What's wrong with my circuit?

I have built this ne556 timer inverter with a few changes, R1 is now 470 ohm, R2 is now 3.3k ohm with a 10k pot, C1 is now a 470pF, and the Darlington transistors are now 2n3055s.

I attached my multimeter to the circuit to measure frequency it should range from 150kHz to 450kHz with the 10k pot. While the pot was at zero (I think) the frequency was about 450kHz but when I started adjusting the pot the frequency went up to 1.1MHz, Why could this happening and how can I fix it?

Also the 2n3055 on pin 9 was getting a lot hotter than the other despite their near exact conditions, why is this happening?

I noticed when adjusting the pot that the frequency does drop to 360kHz then shoots up to 1.1MHz.


Picture of What's wrong with my circuit?
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iceng6 years ago
Nicely drawn diagram the 556 is a dual 555 the only commonality is
pin 7 gnd and pin 14 plus. Output left side is pin 5, output right is pin 9.
You are driving the right Threshold and Trigger both analog input pins by the same output pin that drives the base of Q1. This circuit may not work for every
556 for the same reason input uP digital pins don't switch at the same voltage
from IC to identical IC. As Steve said A scope is much better then my conjecture.
See if someone will loan you one and show you how to use it.
Without one, I would use a 555 to flipflop a JK and drive a pair of MOSFETs
in place of your darlingtons.
556 PIN.bmp
Yes, at the very least you may have to buffer the drives to the transistors.
And get a scope ;-)

And you should add D1 & D2 .
Closing barn door after cow has got out :-)
The MadScientist (author)  iceng6 years ago
I managed to get the circuit working, the problem I found was actually in another part of the circuit which I had not anticipated (circuit we discussed below). The capacitors weren't charging fully and I was getting a high neutral low signal that my multimeter read as 3 times greater than the actual frequency.
GDT circuit.jpg
Never trusted a meter for frequency except for smooth sin waves.
A square wave has a fourier transform harmonic that can manifest
as three times the fundamental in the hidden sadw of a frequency
seeking meter.  This is where a human eye is supreme and can
easily detect real Fq. . .  . . . . .  .  A
If you're messing around with complex circuits like this you NEED a scope ! 5 seconds with that will tell you what's wrong.
The MadScientist (author)  steveastrouk6 years ago
How much do you need to pay to get a decent one?

I've been wanting to get one of those for a while but I don't know anything about them
Take a look on Ebay, I know my dad bought one only about 6 months ago for less than 100 USD, which was a rather beautiful Phillips Dual beam job.

You really need a FAST scope, not less than 20 MHz

Where abouts are you in the world ?

The MadScientist (author)  steveastrouk6 years ago
Sorry about the delay, I'm in Australia.
rickharris6 years ago
Your only giving us part of the story here

The 3055 is getting hot - why? It is drawing too much current - why i don't know from the data you have supplied.

IF your values are correct AND you have done the necessary calculations (search for them) then your oscillator should work OK.

IF something is wrong then either your values are wrong or your calculations are incorrect.

Instruments rarely lie.
The MadScientist (author)  rickharris6 years ago
I have checked all my values and they are correct, and I have entered those values into three different calculators.

I edited the question a bit "I noticed when adjusting the pot that the frequency does drop to 360kHz then shoots up to 1.1MHz." that just doesn't make sense.
as Steve says or a faulty pot?
Good chance he's saturating the inductor I suspect.
What do you THINK you are doing here ? Post a link to the original circuit and I'll take a look at it.

Odds on, either you haven't driven Q2 on hard enough, or its not turning off when
you think it should, and you have a lot of DC flowing.

The MadScientist (author)  steveastrouk6 years ago
I edited the question a bit "I noticed when adjusting the pot that the frequency does drop to 360kHz then shoots up to 1.1MHz." that just doesn't make sense.
The MadScientist (author)  steveastrouk6 years ago
That is the original circuit.
My guess is that the second timer, the one whose output is pin 9, is not oscillating for some reason, and also that it is stuck "high" so that Q2 is always on, and that's why Q2 gets hot.

One thing that looks kind of suspicious about this circuit diagram is that pin 10, the reset pin for the second timer, is not shown anywhere. It should be connected high, same as the reset pin on the first timer, pin 4. That is to say I expect pins {4, 10, 14} should all be connected together but the diagram just shows {4,14} connected together.

BTW, if you do not already have a copy of the data sheet for this ic, NE556, you should get one. Because it is good to know which pin does what. I found mine here:

If you are interested in knowing how this thing is intended to work, it looks like the first timer is wired as an oscillator, and the first one drives the second one. The output from the first timer, pin 5, is connected to the threshold and trigger inputs of the second timer, 12 and 8 respectively. So pins {5, 8, 12} are all connected together. 

Am also guessing that outputs of these timers are 180 degrees out of phase, and the inverse signal of each other... when it's working correctly.  That is, timer 2 is off when timer 1 is on, and timer 2 is on when timer 1 is off. 

The MadScientist (author)  Jack A Lopez6 years ago
I just tried that and there was no difference.

I did play with the pot again and I noticed that the frequency drops to 360kHz then shoots up to 1.1MHz.