Timer circuit

Posted on Feb 7, 2014

Timer circuit has been used in many projects and there are basically 2 types that are used these days. One of them is the use of analog RC circuit where charging of the capacitor circuit determined the T of the circuitry. This type of circuitry has larger tolerance and is used in applications where the T is not so critical as the T is affected by

Timer circuit
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the tolerance of the RC components used. The other is the use of crystal or ceramic resonators together with microprocessor, microcontroller or application specific integrated circuit that need higher precision T in the tolerance of up to 5 ppm (parts per million). One commonly used circuit is the 555 IC which is a highly stable controller capable of producing timing pulses. With a monostable operation, the T(time) delay is controlled by one external resistor and one capacitor. With an astable operation, the frequency and duty cycle are accurately controlled by two external resistors and one capacitor. The application of this integrated circuit is in the areas of PRECISION TIMING, PULSE GENERATION, TIMING DELAY GENERATION and SEQUENTIAL TIMING. In this mode, the device generates a fixed pulse whenever the trigger voltage falls below Vcc/3. When the trigger pulse voltage applied to pin 2 falls below Vcc/3 while the its output is low, its internal flip-flop turns the discharging transistor Tr off and causes the output to become high by charging the external capacitor C1 and setting the flip-flop output at the same instant. The voltage across the external capacitor C1, VC1 increases exponentially with the T constant T=RA*C1 and reaches 2Vcc/3 at td=1. 1RA*C1. Hence, capacitor C1 is charged through resistor RA. The greater the time constant RA*C1, the longer it takes for the VC1 to reach 2Vcc/3. In other words, the time...

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