I searched the forum first before posting. I found this circuit in the book Timer, Op Amp, & Optoelectirc Circuits & Projects by Forrest M. Mims III. I set it up on a breadboard and it works great. I just don't know why it works. I have a few theories, and I've tried measuring with a digital vom, but I'm still boggled. If anyone has the time, please explain what's happening.

 

I am running the circuit at 5v. Measuring accross R2 or R3 to read the voltage at the base of Q1 or Q2, the meter ranges between a low of 4.1v (led on) and spiking to 5.9-6.0v (led off).

 

I would probably butcher the explanation however here is a detailed astable oscillator one using NPN transistors (way more common one).
http://www.electronics-tutorials.ws/waveforms/astable.html

 

Thanks Sinus23. I'm offline until tomorrow afternoon.

 

No problem. Ask away when you can tomorrow. There are far more experienced people here than me.

 

A DVM is the wrong meter to use on such a circuit. The rise and fall times of the circuit will randomly coincide with the sampling (or gating) times of the meter giving you gibberish. A high impedance analog meter, or better a digital storage scope will display the circuit's happenings much better. E

 

Yes, I wish I had the test bench equipment that I used in my high school electronics classes (class of '74). I wish that I would have paid more attention in class too. The electronics-tutorials article has a great explanation. I had to read through it a few times, but it is starting to make sense. I have copies of several other tutorials from that site, and have added this one to the collection. I set up the circuit on a breadboard using NPN (BC547) transistors this time. I set up a row of 8 leds and connected them alternately so as to make marquee chase lights (and it works). The plan is to make a small display (sign) shaped as an arrow, outlined with lights. It could be attractive (or really annoying).

 

Do you understand how an RC circuit works to charge and discharge a capacitor?
Do you understand how a PNP transistor works?

This should get you started
https://en.wikipedia.org/wiki/RC_circuit

 

You may want to look at explanations of how an Astable Multivibrator works:

An Astable Multivibrator or a Free RunningMultivibrator is the multivibrator which has no stablestates. Its output oscillates continuously between its two unstable states without the aid of external triggering. The time period of each states are determined by Resistor Capacitor ( RC ) time constant.

Consider what Papabravo mentions with RC timing and turning the transistors On/Off. When power is applied one transistor will conduct before the other as both sides will never be perfect and thus it begins to toggle between the two unstable states. The LEDs merely show which state is on at any given time.

Ron

 

Hi Papabrovo and Relaodron. Yes, I have been playing with 555 and 556 timer/oscillators. I skipped the tutorial mentioned above because I was anxious to learn about 555s. I think what was really throwing me was the wacky voltage readings using a digital meter. It made it hard to discern what was happening. And, using NPNs helped make sense of it too. PNPs seem backward.

 

I just ordered a bag of 100 bright yellow LEDs to make marquee chase lights and other flashing things. I just really liked this dual flasher circuit because it is so beautifully simple. I could accomplish the same effect using a 555, or even program an AVR chip.

 

If I'm following you correctly, the circuit you posted will flash one LED then the other. It won't chase through 100 LED's. Using a 555 will do the same thing. You're going to need a couple decade counters and 10 transistors. The first decade counter counts from 0 to 9. When it carries out the second decade counter switches from 0 to 1 (and so on with each successive carry-out). One after the next, each transistor turns on one at a time, providing ground for the first 10 LED's, then the second set of 10, then the third and so on. When you reach the 100th LED the second decade counter returns to its first output (zero) and the chase begins again.

Here's a drawing showing a 20 LED chase. Notice that I set output #2 to the reset. If I set the reset to a higher output # then it can chase more lights. It can chase up to 100 LED's. Each set of 10 has to have its own transistor provide ground. To achieve a 100 LED chase you hold RESET low and just let it run.

[edit] I forgot to draw in the current limiting resistors. Sorry. I'll fix it later but now I have to get ready for college. Taking a 2 day Composits class for work.

 

Thanks Tonyr1084, that is a much more sophisticated way to do it. The marquee chase works. The BC547s can handle about 800ma each, and could theoretically light up 40 leds (20ma each). My display will be much more modest than that. I'm thinking about 20 - 24 leds total (10 - 12 per transistor) outlining an arrow (about 10 - 12 inches long). I just arrange the leds in a line, alternately from connections on Q1 and Q2. I have 8 leds on the breadboard running right now. Simple. I've been toying with the idea of having three arrow tips extending outward, lighting in succession using three 555s in a mono-stable chain. That might be a neat effect (or be even more annoying).