Astable Oscillator Circuit with 555 Timer

As we already analyze one simple Astable circuit with 555 Timer, now we will analyze one improved circuit with the same 555 timer. The oscillator circuit is shown on Picture 1. The main difference in this circuit is that resistor R3 now is connected to the trigger pin of the 555 timer and to the output of the circuit via the capacitor C3.

Picture 1: Astable oscillator circuit with 555 Timer

Time-domain analysis

The results of the transient analysis for this circuit in time domain is shown on Picture 2. These results are for the configuration of the circuit as shown on Picture 1. The power supply voltage is Vcc = 5 V DC. Resistor R3 = 270 Ω, R1 = R2 = 500 Ω, while the capacitors are C1 = 100 nF and C2 = C3 = 1 nF. As measured in the simulation, the frequency of the output signal is about f = 1.826 MHz.

Picture 2: Transient analysis - output voltage Vo wave form (time-domain)


The frequency of the output signal of this circuit can be changed, if we change the values of the components of the circuit. For example, we run simulations with three different values for resistors R1 and R2, and we got these results:

>> For R1 = R2 = 500 Ω -> f = 1.826 MHz;
>> For R1 = R2 = 820 Ω -> f = 1.662 MHz;
>> For R1 = R2 = 1 KΩ -> f = 1.632 MHz;


Just to notice here, the model of the NE555 timer circuit used in these simulations is idealised. In practice, this circuit has some upper limit for frequency of operation. This limit depends on the model and technology of the manufactured circuit. Usually, the older circuits had limit below 1 MHz, but the newer 555 Timers can have upper limit frequency of few MHz-s. Above the upper limit the circuit will be unstable.