# 555 Timer Mono-stable and Bi-stable Modes

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This is a demonstration of the mono-stable and bi-stable modes of the 555 timer IC.

For the mono-stable circuit you will need:

For the bi-stable circuit you will need:

*** I like to use different color 22 gauge hook up wire and make my own.

** This resistor kit will give you all the resistors you need and more. You will want to experiment with different resistors in the mono-stable circuit. http://www.dipmicro.com/store/SET-RF405

* This electrolytic capacitor kit will give you all the capacitors you need and more. You will want to experiment with different capacitors in the mono-stable circuit. http://www.dipmicro.com/store/SET-EC2

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## Step 1: LED and Resistor

The picture shows a resistor soldered onto an LED. This is optional, but I found that having a bunch of these made up makes breadboarding a lot easier. Solder a resistor to the cathode lead of some LEDs. The cathode lead is the shorter negative (ground) lead. 270 - 560 ohms works good for on the RaspberryPi, for an Arduino use 330 - 680 ohm resistors. I usually make them with 470 - 560 ohm resistors so they will work with both.

## Step 2: The Mono-stable Circuit

The 555 in mono-stable mode can also be called a "One-shot" or a "Pulse extender".

When the output pin (3) is low the red LED is on and when the output is high the green LED is on.

When the button is pushed the output pin (3) goes high for an amount of time set by the capacitor and the resistor. If your finger stays on the button longer the output stays high. If you press the button quickly, release it, and press it again quickly the second press does not count.

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When a current is applied the output pin (3) will be low.

When the button is pressed pin 2 senses the current through the 2.2 K resistor and turns the output high and the capacitor starts charging through the 10 K resistor.

When the capacitor is charged to 2/3 of VCC pin 6 detects it, turns the output low, and discharges the capacitor through pin 7.

The 10 K resistor and 220 uF capacitor are just starting points. You should experiment with these values.

When the output pin goes low pin 7 also goes low. You should not replace the 10 K resistor with anything lower than 1 K, or you can damage the chip. When pin 7 goes low it is a direct short to ground.

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You can calculate the time by multiplying the resistance (in Ohms) by the capacitance (in Farads) by 1.1.

Since the capacitance needs to be in farads you will have to divide micro-farads by 1,000,000, or nano-farads by 1,000,000,000.

## Step 3: The Bi-stable Circuit

The 555 in bi-stable mode can also be called a "Latch".

When the output pin (3) is low the red LED is on and when the output is high the green LED is on.

When power is applied the output pin (3) will start low. (Red LED on)

Pressing the switch on the left sends a signal to pin 2 which sets the output pin to high. (Green LED on)

Pressing the switch on the right grounds the reset pin causing the chip to revert to it's original state.

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