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Astable Multivibrator

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I made an astable circuit on the breadboard with 2 LEDs and it works no problem.

Is it possible to have both LEDs have power at the same time for the same duration without using a microcontroller? So instead of 1 - 2 - 1 - 2... it would be 1 - 12 - 2 - 12 - 1 - 12 - 2...

If so, what would this be called if it does have a specific name?


You can do it with simple ttl logic and 555 timer

It's quite tricky.

You need an oscillator, counter and some gates to switch on/off LEDs depending on the count and resetting it when it reaches the maximum value.

The first step is to build a truth table, by convention inputs to the logic gate, from the counter are A, B, C, D etc. and the outputs from the gates driving the LEDs  are some other letters.

For example in your case:

There are four states, so you need a two bit counter 22 = 4, if you needed five states, you'd need three bits 23 = 8 and reset the counter when it reaches five in binary. Fortunately you don't need any reset circuitry.

A B | X Y
0 0 | 1 0
0 1 | 1 1
1 0 | 0 1
1 1 | 1 1

X = (A.B')'

Draw the circuit (see attached).

Do the same for Y.

Plenty of tutorials on Boolean algebra can be found using Google, here's a good one.

I think the 74HC series is best for what you want because it can drive LEDs directly, is low power and will work from two AA cells connected in series. TTL is rubbish, it's fussy about the power supply (4.5V to 5.5V) and uses lots of power, CD4000 is slightly better but you'll find that if you've connected an LED to an output it'll be dim and you can't use the same output pin to drive other gates because the LED will reduce the voltage to much.

Rather than messing around with a 555 timer and counter, I'd recommend the 74HC4060 which has an oscillator built-in. All you need to do is use two outputs adjacent to each other for A and B, for example, Q5 and Q4.

I hope you can now see why it's easier to use a microcontroller for tasks like this.

Alternative approach would be 4017 and some diodes, but that would require an oscillator (a tradeoff for simple output decoding).


I'm messing around with some basic circuits on the breadboard getting familure with the basics.

Some bicolor LEDs arrived yesterday and I thought I would test them out by making an astable multivibrator. I found this circuit ( http://www.reprise.com/host/circuits/transistor_flasher.asp ) and made the one on the right with two single color LEDs. Once I got it working, I removed the 2 LEDs and put one of the bicolor LEDs in. I noticed that when there was power for both color pins, it turned yellow - maybe less than a half a second by eyeballing it. So that is where the idea came from.

Now I do have a question where I haven't been able to find a real answer for.

Every circuit that I have seen either has a schematic with the values included or not included. For example, I am reading the Make: Electronics Learning by Discovery book and it does a great job explaining what and how a resistor, capacitor, diode, etc. works. What it isn't telling me is how to figure out the values of everything. If I wanted to know what resistors to use with how many of a certain color LED, I can open up a website, plug in a couple of numbers and it tells me exactly what I should use. I would like to know how I would be able to do that on my own.

Going back to the astable multivibrator circuit ( http://www.reprise.com/host/circuits/transistor_flasher.asp ), say I didn't have the 2N3906 but used a SS8550 instead (random transistor out of parts box). How do I figure out which resistors and capacitors to use to get the same result to the point where if both circuits were next to each other on the same breadboard, they would be at the same rates?


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