either hourglass, timepiece, etc… One of such categories of the clock, that measures time intervals, by counting down the time from a specified time interval is popularly known as Timer. Commonly used timer in the projects is the 30 Minute Timer.

30 Minute Timer Project

Timers are the clocks used for measuring time for a specified time interval. These devices are usually used for measuring countdown, as they work by counting down from a specified time interval.

These timers can be implemented in two types – either as a hardware device or as a software program. For many engineering applications, 30 Minute Timers are frequently used. This timer starts from point 30 and makes a count down to zero. This timer is also used as a time switch which can activate ant device when the specified time is reached.

In 30 Minute Timer project, a timer is constructed which make a count down from 30-minute mark to the 0-minute mark. 555 timer IC is used in the timer circuit. This IC when used as an oscillator, provides time-delays. 555 Timer operates in three modes – A- stable, Monostable, and Bistable modes.

For 30 Minute Timer circuit, 555 IC is operated in Monostable mode. In this mode, the output of 555 IC has two states – a stable state and unstable state. When the user sets stable output as high, the output of the timer will be high until any interrupt occurs. When an interrupt occurs the output enters into an unstable state.i.e. the output turns low. As this state is unstable, the output goes high as soon as the interrupt passes. This feature of 555 Timer is used to design the Adjustable Timer circuits.

Circuit Diagram

30 Minute Timer Circuit can be designed using a 555 Timer IC in Monostable Mode. The output from the 555 IC is drawn from the Pin-3. By adjusting the values of external resistor R1 and Capacitor C1, adjustable timer circuits can be designed.

The time duration for which the output from the pin 3 remains High can be calculated from the formula T = 1.1 ×R1 × C1. Here R1, C1 are the external resistor and capacitor elements attached to the timer IC. For designing a 1-minute timer the R1 value has to be set to 55kΩ and capacitor C1 value has to be set to 1000µF. T denotes the time interval of the timer circuit.

T = (1.1 × 55 × 1000×1000)/1000000 ≅ 60 seconds.

For designing a 30 Minute Timer circuit, from the above equation,  either R1 value has to be changed or C1 value. The R1 value when designing 30 Minute Timer is calculated as –

30×60 = 1.1 × R1 × 1000 µF.

For designing an adjustable timer circuit, in the circuit replace R1 with a variable resistor.

5 To 30 Minute Timer Circuit Using 7555IC

7555 IC is the CMOS version of 555 IC. It is capable of producing accurate time delays and frequencies. When used in monostable mode, the pulse width of the output wave can be controlled using an external resistor and capacitor.

7555- Timer is available as an 8-pin package. The specified time is set using the external resistor and capacitor. 7555 works as a monostable multivibrator. For designing a 30 Minute Timer using 7555, five resistors, each of 8.2 M are used along with a capacitor of 33µF. By changing the switch positions adjustable timers for 5,10, 15, 20, 25, 30 minutes can be formed.

Pin Configuration of 7555-

• Pin-1, GND, is the ground pin also used for low level 0.
• Pin-2, TRIGGER, is the start timer input pin.  This pin is active LOW.
• Pin-3, OUTPUT, is the timer logic output pin.
• Pin-4, RESET, is the timer inhibit input. This pin is active low.
• Pin-5, CONTROL_VOLTAGE, this pin is for setting the upper voltage sense of the timing capacitor.
• Pin-6, THRESHOLD, is the input pin for lower voltage sense of the timing capacitor.
• Pin-7, DISCHARGE, is the discharge output of the timing capacitor.
• Pin-8, Vdd, is the supply voltage.

Pin-3 of 7555 is connected to the 2N2222 NPN transistor using a 4.7k resistor. The transistor goes into saturation state when the output of the 7555 goes high. When the transistor goes into saturation state, the relay is activated. This relay can control any small mechanical device or electronic system. The diode attached parallel to the relay protects the transistor when the relay is deactivated.

Compared to 555 timers, use of 7555 timer works with 8.2 M resistor smoothly. In this circuit, the relay voltage should be the same as the source voltage. Power supply between voltages 5v to 15v should be used. Due to the degradation of resistor and capacitor performances over time, the timer value may not be accurate.

7555 is usually preferred as timer IC in applications where Precision timing is needed. This IC is also applied for pulse generation, sequential timing and time delay generation. For modulations such as pulse width modulation and pulse position modulation, 7555 is preferred over 555 IC. 7555 is also applied as a missing pulse detector.

Timer circuits are very useful in automation systems where human involvement is not desired. This circuit is used in various day-to-day applications. This circuit can be found in automobiles to control the wiper speed, automatic operation of alarm after set time intervals, for automatic dimming of LED in lamps after certain time, in automatic air-coolers, and various applications where certain automatic action has to be done after set time intervals.

Timers can be designed either with 555IC or 7555 IC. But there are certain differences caused in the circuit based on the IC used. 555 IC cannot go rail to rail and it is rated up to 2Mhz. CMOS version of 555 IC is 7555 IC. 7555 IC output is compatible with TTL circuits. Besides these differences, other timing function values remain the same, regardless of which IC is used in the circuit. Which of the Timer IC have you preferred for your application?