![]() When the Figure 4 timer circuit is in its quiescent state, pin 2 is held high via R4, Q1 is saturated and forms a short across timing capacitor CT, and pin 3 (output) is driven low. The diagram also shows the connections for using the 555 as a basic monostable multivibrator or timer, and the following explanation assumes that the IC is connected in this configuration. ![]() The comparator outputs control the R-S flip-flop which, in turn, controls the output stage and slave transistor Q1 the flip-flop state can also be controlled via pin 4. Functional block diagram (within the double lines) of the 555 timer IC, with external connections for use as a ‘timer.’įigure 4 shows (within the double lines) the functional diagram of the bipolar 555 IC the supply-driven 3 x 5k0 potential divider generates a 1/3 V cc reference voltage on the non-inverting input of the lower voltage comparator and a 2/3 V cc reference on the inverting input of the upper comparator. Typical bipolar 555 and CMOS 7555 parameter values.įIGURE 4. Outline and pin notations of the 14-pin DIL version of the 556 (or 7556) ‘dual timer’ IC. ![]() Outline and pin notations of the eight-pin DIL versions of the 555 (or 7555) timer IC.įIGURE 2. Figures 1 to 3 give basic outline and performance details of the bipolar 555 and 556 ICs and their CMOS counterparts, the 75.įIGURE 1. When used in the monostable mode, its output can be pulse-width modulated (PWM) and, in the astable mode, it can be subjected to frequency-sweep control, to frequency modulation (FM), or to pulse-position modulation (PPM). The 555 is a versatile timer IC that generates stable timing periods from a few microseconds to hundreds of seconds via a simple C-R network, and gives good output waveforms with typical rise and fall times of 100ns. This article explains 555 basics and shows ways of using the IC in monostable applications. The best known timer ICs are the ‘555’ family of devices, which are available in both single (555) and dual (556) bipolar packages and also in CMOS forms (75) they use a mixture of linear and digital IC technology. Timer ICs are designed to generate accurate and stable C-R - defined timing periods, for use in monostable ‘one-shot’ pulse generator and free-running astable squarewave generator applications.
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