Auto Timer Circuit with Extensive Limit by LM555

Figure 1 

The circuit was designed to produce a timing scheme that will provide a longer span of time to become activated.

• LM555 – a highly stable device for generating accurate oscillation or time delays with additional terminals provided for resetting or triggering, used in linear ramp generator, pulse position and width modulation, time delay & pulse generation, sequential &precision; timing, with features such as adjustable duty cycle, temperature stability, TTL compatible output, availability in 9-pin MSOP package, timing from microseconds, and operates in both astable & monostable modes
• 4017 – a decade counter where the count advances as the clock input becomes high that may be combined with diodes for some functions such as flash sequences
• 4020 – a 14-bit ripple counter where the slight delay may cause glitches to occur in any logic gate systems connected to its outputs before a clock pulse is responded by the later counter outputs
• BC214 – a complementary silicon planar epitaxial transistor used in AF small signal drivers and amplifiers as well as for low noise preamplifier applications due to its feature of good linearity of DC current gain

There are three ranges of timing being covered by the circuit. This is made possible by using two switches with three selections. The wide scope of time covered by the circuit would start from 1 minute to 20 hours before automatically turning OFF. The circuit consisted of a 555 timer IC1 in the initial stage, which provides oscillation with the application of power. The IC1 functions in astable mode as an oscillator where the duty cycle and free running frequency are accurately controlled by 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 200 mA. The astable mode output provides a continuous stream of rectangular pulses with a specified frequency.

The clock pulses from 555 timer IC1 are fed into the decade counter IC2 4017. This is then combined with a ripple counter IC3 4020 to provide a joined count rate of 81920. This results to Q1 being turned ON after the output of IC3 goes high. To adjust the timing option, a 470K ohm potentiometer RV1 is used to obtain the desired output.

Figure 2 

There are several varieties of timer circuits that are being sold in the market. Some of them will come in the form of hydroponics timers where they can be adjusted to 120 minutes or 480 minutes which can be used in applications that requires a device to be turned ON and OFF at precise intervals. They can also be constructed as heavy duty outdoor timer or indoor digital timer, programmable wall timer, outdoor security light timer, spring wound timer, and multi setting appliance timer.

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