Autosport Labs

Installed a hall effect sensor for a tach signal. It's more or less working. It does correctly report the RPMs but it flashes back and forth between zero and the RPMs when the sampling is set to 1Hz. When I set it to 25Hz it goes back and forth between zero, the actual RPMs and a random crazy high number that is greater than 25K.
I have it attached to the first RPM channel and I've tried powering the sensor from 5VDC and 12VDC with the same result.
Attached is the image from the scope. It looks like the signal is good. Any ideas what could be going on here?

Hi,

Sorry for the late reply.

Is this MK1 or MK2 RaceCapture/Pro you're working with? MK2 has some additional filtering and hysteresis built-in. However, note there are no pull-up or pull-down resistors built in - you need to provide those externally.

Your hall effect sensor might benefit from a pull up resistor tied to 5v.. Can you describe your circuit in greater detail?

Thanks for the reply. This is for a RCP MK1.

I'm using the Hamlin #55505 sensor that is listed in your sensor guide. The circuit is pretty straight forward. I've connected the power to 12VDC and 5VDC with the same result. The signal wire from the HE sensor is going into the 1st RPM/Frequency port.

I'm a little confused about your suggestion to use a pull up resistor. I'm thinking it isn't needed for a HE sensor.

Many Hall effect sensors require a pull-up resistor so that a strong high and low signal is generated.

By itself, a hall effect sensor is like a normally open switch. Some sensors include a built-in pullup resistor. Check the sensor datasheet you have for information.

Here's one topic discussing concepts around it: http://forum.arduino.cc/index.php?topic=74007.0

If you still experience noise then you may want to buffer it with a schmitt trigger buffer circuit, which will clean up the signal and eliminate the false readings).

I installed the pull-up resistor and the tach is working great. On to the next sensor.

Hooray! Glad you figured it out!

No figuring it out involved on my side. It was your simple suggestion that fixed this. Thanks for responding to the post.

I'm revisiting the tach since I occasionally get a random reading that is impossibly high, 13K+. The sensor I am using is the Hamlin 55505. http://www.littelfuse.com/~/media/elect ... =hamlin-lf

According to the sensor guide this shouldn't require a pull up resistor and it also has some logic built in to take out noise.

It definitely works better with the pulp resistor so I am keeping that. My goal now is to get the signal stable so I don't get the intermittent high spikes.

This is important since I am going to use the tach signal for a rev limiting system. This is a diesel so standard rev limiters don't work.

Any ideas what could be going on here?

RaceCapture/Pro has hysteresis built in to the timer inputs for even further noise filtering after the noise filtering that the sensor may provide.

I would put an oscilloscope on the output of the sensor to see what the waveform looks like, to see if there is any additional noise signals. That would be the next best step to take.

Let us know what you find out!

I'm using the MK1. Does the MK1 have the hysteresis rejection built in? The signal looks pretty clean to me but I really don't know what I am doing here.

Actually, MK2 has the hysteresis on the input, MK1 does not. You could add a similar circuit to MK1 to buffer the input. That should clean things up.

torkey wrote:I'm using the MK1. Does the MK1 have the hysteresis rejection built in? The signal looks pretty clean to me but I really don't know what I am doing here.

If you zoom in the timebase on your scope, you might see some noise on the falling edge, or the rising edge. If there's noise, even one really brief spike, it could be detected as an impossibly close pair of pulses, which would result in an impossibly fast RPM reading.

From the rev G schematics, RCP's 3 frequency inputs have a little surge protection, then a 74HC14 Schmitt trigger chip, which gives 0.8V of hysteresis (noise rejection). But if your signal has bigger/fast noise, it might still get through. The 74HC14 chip specifies a switching time of 22nanosec high-low and low-high, so theoretically it could pass a 20MHz signal nice and clean. But fast noise is still noise...