![]() Q- goes high and output goes low turning on a PNP transistor.Ĥ. As soon as the capacitor reaches 2/3 Vcc, the comparator goes high and resets the flip-flop. With Q- low, the NPN transistor internal to the 555 is turned off, which lets the capacitor C1 charge toward Vcc through R2 and R1.ģ. For PNP it will use the opposite cycle.Ģ. This in turn sets Q- low and since there is an inverter on the ouput, sets pin 3 high which turns on an NPN transistor. This means that 0V is on pin 2, forcing its comparator high. When power is first applied the capacitor C1 is initially uncharged. The 555 timer above works in the following way.ġ. ![]() In this circuit I will be using the 555 timer in a very basic astable mode. Using external RC circuits we can control the frequency of the output pin. The flip-flop looks at the S and R values and produces either a high or a low based on the voltage states at the inputs. These are fed into the Set and Reset inputs on the flip-flop. A comparator is pretty simple, it looks at its terminals + and – and if + is greater than the – input, it drives the output high or low. The outputs from each divider represent 2/3 Vcc and 1/3 Vcc which are then fed into two comparators. These resistors act a a three step voltage-divider between +Vcc and Ground. If you look at the image above, you can begin to see just where the 555 timer gets its name from, the three 5k resistors in series. In this mode the 555 acts as a square wave generator whose waveform can be adjusted by two external RC circuits. I rarely use the 555 timer in monostable mode but have had many applications where I have used the IC in astable mode. In monostable mode it resembles a one-shot timer where a trigger voltage is applied and the chips output goes from low to high based on a time set by an external RC circuit. The 555 Timer is a fantastic precision timer that can act as either an oscillator (astable mode) or as a timer (monostable mode).
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