Monostables Essay, Research Paper

When a beginner to electronics first looks at a circuit board full of components he/she is often overwhelmed by the diversity of do-dads. By looking at the diagrams bellow you can identify some of the simple components and their schematical symbol. Now you are able to call them resistors and transistors instead of “Whatchamacallits”.

Electronic component are classed into either being Passive devices or Active devices.

A Passive Device is one that contributes no power gain (amplification) to a circuit or system. It has not control action and does not require any input other than a signal to perform its function. In other words, “A components with no brains!” Examples are

Resistors and Capactitors.

Resistors:

This is the most common component in electronics. It is used mainly to control current and voltage within the circuit. You can identify a simple resistor by its simple cigar shape with a wire lead coming out of each end. It uses a system of color coded bands to identify the value of the component (measured in Ohms) *A surface mount resistor is in fact mere millimeters in size but performs the same function as its bigger brother, the simple resistor. A potentiometer is a variable resistor. It lets you vary the resistance with a dial or sliding control in order to alter current or voltage on the fly. This is opposed to the “fixed” simple resistors. Active Devices are components that are capable of controlling voltages or currents and can create a switching action in the circuit. In other words, “Devices with smarts!” Examples are Diodes, Transistors and Integrated circuits

Capacitors:

Capacitors, or “caps”, vary in size and shape – from a small surface mount model up to a huge electric motor cap the size of a paint can. Whatever the size or shape, the purpose is the same – It storages electrical energy in the form of electrostatic charge. We will get into the mechanics and further properties of this later. The size of a capacitor generally determines how much charge it can store. A small surface mount or ceramic cap will only hold a minuscule charge. A cylindrical electrolytic cap will store a much larger charge. Some of the large electrolytic caps can store enough charge to kill a person. Another type, called Tantalum Capacitors, store a larger charge in a smaller package.

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A monostable multivibrator is a circuit that produces a single output pulse when “triggered.” It is also known as a one-shot multivibrator. The output pulse is produced by an input signal known as the trigger input. The output pulse of a monostable multivibrator is positive (i.e., an increase fom zero voltage to a desired voltatge level). The trigger input involves switching a constant positive voltage to zero. Once triggered, the output pulse begins and further trigger inputs will be ignored until the output pulse is completed. The length of the output pulse has no relationship to the length of the trigger input.

This circuit uses the 555 timer IC. The length of the output pulse depends on the values of R and C. The formula for determining the length of the output pulse is

output pulse length = 1.1RC

MONOSTABLE MODULE

The monostable module controls the width of the trigger pulse, which is synchronised to the clock signal of the data acquisition board. It is based on dual monostable multivibrators with schmitt-trigger inputs (74LS221). It features positve-transistion-triggered as an inhibit input.

Pulse triggering occurs at a voltage level and is not related to the transition time of the input pulse. Schmitt-triger input allows jitter free output with excellent noise immunity. Furhthermore, 74LS221?s internal compensation makes the pulse width virtually independent of the temperature.

This moule provides three indpendent outputs (1?s, 1.5?s and 2.5?s). The 1?s ouput is used for pulse modulation where as 1.5?s and 2.5?s pulses are used for switching purpose. These pulse widths can be adjusted independently by using potentiometers. Note that the ouput stage of this module is buffered to provide suffcient current to drive the RF switches.

Monostable makes low-cost F/V converter.

Author/s: Mark Brinegar

Issue: Feb 18, 1999

The circuit in Figure 1 is a low-cost frequency-to-voltage (F/V) converter. Using a monostable (one-shot) multivibrator, the circuit accepts an open-collector square wave that varies in frequency from 0 to 10 kHz. The one-shot produces a pulse of a fixed width each time the input signal triggers it. The result is a variable-frequency, variable-duty-cycle signal at the output of the one-shot. The time constant determined by [R.sub.2][C.sub.1], 100 [micro]sec, determines the width of the pulses the one-shot produces. This time matches the period of the maximum input frequency (10 kHz). The duty cycle of the one-shot’s output is thus 100% when the input is at its maximum frequency.

[Figure 1 ILLUSTRATION OMITTED]

The variable-frequency, variable-duty-cycle output of the one-shot is the input for the lowpass filter comprising [R.sub.3] and [C.sub.3]. The net result is, as the input frequency varies from 0 to 10 kHz, the dc output signal varies from 0V to [V.sub.CC]. You can alter the circuit to accommodate different input frequencies by simply adjusting the [R.sub.2][C.sub.1] time constant to match the period of the desired maximum input frequency.

output pulse length = 1.1RC