Multi-vibrators http://www.ee.ed.ac.uk/~kap/Hard/555/node1.html

Multi-vibrator A multi-vibrator is an electronic circuit that can exist in a number of “states” (voltage and/or current outputs) A flip-flop is a bi-stable multi-vibrator, bi-stable means it has two stable states A state is stable if it is robust against the fluctuations (noise) that are always occurring

Mono-stable multi-vibrator A mono-stable multi-vibrator has one stable output (usually zero). It also has an unstable state. Certain input will put the circuit into its unstable state, which lasts for a set period of time before returning to the stable state. Unstable states are still robust to noise In wave terminology, this provides one with a single pulse.

Pulse STABLE STABLE UNSTABLE

One shots One purpose of a mono-stable multi-vibrator is to output a signal of a specified duration. The input (trigger) may be short (or unknown) in duration, but the output pulse has a predictable duration (can be controlled by the time constant of an RC circuit).  = RC The time constant and duration are not equal but are proportional. Such a circuit is called a “one shot.”

Shapers Another purpose of mono-stable multi-vibrators is to “shape” input signals. Recall in digital circuits we want signals to be clearly high or low; a mono-stable multi-vibrator can take signals which are not of this form and create signals which are.

Schmitt trigger

Schmitt trigger If the voltage is above a certain value (the upper trip point) and rising, the output is high. If the voltage is below another value (the lower trip point) and falling, the output is low. Otherwise, it remains whatever it was.

Schmitt trigger The upper trip point Above the upper trip and going up Below the lower trip and going down The lower trip point

A-stable multi-vibrator In an a-stable multi-vibrator, there are typically two states, neither of which is stable. The circuit repeatedly flips back and forth between the states.

A-stable multi-vibrator

A-stable Multi-vibrator Assume a state where the transistor on left is ON and transistor on right is OFF and the capacitor on the left has no charge. Since the left transistor is on (hard) it is not dropping much voltage, therefore “all” the voltage is being dropped by the resistors The capacitor on the left begins to charge through the 10K resistor on the right

A-stable high low OFF ON Charge building up

A-stable Charge builds up on the left capacitor, “pulling-up” the voltage presented to the base of the transistor on the right. When the base reaches about 0.7v the transistor on the right turns on. Current now starts to flow through the 1K resistor on the far right, thus dropping the voltage level at the collector. That low voltage makes its way to the base of the transistor on the left turning it off. The cycle repeats itself.

A-stable low ON Turns off

Duty cycle In a square wave (e.g. a computer’s clock), the wave is characterized by its frequency, its amplitude and its duty cycle. The duty cycle is the percent of time that the signal is high. Duty cycle = thigh/(thigh+tlow)*100%

555 Timer A similar circuit uses the 555 chip (Integrated circuit) The resistors and capacitors are external to the chip so that the period and duty cycle of the circuit can be controlled.

555

555 as Monostable multivibrator

555 as Astable Multivibrator

555 Timer (WorkBench version)

Crystals The very high frequency square wave used for the CPU clocks are not generated in the manner described on the previous slides. The high frequency signal is supplied by crystals subjected to an electric field.