boosting output of oscillator

Thread Starter

3dchipmaker

Joined Jul 15, 2006
23
thanks ,
let me see if i get this.put 1k resistor - led- 120 resistor before 12v rail entering collector of transistor.then 4k7 resistor between pin3 and base of transistor. do i attach r2 10k before or after 4k7
 

hgmjr

Joined Jan 28, 2005
9,029
pebe said:
The original circuit shown for the 555 will work OK. It does not need the discharge pin connected. It is an astable oscillator with a unity mark-space ratio. The problem, as I said before, is that the output will not go to the +ve rail.

Wiring the driver transistor as in the attached circuit will ensure that the transistor is switched hard on and off and the output will swing between about 0.3V and 12V. The fact that the transistor inverts the signal doesn't matter.
My bad, pebe is right. The circuit in the schematic should work. It is hooked up to produce an output squarewave with an approximate 50% duty-cycle.

The problem is that if the output does not swing above 66% of the supply voltage (8Volts for your 12V supply) and below 33% (4Volts for your 12V supply) the output will not oscillate.

Like joejester, I am puzzled that the LM555 pin 3 is not swinging between 0 and at least 10V.

hgmjr
 

Thread Starter

3dchipmaker

Joined Jul 15, 2006
23
thanks,
did anyone look at schematic and assembly attachments i posted to see if there were any differences that would effect operation?
 

Thread Starter

3dchipmaker

Joined Jul 15, 2006
23
pebe thanks,
tried new output circuit the only thing i was not sure of was if led was wrapped in 1k or not. tried both ways voltage at output was 3.2 voltage before120 was 10v.but circuit would not oscillate
 

Thread Starter

3dchipmaker

Joined Jul 15, 2006
23
thanks joe,
i guess my main problem is getting from a schematic to the real world.the schematic you gave me does not look like the one supplied with the kit.you made that off of pcb layout.without a pcb layout i could not build from schematic or tell if mistakes were made on either. its easy for me to read blue print i can tell what part looks like in my head.but the components do not look like schem soldered on board.what is easiest way to pull both together?
 

pebe

Joined Oct 11, 2004
626
3dchipmaker said:
pebe thanks,
tried new output circuit the only thing i was not sure of was if led was wrapped in 1k or not. tried both ways voltage at output was 3.2 voltage before120 was 10v.but circuit would not oscillate
Going back to your post #22, R2 (10K) should be connected to pin3 - not the transistor base, otherwise the 555 will not oscillate.

You have not said how you are measuring the output voltage. Are you using a 'scope?
 

JoeJester

Joined Apr 26, 2005
4,390
I sure hope he has been using an oscilloscope. Otherwise, the 6 Volt's he's been seeing is the pulses at their nearly 57.6% duty cycle.
 

Thread Starter

3dchipmaker

Joined Jul 15, 2006
23
thanks again,
you are probably going to laugh but i will tell you how i measured output.voltage i measured with a multimeter but pulse rate i measured by attaching speaker and recording sound then cutting 1 second out of recording and decreasing time and counting pulses.
 

pebe

Joined Oct 11, 2004
626
3dchipmaker said:
thanks again,
you are probably going to laugh but i will tell you how i measured output.voltage i measured with a multimeter but pulse rate i measured by attaching speaker and recording sound then cutting 1 second out of recording and decreasing time and counting pulses.
Now things are beginning to get clearer. Another couple of questions and it may be possible to come up with a rational explanation. When measuring voltage:
1. Did you use a digital or an analogue meter?
2. Was it set to its DC or AC range?
 

pebe

Joined Oct 11, 2004
626
3dchipmaker said:
Thanks,
It Was Digital And Was Set To Dc
Then as JoeJester said, your meter is reading the mean level of the waveform.

I was surprised you did not get an erratic reading when measuring a varying waveform with a meter set to DC, but I have just tried it myself. I measured 24vAC on the DC range and read a steady 0V, so I asume the meter has a filter included on the DC ranges.
 

Thread Starter

3dchipmaker

Joined Jul 15, 2006
23
thanks for all your help!

pebe,
would it be safe to say that the way im taking measurement would be an accurate reading of voltage output.and turning signal into audio and counting pulses per second an accurate hertz measurement?
 

pebe

Joined Oct 11, 2004
626
3dchipmaker said:
thanks for all your help!

pebe,
would it be safe to say that the way im taking measurement would be an accurate reading of voltage output.and turning signal into audio and counting pulses per second an accurate hertz measurement?
You cannot make an accurate measurement of peak-peak levels of a waveform by measuring the mean level. a 0-12v pp level will give a 6V mean level - but so will a 5-7v pp signal. If the duty cycle is not known that will also give a false impression. The best way to take measurements practically is to use a 'scope. That will tell you the shape of the waveform, its pp levels and, if a square wave, its duty cycle. There are other ways to measure pp levels with a multimeter but they are laborious.

Your method of measuring frequency is quite unique! A frequency counter would be easier - you can now get mutimeters with the function built-in that give a direct readout of frequency.

If you want to have it all calculated rather than measured, then a simulator like the one JoeJester used would be ideal.
 
Hi, Malcolm here.
Your circuit is called an astable oscillator or sometimes it is called a multivibrator.
As you can see from the diagram, the circuit is symetrical and you will not be surprised to learn that the capacitor and base resistor for each half determine the time that the circuit keeps the collector voltage for that half high.
so... if you change the cap on the LHS to be a larger value, for example, the time that the LHS will remain high at it's collector will increase. Ditto, if you increase the base resistor on that half.
This will reduce the frequency but in a way that you will probably not want as the mark-space ratio is no longer 1:1. So both caps or both resistors need to be increased (I'll come to decreased presently) together to maintain the 1:1 ratio of the output voltage.
The output can be taken from the collector of either transistor b.t.w.
A dual gang potentiometer (a linear rather than log law) is what is needed with the required variable resistors taken from wiper to one end of each pot.
BUT... there is a caveat. If you reduce the base resistors of the transistors too much (eg zero ohms if you turn the pot fully to one end) then you will kill the transistors. For that reason, take the existing base resistor and replace it with 1k ohm fixed resistor in series with the remaining resistance as the variable pot resistance (the pot needs to only be roughly of correct value). Do this for both halves of the circuit.
If you want to do some sums, the time that each half of the astable is high will be 0.7CR where C is in farads and R is in ohms.
Hope this helps, Malcolm.
 

SgtWookie

Joined Jul 17, 2007
22,220
Hi Malcom - welcome to the Forums.

I've been around for over a year, but this thread predates me.

It's a good idea to check the last reply date before replying to a thread, as you may unintentionally bring up a topic that has been "dead" for quite a while; generally considered to not be beneficial.
 
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