Help .. need SIMPLE circuit for tach light

[rexlan]

555 tachometer circuit

I have an electronic tach on my small race car. I changed engines and now need to control an intake motor based on engine RPM.

I have a 12v square wave output from the engine ECU that goes to an existing tachometer. I can connect to that line. I have a 4 cylinder engine so I assume 4 pulses/rev.

I need a very, very simple circuit (preferalby a 555 or something even easier, that can count the pulses to figure out the RPM and then at about 3,000 RPM give me a switched 12v output (very low current required).

I also need to be able to program or set the exact RPM to switch at ~ 2500-3500

So if the RPM is > 3,000 turn the little motor on.
RPM < 3,000 RPM turn the motor off.

I have very basic stuff, R, C and a few diodes, FET's. I need a schematic please.
I also have to make this thing very small and I can not use a microcontroller because I need to do this myself.

Thanks for reading and any help!

 

[IanP]

tachometer circuit

Of course, you can build a simple achometer with 555 timer and a few external components + a meter ("mechanical integration") .. see picture below ..
https://www.electronics-tutorials.com/devices/555-tachometer.htm
As the integration is performed by the meter there is no easy way of getting dc voltages to compare with a reference for certain frequencies ..

However, I would go for a PICAXE microcontroller .. also 8-pinner ..
It is extremaly easy to program in BASIC, so building a tachometer around this gizmo wouldn't be simpler .. see another picture below ..
If you need more details on this - ask ..

Regards,
IanP

 

[on1aag]

led tach circuit

Hi Rexlan,

This circuit can detect the speed of your motor and will give you an
output command of 12 volt. The speed can be set between 6,35 mS
(2362 rpm) and 4,13 mS (3632 rpm).
The input resistor with both diodes and the led are optional.

on1aag.

 

[rexlan]

led tachometer circuit

WOW .... you folks are terrific and thank you very much.

on1aag
this is exactly what I am looking for.

I have a spec racer ford (see picture) and I am building an enduo motor for it with a different engine (12 hour race). The new engine uses a split port intake and has a motor control to open the second half of the ports at 3-3500 RPM.

My existing ECU does not have an output for this new engine so I need a way to sense the RPM and then send 12V to the SP controller to open it. When the RPM drops below the set point I need it to close (remove voltage).

Ideally, it should open and close pretty fast.

I measured the coil and it looks like 1.5K but I actually measured the current and it draws 3.6 amps when it is locked full open!

So, I will need a relay and some current limiting as well I think.



IanP
This circuit won't work for me. Using the PIC is ideal but the problem is that I would have no way to work with it because I don't understand how it works!!! I can handle a basic resistor but the 10011100011 and I'm outta here! Thanks

 

[Buriedcode]

simple tachometer

Hi,

Hope you don't mind me joining the party here, but I've been asked to do a similar thing, with one small exception. The 'output' is simply an indicator, LED, that starts to flash at a predetermined rpm, then flashes faster, and finally is continually when another rpm is reached.

Now, it would be good practice for me to use 'logic' or timers, but I might as well just use a small PIC, as IanP suggested. They all come with at least one timier, so it won't have to be a flagship PIC or anything.

However, although I think I can sort out measuring the rpm from the engine (depends on where it getting timing from, ECU or sparks/sensors) using a timer, I'm not sure where to being for the flashing LED. Say I had the lower limit as 3000rpm, and the upper as 5000. I would like this to be used entirely for the 3000-5000 range. Would anyone recommend 'waiting' a set time, before starting timing? eg, wait 12ms, then start a timer so I don't waste the resolution of it for speeds of >5000.

I must say, although I've been using PIC's for a while, I'm a tad indicisive about using a 'counter' or a 'timer' (or both). IanP or anyone else, please advise.

Thankyou,

BuriedCode.