Experiment 14: The 555 Astable Timer & the 74123 One-shot Multivibrator

Objectives:

1. To build and test a 555 timer.

2. To build and test a 74123 one-shot multivibrator.

Discussion:

A capacitor and resistor circuit charges at a specific calculable rate depending on the values of the respective components. This constant can be used to create a timed switch that is a latched (one-shot) or a toggled (astable) multivibrator.

The 555 timer is a type of IC that, using a external resistor-capacitor (RC) circuit, will toggle high and low without using a clocked input. It uses a Trigger input to set the latch, and a Threshold input to reset it. Knowing by experimentation that the capacitor will charge to 2/3 of the source voltage in

t_on = (.69) R_total x C seconds,

and that at peak charge the current through the capacitor will approach zero, the current will then trip the Threshold, and this will cause the output to go low. But, if one of the two resistors used is connected to the Discharge pin, the time off can be limited to:

t_off = (.69) R₂ x C,

which causes the square wave output to take on a timed, regular appearance. Thus the "time on" and "time off" can be preset by component selection, or derived using basic calculations based the values of the components at hand.

The 74123 one-shot multivibrator is similar to the above IC in many respects. When used singularly the 74123 is monostable; once it is latched it can not return to its previous state. It also uses an external RC circuit to control the timing, but is governed by the equation:

t_on = (.25) R_total x C (seconds).

In this lab two 74123s were wired together to produce an overall astable output. One multivibrator's output controlled the others input.

Procedure (a):

For the 555 timer, the circuit below was wired:

Figure 14.1: The 555 timer

When tested: R₁ = 9.80kW R₂ = 17.88kW.

Calculations: t_on = (.69) R_total x C = (.69) (9.80kW + 17.88kW) (0.01mf) = 190.92msec

t_off = (.69) R₂ x C = (.69) (17.88kW) (0.01mf) = 123.37msec

Period: p = t_on + t_off = 190.92msec + 123.37msec = 314.36msec

Frequency: f = 1 / p = 1 / 314.36msec = 3.18kHz

Duty cycle: t_on / (t_on + t_off) (100)= 190.92msec / (190.92msec + 123.37msec) (100) = 60.70%

Procedure (b):

For the 74123 one-shot multivibrator, the circuit below was wired:

Figure: 14.2: 74123 one-shot multivibrator

When tested: R₁ = 9.80kW R₂ = 23.04kW

C₁ = 0.01mf C₂ = 0.001mf

Calculations:

For U1a: t_on(U1a) = (.25) R₁ x C₁ = (.25) (9.80kW ) (0.01mf) = 2.45msec

For U1b: t_on(U1b) = (.25) R₂ x C₂ = (.25) (23.04kW) (0.001mf) = 5.76msec

For U1a: t_off(U1a) = t_on(U1b) = 5.76msec

For U1b: t_off(U1b) = _ton(U1a) = 2.45msec

Period: p = _ton(U1a) + t_on(U1b) = 2.45msec + 5.76msec = 8.21msec

Frequency: f = 1 / p = 1 / 8.21msec = 121.80kHz

Duty cycle:

For U1a: t_on / (t_on + t_off) (100)= 2.45msec / (2.45msec + 5.76msec) (100) = 29.84%

For U1b: t_on / (t_on + t_off) (100)= 5.76msec / (2.45msec + 5.76msec) (100) = 70.15%

This circuit's output was viewed on the oscilloscope, and the results were as follows:

Period: 8.28msec

Frequency: 120.75kHz

Duty cycle (U1a): 30.12%

Duty cycle (U1a): 69.88%

Percent difference (measured versus calculated):

Period: (8.28msec - 8.21msec) / (8.21msec) (100) = 0.85%

Frequency: (120.75kHz - 121.80kHz) / (121.80kHz) (100) = -0.86%

Duty cycle (U1a): (30.12% - 29.84%) / (29.84%) (100) = 0.94%

Duty cycle (U1b): (69.88% - 70.15%) / (70.15%) (100) = -0.38%

Conclusion:

The 555 Timer is a versatile IC to use, because it acts as a clock whose duty cycle is governed by the external RC circuit that is built into it.

The 74123 Multivibrator is a versatile circuit for the same reasons as the 555 Timer, with the exception that its full cycle is not symmetrical; that is it is not a 50% duty cycle.

Depending on the needs of the designer, the multivibrator and the components chosen should be considered accordingly.

The percent difference between the measured and calculated values are most likely due to internal resistance of the circuit.