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Dual 555 Oscillator

This circuit takes the outputs from two different 555 multivibrators and combines them using a summing Op Amp. Shown only to depict another implementation of a 555 timer, most of the back ground calculations being covered under the other topics in this section.

Standard 555 oscillator circuit schematic
555 Osc

Astable oscillator circuit
The standard configuration for a 555 Astable multivibrator is shown to the left. The circuit is shown again to reference the next schematic which contains two individual astable circuits.

Resistors R1 and R2 in combination with capacitor C1 select the output frequency, however the frequency is not relevant to the following discussion [adding two outputs together].

This is one topic of a number covering the 555 timer, with this page covering a combination of 555 timers and operational amplifiers. The circuit is composed of three ICs; two 555 timers and an operational amplifier. However a dual 556 timer could have been used, but a single 555 timer was used to provide the same pin out given on the other pages covering the 555. The operational amplifier, which is labeled Adder, could be almost any Op Amp including a single, dual or quad package; although only one amplifier is required.

555 tone adder circuit schematic
555 Multi-Tone Adder circuit

The circuit consists of two individual pulse generators, operating at different pulse widths, and connected to a three input summing amplifier. The third input shows a symbol for a pulse generator, and only depicts the fact that any number of inputs could be used The timers could be set to operate at the same pulse rate, but that couldn't serve much purpose, as there addition wouldn't change anything. The addition of two identical waveform produces the same output as the input, but at a different voltage level. However if the output was required to be at a different voltage level the Vcc could just be taken to a higher voltage, saving the Op Amp.

Dual 555 output generator circuit schematic
Dual 555 circuit

This is the same circuit used above but configured differently and with out using the Op Amp adder circuit. This circuit generates a single output from two 555 timers concatenated together. Both timers are set up just as all the other astable circuits are, with the resistors and capacitor selected to define the output frequency. The difference here is that the output of the first circuit feeds into the reset line of the second circuit. So when ever the output of the first timer goes low, the output of the second timer is reset or forced to go low.

This configuration could be changed slightly so that the output of the first stage feeds the threshold of the second stage. The first configuration used the low level of the first stage to force the output of the second stage low, while this second configuration [not shown] uses the voltage level of the first stage to change the output of the second stage. Either way, both circuit would use the pulse width of the first timer to change the pulse width of the second timer.

Note the 555 will not oscillate if RB is greater than 1/2 RA because the junction of RA and RB cannot bring pin 2 down to 1/3 VCC and trigger the lower comparator.

Timing Equations:
Charge Time; t1 = 0.693*R1*C1
Discharge Time; t2 = [(R1*R2/(R1 + R2)]*C1 lin [(R2 - 2*R1)/(2R2 - R1)]
Total period; T = t1 + t2
Oscillation frequency; f = 1/T
Duty cycle = D = 50% with R1=51k, R2=22k ohms, C1=0.01uF

Related Function; 555 Decreasing Frequency, Astable Multivibrator
Related Circuit; 555 Monostable multivibrator, standard circuit

Return to the main 555 Functional Description page.

Regardless of the circuit configuration used, the single output could be used to drive a speaker; as in a Mono Speaker system.

PC motherboard

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