Rpm signal generator

[Turboara]

Hello, I am student still learning about electronics. I am trying to build a rpm signal generator to be able to test a box of tachometers I have laying around. I was trying to get some pointers as to what board I would need, I was wondering if a 555 timer with a potentiometer would be a good starting point? Will that be able to create the pulse signal for the pick up to capture?

 

[Nigel Goodwin]

A 555 would probably OK, it should cover the frequency range you need - however, what specification do the pulses need to be?.

 

[Turboara]

The tachs I have go up to 5-6,000 tops. They are 4 cycle. Trying to get the pulse from 0-6,000 adjustable with the potentiometer. I don’t know if I would need a different set of resisters or capacitors to get the pulse where I want it.

 

[Pommie]

How will you calibrate your 555 circuit? What voltage do your tachometers require? Are you able to program?

Mike.

 

[Turboara]

I dident think I would need programming, I’m going to use a inductive pick up coil clamp to send the signal to the rpm meter, the inductive pick up coil I believe is just looking for a pulsing signal DC. I was thinking I can use the potentiometer to adjust

 

[gophert]

I dident think I would need programming, I’m going to use a inductive pick up coil clamp to send the signal to the rpm meter, the inductive pick up coil I believe is just looking for a pulsing signal DC. I was thinking I can use the potentiometer to adjust

If your only goal is to check if the tachometer will record a reading if you feed it pulses, then a 555 timer (with additional circuitry) is fine to generate the pulses. If you want to know if the tachometer reads the "correct" rpm based on the pulse train sent to it, then you need some type of calibrated stream of pulses to put into the tachometer - so a microcontroller or a crystal based timer and frequency divider circuitry should be used.

 

[Turboara]

My goal is to see to just send it pulses and see if I’m getting readings. I can go to my electronics store and pick up the 555 timer. Anybody know what else I might need?

 

[Pommie]

Depending on where you live, you may be able to use an AC wall wart rectified to give 100 or 120 Hz to test them - 6000rpm = 100Hz . Can you measure the static resistance of the tachos?

Mike.

 

[Turboara]

I wanted to control it with the potentiometer to be able to send it through it’s full sweep.

 

[gophert]

I wanted to control it with the potentiometer to be able to send it through it’s full sweep.

His setup will get you from about 800 rpm until 8600 rpm. But pots are not very accurate so if your pot only goes to 90k ohms, then you'll have trouble getting down to 800rpm but should get most of the tach range.

Some tachs are only connected to one cylinders ignition and, therefore, only fire once every two revolutions of the engine so this circuit may show 1600 rpm as the slowest and 16000 as fastest (tach would be internally calibrated to double pulse speed). Other tachs use some flywheel position to fire the tach so it should fire correctly on those.

Note the pot is a 100k linear version. The 10k is there to prevent the wave from getting too "non-square" at high rpms.

 

[Turboara]

Nice. Going to the electronics store this weekend to pick up some parts. Thank you.