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Lesson 5: The finished circuit

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It's day 5! You've nearly designed your circuit.

Let's take a quick look at the circuit you had in Lesson 4:



So...

Remember from the previous email how the inverter reverses what is on the input so that the output becomes the opposite?

If there is no voltage on the input, then the inverter outputs a voltage. That voltage switches on the LED.

Recall that 'Voltage' is called HIGH or 1 in inverter-speak.

'No voltage' is called LOW or 0 in inverter-speak.

So imagine if, instead of having the new resistor connected to the switch - as in the previous diagram - you connected it to the output from the inverter - as in this diagram:



Now, let's see what we have:

There's no voltage on the inverter input to begin with... the inverter turns that into a voltage on the output... which turns the LED on.

We also feed the voltage back 'round to the delay element (the resistor + capacitor)... which after a short delay becomes a voltage on the inverter input… which turns the voltage into no voltage on the output. Which turns the LED off.

Back 'round to the delay element, there's no voltage... which after a short delay becomes no voltage on the inverter input… which is converted to a voltage on the output - and on goes the LED again.

This could go on forever!

When the output is HIGH - that is, there's a voltage - the input becomes HIGH after a short delay since they are connected through the delay element.

But when the input is HIGH, the inverter makes sure to change its output to LOW.

Since the output is connected to the input through the delay element, that means the input soon becomes LOW.

Which changes the output back to HIGH again.

So this means the output will keep switching between HIGH and LOW.

Are you with me on that?

And on the output, we have the LED connected...

...which means the LED will blink on and off continuously!


Where is the battery connection??

In lesson 3 you learned that the inverter is always connected to plus and minus. But to simplify, this is usually not shown in the circuit diagram.



Want to build it?

Then look out for tomorrow's email!

I'll show you exactly how to get the components you need - and how to build the complete circuit. Until then, if you want to see it in action, check out this video.

SUMMARY

In Lesson 4 we created a delay element in our circuit that delayed turning the LED on and off as we turned the switch on and off. In this lesson, we fed the circuit back into itself, so we didn't have to keep flipping the switch; once we connected it the action of the delay element and inverter creating alternating voltages turned the LED on and off itself.


Keep On Soldering!
Oyvind @ build-electronic-circuits.com


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