"Heavy Duty" Square Wave Oscillator

[Hayato]

Hello,
I'm projecting an square wave oscillator that needs to function almost 24h/day under temperatures ranging from 80°C to 50°C.

I'm thinking of Astable Multivibrators, but which kind of those multivibrators are adequated for the job? The old and good TR type, does the job? Or an IC type would be more adequated?

Thanks.

 

[Russlk]

Go with the IC, it will be simpler and better. Use an automotive rated one such as SA555, rated for -40C to +85C.

 

[Nigel Goodwin]

Another concern is how accurate does it need to be?, any RC type oscillator (including a 555) isn't going to very stable over that temperature range.

 

[Dr.EM]

Yeah, any RC oscillator won't be extremely accurate, but you can maximise stability by using metal film resistors and either polystyrene or silvered mica capacitors for the timing components. These parts are more stable.

 

[Nigel Goodwin]

Yeah, any RC oscillator won't be extremely accurate, but you can maximise stability by using metal film resistors and either polystyrene or silvered mica capacitors for the timing components. These parts are more stable.

Yes, or (much better) use a crystal to generate the required frequency, divided down as required - or an 8 pin PIC with an external crystal.

But it really depends what it's for?, what the require frequency is?, and if it needs to be stable and accurate?.

If it's not at all critical, a simple 555 would be fine - or even a standard astable multi-vibrator.

 

[Hayato]

Thank you all.
Well, I'm avoiding the 555, because I need "constant" duty cycle.
If I add an potentiometer to vary the 555 oscillation frequency, its duty cycle will vary too.

The frequency ranges from 60Hz to 15kHz. It's not needed an extreme accuracy, minimum of about 90%. But it needs to be stable.

Are you sure xtal will be better? The vibration of car will not affect its physical structure?

I'll use to drive the ignition coil.

 

[Nigel Goodwin]

Thank you all.
Well, I'm avoiding the 555, because I need "constant" duty cycle.
If I add an potentiometer to vary the 555 oscillation frequency, its duty cycle will vary too.

The frequency ranges from 60Hz to 15kHz. It's not needed an extreme accuracy, minimum of about 90%. But it needs to be stable.

Are you sure xtal will be better? The vibration of car will not affect its physical structure?

I'll use to drive the ignition coil.

Well you've now completely altered your original question! - you never mentioned it had to be variable, obviously a crystal would make that difficult.

For a start I would suggest you try telling us EXACTLY what you are trying to do?.

 

[Hayato]

Well you've now completely altered your original question! - you never mentioned it had to be variable, obviously a crystal would make that difficult.

For a start I would suggest you try telling us EXACTLY what you are trying to do?.

A-ha! Because of that I wanna build an astable oscillator!!

Well, as I've said before, I'll build a driver for ignition coils. I need a variable frequency one, because I wanna find the "optimal oscillation frequency" of the ignition coils.
In my mind, the optimal frequency is around 500 Hz ~ 1kHz, but I wish to find.

But I haven't changed my question, I still need that it be stable functioning almost 24h/day in temps ranging from 50°C to 80°C.

Thanks

 

[philba]

To get the stability, the uC solution is going to be the best though how you adjust the frequency with stability is going to be tricky.

 

[justDIY]

a pic with a hardware PWM should give you want you desire, just program in some buttons "UP/DOWN"

12F683 I believe has one hardware pwm and an onboard (iirc temperature compensated) rc clock

 

[Hayato]

Philba,
After I find the "optimal frequency", I'll lock the frequency.

justDIY,
A phantom named uC is meeting me every time. Just kidding.
But I need to do some PIC courses, my knowledge in that area is very low. And all material that I find about PIC programming in the internet is very weak.

Thanks

 

[Hero999]

90% accuracy?

Do you know that means?

It means is can vary by 90% and you've not said whether it's +/- 90% or both.

