How to Turn ON a relay when a frequency is reached using a hall effect sensor

Thread Starter

shqip999

Joined Jun 20, 2017
9
i did a manual transmission swap in my car. everything works perfect except the reverse lock out. The reverse lock out is a simple solenoid that activates when the car goes more than 10 mph and prevents you to go into reverse. I want to design a circuit that takes the input from my speed sensor (hall effect sensor) and when the car goes above 10 mph, it will trigger a relay that turns on the solenoid. when the car goes under 10 mph the solenoid would turn off. I did some testing and uploaded a picture of the hall effect sensor when the car reaches about 10 mph. Any help is appreciated.hall effect sensor @10 mph acura tl.JPG thanks
 

LesJones

Joined Jan 8, 2017
2,662
I think you mean 400 hz (Not 400 Khz.) If you don't want to use a micro controller you could use a frequency to voltage converter IC such as an LM2907 and feed it's output into a comparitor IC such as an LM393.

Les.
 

Thread Starter

shqip999

Joined Jun 20, 2017
9
thank you all for the reply. i decided to go with a 555 timer to convert the frequency to voltage and use and comparitor to compare the voltages.
 

jpanhalt

Joined Jan 18, 2008
9,663
thank you all for the reply. i decided to go with a 555 timer to convert the frequency to voltage and use and comparitor to compare the voltages.
Can you post the circuit you plan to use? One circuit I saw has this caution,
The output of the circuit is not a pure DC but a PWM waveform. Additional circuitry is required to convert this PWM waveform to pure DC.
Dedicated F to V converters, like the LM2907 do that too and don't require additional circuitry like a low pass filter/integrator.
 

jpanhalt

Joined Jan 18, 2008
9,663
I am familiar with that chip and perhaps wasn't as clear as I needed to be. Yes, the chip requires external connections and components as do most chips. I felt that was implied. But the integrater/LPF function (C1, R1) is designed in and how to calculate the RC values for those components is described in detail in the datasheet. The same cannot be said for the 555 circuit that I found. Maybe the circuit proposed by the TS includes that information. We don't know.
 
Last edited:

Alec_t

Joined Sep 17, 2013
11,387
Here's a sim of a simple circuit which should do the job :
SpeedTrigger.PNG
I have assumed the Hall sensor is an open drain/collector type with a pull-up resistor to 5V. R1 limits the C1 charge/discharge current to reduce the load on the sensor. The trimpot provides adjustment of the cut-in speed. The TL431 should be ok driving a small relay, but will run warm. If the relay is a beefy one then it could be driven from the TL431 via a power transistor.
The asc file is attached if anyone wants to play.
 

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Tonyr1084

Joined Sep 24, 2015
4,691
Late entry here, but isn't the hall sensor putting out what amounts to PWM locked at 50%? The percentage of time the sensor is high is the same as the percentage of time it is low regardless of speed. So if you're using an RC circuit, suppose you stop at a red light and just by coincidence, the hall is sitting high. Doesn't that charge up the RC circuit and trigger the TL431?
 

eetech00

Joined Jun 8, 2013
1,935
Late entry here, but isn't the hall sensor putting out what amounts to PWM locked at 50%? The percentage of time the sensor is high is the same as the percentage of time it is low regardless of speed. So if you're using an RC circuit, suppose you stop at a red light and just by coincidence, the hall is sitting high. Doesn't that charge up the RC circuit and trigger the TL431?
umm...no...PWM implies constant frequency while varing the width (on/off time) of the pulse. If the frequecy is constant, then a 2907 isn't applicable here.

eT
 

ian field

Joined Oct 27, 2012
6,539
Basically you do a frequency counter and a small amount of code.

Do you write code, work with processors ?

You could also do this with a filter and a comparator.

Or a 567 tone detector http://www.ti.com/lit/ds/symlink/lm567c.pdf

https://archive.org/details/bitsavers_signeticsdcsPLLApplications_5800304



Regards, Dana.
the 567 would be my first choice. There were some interesting 8-DIL F/V converters in early VGA multisyncs, but I doubt any of them are still available.
 
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