# How To Use A Cmos 4017 As A Toggle Switch

Posted on Feb 6, 2014

This circuit uses a Cmos 4017. The 4017 is a decade counter. The count starts at zero. And it advances by one - each time pin 14 is taken high. When the count reaches nine - it goes back to zero - and starts all over again. = As the count progresses - each of the output pins goes high in turn. The first is pin 3 - it represents zero. The second is

pin 2 - it represents one. Pin 4 represents two - and so on. The number represented by each output pin is shown in red. = Although the 4017 will count up to nine - we are only using it to count up to two. Consequently - most of the output pins are unused. Unused Cmos output pins should always be left unconnected. = The circuit uses two output pins and two input pins. The outputs are - one and two - at pins 2 & 4 And the inputs are - Clock & Reset - at pins 14 & 15. = The output at pin 2 is used to control the relay. When the count reaches one - pin 2 will go high. When pin 2 is high - it`s connected internally to the positive line. And it supplies base current to the transistor - through R2. = The resistor limits the base current to under 5mA. This is more than enough to switch the transistor on. When the transistor switches on - it connects the negative side of the relay coil to ground. And this causes the relay to energize. = The transistor does a second job. It connects the negative side of the LED to ground. The LED is acting as an indicator. If the LED is lighting - you know that the relay is energized. The 4mA or so - provided by R1 - should make it bright enough for this purpose. = When the button is pushed - and the count moves on to two - pin 2 will go low again. When pin 2 is low - it`s connected internally to the negative line. As a result - it can no longer supply base current to the transistor. So the...

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