If a 1Hz oscillator has an accuracy of 90% that means it can drift between 1000 + 1000 * .9 and 1000 - 1000 * 9, that's a very big range! Are you sure you didn't mean 10%?

 

[Hayato]

90% accuracy?

Do you know that means?

It means is can vary by 90% and you've not said whether it's +/- 90% or both.

If a 1Hz oscillator has an accuracy of 90% that means it can drift between 1000 + 1000 * .9 and 1000 - 1000 * 9, that's a very big range! Are you sure you didn't mean 10%?

Ooops...
I forgot a decimal separator there, 9.0%

Sorry for that error.

 

[panic mode]

so you want circuit that will stay stable within 9%, have adjustable frequency and duty cycle but not depend on each other?

there are several ways to do it, you can try mcu, use oscillator followed by monostable or use sawtooth oscillator followed by comparator with adjustable preset but there are also CMOS or TTL solutions...

 

[Hayato]

so you want circuit that will stay stable within 9%, have adjustable frequency and duty cycle but not depend on each other?

there are several ways to do it, you can try mcu, use oscillator followed by monostable or use sawtooth oscillator followed by comparator with adjustable preset but there are also CMOS or TTL solutions...

Yes, for example,
It keeps the duty cicle of 50% at 60 Hz.
It keeps the duty cicle of 50% at 500 Hz.
It keeps the duty cicle of 50% at 10000 Hz.

And so on. I was looking for a fixed duty cycle astable multivibrator mode of 555 and I've just found what do I want.

It's a 555 cofiguration that uses only a capacitor and a resistor to oscillate.

 

[Hayato]

Just one more thing.
Does anyone knows a "pulse expander" circuit?
Like.
If I input a 1 ms single pulse in a device, the device will output a 3 (1*3) ms pulse.
If I input a 2 ms single pulse in a device, the device will output a 6 (2*3) ms pulse.
If I input a 3 ms single pulse in a device, the device will output a 9 (3*3) ms pulse.

Thanks.

 

[Russlk]

There is a problem with your project: the automotive ignition coil has already been optimized and it cannot sustain 50% duty cycle. You will have to design the ignition coil also.

 

[Hayato]

There is a problem with your project: the automotive ignition coil has already been optimized and it cannot sustain 50% duty cycle. You will have to design the ignition coil also.

Hello, I'll use for old cars.

It may not sustain 50% duty cycle at low frequencies, like about 84 Hz (breaker point rate at 2500 rpm, for 4 cylinders cars).
But if I use higher freqs. like 5kHz, it will sustain.
50% duty cycle of 200 Hz = 2.5 ms
50% duty cycle of 5000 Hz = 0.1 ms

But first, I need to make some tests, to see if the traditional ignition coil core sustains freqs. in the range of kHz.

 

[Nigel Goodwin]

To be honest, your entire scheme makes no sense at all? - and why would you want 50% duty cycle to feed an ignition coil? - essentially the coil charges up while the points are closed, and generates the spark (by it's back EMF) when the points open.

 

[Hayato]

To be honest, your entire scheme makes no sense at all? - and why would you want 50% duty cycle to feed an ignition coil? - essentially the coil charges up while the points are closed, and generates the spark (by it's back EMF) when the points open.

I think we are being off topic here.
We are discussing if it will work or not, let this subject to me!
But it's my fault, because I'm not making myself too clear.
I'll try to clarify.

The main purpose of 50% duty cycle is to have multiple sparks while the breaker point is shorted.
For example, in a traditional ignition system, I'll have 1 pulse (driven by the breaker point), consequently I'll have 1 spark.

If I use the breaker point to drive a circuit, to deliver several pulses, consequently I'll have several sparks.

I made a drawing:
**broken link removed**

There is why I need a "pulse expander" too.

If the Breaker Point keeps 2ms shorted, I need a "pulse expander" to make the oscilator oscillate for 6ms period.

If the Breaker Point keeps 3ms shorted, I need a "pulse expander" to make the oscilator oscillate for 9ms period.

And so on...