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Detect above or below set frequency

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schnauzerpup

New Member
I am a total newbie and I appologize in advance for the simple minded nature of this question, but I am hoping someone can give me a push in the right direction.

I need to detect when a square wave falls below 1.5khz and close a relay and then open it again when when frequency is above that. I would also like to have a pot to adjust the cutoff point.

Am I right in thinking that something like a 555 set up as a "missing pulse detector" with the output sent to retriggerable one shot (can you use a 555 for that? If not what is easiest to use?) is the way to go?

Sorry for these dumb questions. As a background I need to close a relay when the pulses from a car's vehicle speed sensor (conveniently available at one of the pins on the radio) says I am under 10mph. It is to make an automated warning light for a rural mail carrier.

Thank you.
 
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crutschow

Well-Known Member
Most Helpful Member
You could use the digital phase detector (phase comparator II) from a 4046 phase-locked loop circuit. It also has a VCO oscillator you can use to provide the 1.5kHz reference frequency.

Instead of connecting the phase detector output to the VCO input as for a normal PLL connection, just use the phase detector output to control the relay (the output will require a MOSFET amp to drive a relay). If the input frequency is lower than the VCO frequency the phase detector output is low, if the input frequency is higher the phase detector output is high.
Note: the detector output is a series of pulses so you will need to add a small cap to ground at the detector output to get a steady dc signal.

A pot connected to Vdd and ground with the wiper to the VCO input will control the frequency of the oscillator to give the adjustment you want.

The data sheet will show you how to calculate the resistor and capacitor values to get a nominal 1.5kHz VCO frequency.

And, of course, if it's to be powered from the car battery, you will need some type of transient circuit to limit the high voltage spikes that can occur in a car electrical system during starting, etc.
 
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MikeMl

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Brevor

Member
Google LM2907 it's a frequency to voltage converter IC. The National Semiconductor datasheet has some application circuits that you may be able to use. You would need to connect the output of this IC to a window comparator circuit made from a couple of op amps. This would give you your upper and lower limits. I did something like this once using just a PIC microcontroller to test the clock output from a digital controller. It was a single IC solution if you are into programming.
 

schnauzerpup

New Member
Thanks for all the great replies. I have ordered an LM2917 after looking at all the replies and will give that a try. Thanks again!
 

ccurtis

Well-Known Member
This circuit has been around for 30 years, or so, in various implementations. I happened upon this implementation in a quick search for you.

**broken link removed**
 

Alexsgarage

New Member
You can use an XOR gate as a phase detector and an SG3524 to generate a reference frequency at 50% duty. This circuit will detect when the frequency strays from 1.5 kHz.
 

crutschow

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