Need help with Oscillator design

Thread Starter

chopperdog

Joined May 20, 2010
10
Good Morning.

I have unsuccessfully tried to build an Oscillator that, when it pulses, closes a relay, causing current to flow through the relay.

It is for an automotive installation, so it is 12v.

I used a 555 chip as the timer.

So here is what I am trying to do, and any help is greatly appreciated.

I want to close a normally open momentary switch that will allow 12v power to the timer or oscillator.
The timer will then send out pulses of 12v to the relay. The length of the pulses should be very short, like .20 seconds.

What I want the pulse to do, is to power a relay. When the switch portion of the relay closes, it allows current to flow for that .20 seconds, pulsing as long as the switch is held closed.

Okay, is it better to use a Transistor instead of the relay?
The output of the timer could active the transistor (B, I think), then allow the current to flow from C to E?

If so, what size transistor would I use?

I tried all this, and it didn't work.

I found a design that I am trying to make work for my purpose. I have attached the design I tried.

Any help is greatly appreciated.
 

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Audioguru

Joined Dec 20, 2007
11,249
The coil of a relay is an inductor that produces hundreds of volts when it is turned off. The hundreds of volts destroy the output of a 555 if the two protection diodes shown in the datasheet are not used.
 

Thread Starter

chopperdog

Joined May 20, 2010
10
Which protection Diodes?
I assumed that the relay would be powered by #3 off the 555 chip, and grounded to the same spot that #1 is.
Is all this correct?

Will it work with a relay if I put in the protection diodes you mentioned?

Thank you,

Chopperdog
 

sgardner025

Joined Nov 5, 2009
79
Are you using a solid state relay? If not, you will want to look at using a transistor if you are pulsing it on and off very quickly. What is the circuit used for?
 

Thread Starter

chopperdog

Joined May 20, 2010
10
It is used to interrupt the ignition momentarily, making shifting in and out of gear easier. It is a bit of an experimental design, but similar designs are in use.
I know it will work, I just have to get the circuitry correct.
I do think a transistor would be better, but I am not sure exactly how to make that work.

I am certainly a novice, but am trying to grasps this.

Thanks.
 

retched

Joined Dec 5, 2009
5,208
What are you running through the load side of the relay?

Are you running the coil wire through the relay?

Ill just ask: How are you interrupting the ignition?
 

jpanhalt

Joined Jan 18, 2008
11,088
It is used to interrupt the ignition momentarily, making shifting in and out of gear easier.
Sounds like a good way to reduce the gas mileage, add VOC's to the atmosphere, and maybe create a combustible mixture in your exhaust.

Of course, if it were used to inject fuel into your cut-outs with a spark, you can get a very nice effect. I am not sure that was legal in the 50's using old technology, but I am pretty sure it isn't today.

My best advice, learn to shift. If it is an automatic transmission, just don't press the pedal to the metal all the time.

John

Ahhh, I see. Chopperdog wants it for a motorcycle. I was thinking of chopper as in helicopter. Same comment applies, except the part about the automatic tranny.
 

Thread Starter

chopperdog

Joined May 20, 2010
10
I appreciate the comments, even the cynicism.
Actually it's for a boat.

The design of boat I have actually uses a circuit that does the same thing I am trying to design. The problem with the existing one is that it is notoriously unreliable.

It's a typical car engine.
The way it shifts is that when you select forward or reverse, the ignition interrupt stumbles the engine, allowing the gears to mesh.
It does this by shorting out the coil in micro seconds at a tiime. This sounds odd, but it is how it is done on 90% of the boats out there.
It does so by a mechanical switch.

My idea is to use a momentary switch instead, because the mechanical switch is a whole different issue.

So, does anyone think this can be done?

Thanks.
 

beenthere

Joined Apr 20, 2004
15,819
One observation - mechanical relays operate in 10s of milliseconds. Kinda slow, but possibly enough unless you're drag racing.
 

retched

Joined Dec 5, 2009
5,208
Umm, you want to use a momentary switch? Do you think your timing as a human will be adequate?

Doing this automatically would require you to receive the shift signal, send a pulse to a relay coil to short the leads for the length of the pulse and disengaging the coil in one sweep.

The hardest part should be getting the shift signal from the ECU.
 

Thread Starter

chopperdog

Joined May 20, 2010
10
The momentary switch will be pushed during the entire shifting process, then released when the gears are engaged. Precise timing by me is not required.
The pulsing will last as long as the momentary switch is held down.
This will definately work.

I already have the momentary switch installed, but as you said retched, the human timing is not accurate.

So, basically, all I need is a relay to open and close at about .2 second intervals, whenever I push a momentary switch.

To make this happen, I think it would be good to make the 555 timer activated by the momentary switch that I push, then the timer output would power the relay.

Thanks,

Chopperdog
 

retched

Joined Dec 5, 2009
5,208
Sure, that would be pretty straight forward.

Build a one-shot 555 circuit that outputs a .2 second pulse when triggered.

The momentary button you press, would shoot the .2 second pulse triggering the relay for .2 seconds.

The relays NO contacts would be the + for the ign-coil and the - chassis ground. That will short the ign-coil for the duration of the 555s time.

Here is what you want to do:
This is from:
http://home.cogeco.ca/~rpaisley4/LM555.html



You want R1 to be 180ohms and C1 to be 1000uF.

That will give you a pulse of ~ .1991 seconds

Thats pretty close to your .2 second requirement. ;)

And with the tolerance of most resistors and capacitors, it will probably be a hair over .2 seconds.
 

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Thread Starter

chopperdog

Joined May 20, 2010
10
Thanks retched, but wouldn't that be just a one shot pulse when I pushed the momentary switch?
I actually want a oscillation, like .2 pulse, .2 no pulse, .2 pulse, etc.....
This way, the engine will stutter with each .2 pulse.

I believe that would be an Astable Oscillator, which is my current design. I actually found a pic of my basic design from the same website you referenced. It is possible that i just had the wrong time built in with the resistors and capacitor.

I really appreciate your help.

Thanks,

chopperdog
 

retched

Joined Dec 5, 2009
5,208
Ahh, I wasn't aware that you wanted a repeating pulse.

In that case, an astable is the way to go.

Your resistor and capacitor values are the most important things in having proper timing. Also, the relay should be rated for low current for the coil to be energized.

So using R1 as 51 ohm R2 as 330 ohm and C1 as 1000uF will give you
.26 sec high and .22 sec low over and over until you release the button.

Also from http://home.cogeco.ca/~rpaisley4/LM555.html:
 

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