I tried making this multivibrator today but can't make it work properly. Can anyone see what I'm doing wrong? It comes from this page. I think I've used all the same components except the caps are 10uF instead of 1uF. I originally tried using the two 1uF caps I had on hand but they were of different types and, when it didn't work the first time I swapped them out in hopes that was the problem. I've verified that the power connector (9v battery) is connected with the right polarity. I tried looked at the circuit with a scope before putting the LEDs in. I connected the probe ground to the negative rail and the probe to the O1 and O2 indicated on the linked page. It looked like something was oscillating but it certainly wasn't a square wave.

 

Looks ti me like you have the base and collector reversed on the top transistor.

 

I double-checked and the emitter pin is in the negative rail. The center pin (the base, right?) is in colum 23 (with the green jumper) and the remaining pin (collector, I hope) is in column 26. Is that the way it should be?

 

A couple of things:

The 100k base resistor might not supply enough current to saturate the transistor with your 1k load. 10:1 or 20:1 might be a better choice.

If you want the timing to come out right keep the power supply to less than 5 Volts so things don’t get complicated because of the emitter-base junction conducting when the junction breaks down in reverse.

 

Is that the way it should be?

You show the LEDs connected to the base of the transistors. They should be connected to the collectors in series with the 1K resistors.

 

You have already received many suggestions for corrections. Hopefully, the circuit will work after you make all these corrections. If still this circuit does not work, I think you'd better change the cap value to 1uF too.

 

Are you sure your power supply polarity is correct? Red usually identifies the +ve pole, but your red wire goes to the breadboard rail labelled "-".

 

Red usually identifies the +ve pole, but your red wire goes to the breadboard rail labelled "-".

I see the RED wire at the correct location.

 

I see the RED wire at the correct location.

I don't. Each side of the breadboard appears to have both + and - rails. I see a red wire in - and a black wire in +.
The NPN transistor emitters are shown connected to the red wire.

@Circuits123
Not all breadboards are the same. Is each power rail continuous from one end of the breadboard to the other, or do successive groups of 5 holes need to be bridged for continuity?

 

I don't.

I see an orange wire on the negative rail and what looks mostly red on the positive rail.
I can't tell 100% if those jumpers are connected correctly at the power jack so it's possible they are reversed but they appear correct.

 

I have some of those same boards and the rails are continuous which is indicated by the solid color lines.

 

Thanks to everyone for responding. It works! I wish I could say what I fixed but I don't know. I just rebuilt the whole thing.

 

That happens frequently on breadboards. Usually a wire or terminal is no fully inserted and not making a good contact.

 

...which is why they are sometimes not recommended.