Who's smart enough to show me how to create high freq swithcing?

[Tewkes]

I've been asking Jaycar, Dick Smiths, Aus, USA & UK component retailers (only because of ease of contact) for help trying to quickly switch a signal.

The circuit in question has a metal sprocket that passes a magnet/sensor which sends a pulse that relates to a speed on a digital display. It currently runs from 0hz to about 1khz, i need to speed this up to about 1mhz although anywhere from 50khz to 2mhz will do.

I have 3 wires i have the choice of switching. Switching either one will create the desired effect. one is 10vdc, one is 0vdc/earth, one is 5vdc. I've tried switching between the wires but it has no effect. I can manually touch the wires but nowhere near fast enough.

I've tried using a 12vdc motor 2khz motor controler but beacuse it sends 12vdc as it's signal it doesn't work.

It seems the sensor only registers if the SIGNAL itself is open and shut. I have 12vdc power supply available or anything i can buy from an electronics shop.

I'm a sparky so i have some reasonable knowledge about electronics, but this is beyond my understanding.

I've spent hours looking up mosfet's, capacitors, diodes etc. Even though maybe converting the signal from dv to ac to give me the pulse i need but i've hit a brick wall.

Any advice would be greatly appreciated as it's driving me nuts lol. i don't know the current available to me but i'm guessing it's reasonlable as it's a large display it powers up.

If I can measure anything to help out, just ask me and i'll do it.

Thanks guys!!!!

Paul

 

[carbonzit]

Let's figure out what you're really trying to do first.

• So you have a sensor near a sprocket that generates one or more pulses per revolution, correct?
• The current setup gives you from 0 to 1000 pulses per second?
• You want this to be converted to 0 to 50kHz or higher, corresponding to the speed linearly?

Sounds like what you need is a frequency multiplier, one that generates n pulses for every incoming pulse. There are several ways to do this.

 

[Tewkes]

the sprocket has four teeth that rotate dependant on speed, yes i guess a frequency multiplier would work but it must be in series for it to work i think. Say the circuit broke once on the input, that would have to be say 10000 times quicker on the output. But it must be eqactly that circuit. I guess it must be that way because there must be a field generated that creates the pulse. I tried using a 72mm hole saw and putting the teeth very close to the sensor but could only get the signal to read 164, need it to be much higher.

 

[Tewkes]

would need to run it for about a hour so heat might be an issue. Not with the machine but with the delicate components.

 

[Diver300]

What do you want the speeded-up signal to connect to? You mentioned 50 kHz to 2 MHz, so I guess those numbers came from somewhere.

 

[Tewkes]

Freqs

Well it currently runs from 50-200 pulses a second, ideally I'd like the ability to bring it up to 2mhz but I'm unsure if the current electronics will deal with that frequency. Could go for 500khz for now to see how it handles.

 

[Reloadron]

The problem you seem to have is measuring rotational speed of a shaft using a 4 tooth sprocket and magnetic speed pickup. The best resolution you can hope for is 4 pulses per shaft revolution. A speed pickup like you have would be fine if you had a 1,000 or more tooth gear but you are working from a 4 pitch gear.

You can screw around and experiment with frequency multiplication but I doubt you will achieve very great accuracy.

Given a choice (I don't know if you have a choice) I would use an optical encoder of this type. The merit obviously being this will afford you a wide range of pulses like 1,000 or 3,600 pulses per revolution. You can pretty much have whatever you want as well as open collector output versions to meet your voltage needs. They come in countless design flavors.

A magnetic pickup using a 4 pitch gear for what I see in your application is just not a good choice, especially if you want accuracy and resolution.

I only linked to BEI as I use them. There are likely some very inexpensive Chinese manufacture versions out there. A Google of optical speed encoders should yield more results.

Just My Take
Ron

 

[crutschow]

Multiplication will not improve accuracy.

One way to get a faster reading with good precision is to measure the time period between pulses and calculate the speed from that (1/period). That's commonly done to improve precision with low frequency pulses. You could use a microprocessor to perform the 1/period calculation to get the frequency or rpm.