I need to make a tachometer for measuring rpm?

Question

I have built a turbo jet engine, I have a spinning shaft off the compressor side of the turbocharger. I need to send a light beam onto the shaft then because the shaft as 6 points I should be able to count the rpms that make and brake the light. what type of light beam? can I use a 555 timer that can be triggered by the light beam and then send it to a digital counter? I have used a 555 timer in my ignition system that pulses a coil to spark the spark plug, that ignites the propane. so how and what type of light beam with the 555 is the question. thanks for all input, dave

any ideas for this project would be greatly apreaciated.
http://www.davesjetengines.com

to:
koki83 - not sure what you are saying? buy what from scrap yard?

Answer 1

First I'll address the counter scaling. If you get one pulse per revolution from your sensor, set the 555 to one pulse per minute. Use it to reset the counter and use the sensor output to increment the counter. Thus, the counter totals how many sensor pulses you get per minute. You need a register to grab and display the count before the 555 pulse resets it.

If you get 6 sensor pulses per revolution, set the 555 to 1/6 of a minute rate. Again the counter will accumulate RPM.

Depending on the RPMs you are trying to measure and the number of digits in your counter, you may have to scale the 555 timer rate appropriately. For example, if you have a four digit counter but want to count up to 11,000 RPM, you will need to set the 555 to one tenth of a minute so 11,000 RPM will display as 1100 and you have to imagine the "0" in the right most position.

As for the sensor, maybe it would be simpler to use a magnetic pickup (assuming your rotating blades are iron based). The magnetic sensors are easy to get, every car has many of them. Sensing engine RPM, sensing drive shaft RPM, etc. They should be very available at the junk yard.

Whether you select light or magnetic, I am sure you will need some type of amplifier to get the signal level up to that needed to count the counter.

Hope this helps.

PeteC

Answer 2

How closely do you want to measure the rpm's? What precision do you want? You could use a completely digital device for "extreme" precision. You need a high speed counter and clock signal. I suggest something that will operate around 4 Megahertz. Collect the data. Feed that to a division circuit or a calculator. You will need some type of RAM as a register. You will also need to slow the data feed to the calculator, if a calculator is used, down to what a fast human would do. Scientific calculators are as cheap as 6 or 10 dollars. The formula is: (Number of increments in time unit * number of cycles timed * frequency of clock signal)/(Number of clock signals counted). Example: (60 seconds per minute * 1 cylce timed * 4000000 Hz) / (4800 clock signals) = 50000 rpm

Answer 3

Use a red laser pointer (under $10) as a light source, you can mount it a long way from the shaft. Use a red-receptive phototransistor mounted in a cardboard tube under or over the laser pointer like a rifle and sight, and put a blue blocker filter in front of them. Bias the phototransistor from your 555's power supply and put about 10k emitter to ground. You want the transistor to turn ON when you get direct reflection from the shaft. Capacitor-couple that with about .1 mike to trigger your 555 circuit . Send me some test results.

Answer 4

why is that buy one from scrab yards