OK this is my confusion:

"The voltage across the transistor decreases because the LED discharges the capacitor."

If you say the voltage across the transistor decreases it's because at some point it was high. Does the V across the T get high when power is applied and since one terminal of the T is to GND and the other to 12V, then as current flows from the Cap discharging the cap, the + side of the T goes down?

 

45+ years ago I made a Do Nothing Box and gave it to a pretty young exchange student. Well, it did not "Do Nothing"!
We have been married for 45 years

The circuit is 3 cross connected NE2 neon oscillators. They flash one after another.
I wonder if the transistor circuit will work the same?

 

Pretty cool story that made me recall doing "never ending" blinking circuits by taking advantage of the "avalanche" behaviour of Neon lightbulbs. That and the countless times I replaced the elusive newfangled "DIAC" with NE-2s on my dimmers... (my hometown was 220Vac, thus it still allowed a good voltage control range after the NE-2 triggered).

 

Ok so Im trying this astable multivibrator. The differences are that instead of a 330Ω for each LED, i only have 220Ω R and that instead of 47kΩ resistors I have 30kΩ resistors. Could that be the reason the circuit doesnt work?:



My wiring came up like this:

 

Your resistor substitutions, though the 220 ohms made me wince a little.

Are you sure the capacitors are in the correct direction? (Can't tell from photo)
What are you using for VCC?
What are the transistors?
And is VCC + or - with respect to the emitters.

 

I would say, that in the circuit the capacitors are backward. With one transistor conducting it's collector is low, and in order to charge thwe cap so that the other transistor can switch on the cap has to have low enough leakage so that the base voltage for the other transistor can turn on.If the caps leak. At least that is what I can see could be the problem.

 

Thanks,

1. Ok, the caps have the negative side facing the 30kΩ. According to the schematic above, they are in the right direction. The white stripe is the (-) side.

2. Vcc is two 9V in series for a total of 18V.

3. Transistors are 2n2222.

 

For a test, if you take the caps out of circuit, do both LEDs light?
If not, have a look at that first.

 

For a test, if you take the caps out of circuit, do both LEDs light?
If not, have a look at that first.

Like I just mentioned, if the cap leakage is excessive then the thing would not work. AND capacitor leakage current in the reverse direction is generally much higher.

 

Ok, the caps have the negative side facing the 30kΩ.

It should be the positive side to the base, negative side to the collector, because the collector never is more positive than the base.

ak

 

I removed the calls and the Leds don't light up...but I tested the Leds and they are fine. The caps and resistors are new. The T are from an old project but they were fine after I used them.

 

I removed the calls and the Leds don't light up...but I tested the Leds and they are fine. The caps and resistors are new. The T are from an old project but they were fine after I used them.

There is a problem.
Check the transistor connections. They are not all the same.
Without the caps, the current via the base resistors should turn the transistors on and light the LEDs.
Do you have a multimeter?
If so, measure the volts on the transistors.

 

18 V supply with 220 ohms for the LEDs is rather cruel. I'd use just one 9 V battery.

Both pins of the LEDs are plugged into the same strip, so they are shorted.

 

Both pins of the LEDs are plugged into the same strip, so they are shorted.

Well spotted. I completely missed that!

 

At 18 volts you are likely to be avalanching the emitter-base junction of your 2N2222 when the other transistor turns on. Not only will that affect timing, it could lead to eventual failure of your transistor to have enough gain to oscillate.

 

According to the photo the collector of the transistors are connected to ground.

 

looks correct if using 2N2222
i seriously question wiring methods though, connections of all components are stretched widely and probably don't make good connections. when breadboarding, avoid stress on connections, try to make the fit more or less straight into the breadboard holes, without too much fanning out. and don't place more than one wire to same hole. don't see negative power or mention of supply voltage. this circuit only needs couple of volts (5V for example). connecting too high voltage will fry it.

 

See

 

My wiring came up like this:

The green LED is shorted, and it looks like the red one is too.



As mentioned previously, you shouldn't insert more than one wire in a hole. If you abuse the spring loaded clips, you could get bad/intermittent connections.

Your lead forming skills need improvement. When you bend leads, you need to avoid putting stress on them where they exit the component body. You can use smooth jaw needlenose pliers to take the wiggles out of the leads. You can also use the pliers to form some 90 degree bends.

 

Ok I fixed the LEDs issue and it works now.

I still dont understand why the caps had to be backwards if the schematics Ive seen all have the caps with the (-) legs facing the base of the transistor?