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Old 21st Mar 2019, 9:01 am   #1
mark_in_manc
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Default Low frequency indicator?

Hi folks

For OT reasons I need to build a simple (I hope) device to give an indication of frequency (in the form of a dc voltage) from a dirty ac signal with a strong fundamental component. The frequency (ignoring harmonics) will vary from about 300-1500Hz, and the amplitude will not be constant. First thoughts are that I will need to amplify and clip to turn this into a constant amplitude sq wave, and then integrate it, perhaps using a simple low-pass filter. If anyone has any thoughts on this or cookbook places to send me, I would be grateful!
cheers
Mark
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Old 21st Mar 2019, 9:18 am   #2
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Default Re: Low frequency indicator?

A possible 'old-style' circuit-source idea: in the 70s and 80s there were various projects in Practical Electronics, Elektor etc. for car rev-counters.

Some were very simple - a 'debounce' circuit [Schmitt-trigger or 555] to square-up the wobbly waveform from the contact-breaker-points followed by a diode/capacitor "charge-pump" as frequency-to-voltage converter, then an emitter-follower to drive the meter.

Others were more-sophisticated - using a PLL. I even recall one that did it all digitally and drove a 3-digit 7-segment display, using CMOS logic.

I guess these days an Arduino and cheap LCD display would be the way to go.
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Old 21st Mar 2019, 10:17 am   #3
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Default Re: Low frequency indicator?

The PLL sugestion is a good one. By strongly filtering the output from the phase detector it is possible to detect a coherent signal within a negative signal to noise ratio.
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Old 21st Mar 2019, 11:32 am   #4
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Default Re: Low frequency indicator?

Trigger a suitable pulse width monostable from the positive going zero crossings of that square wave (or the original dirty signal if it's at least clean around the crossovers) and simply hang an analogue meter on the output.
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Old 21st Mar 2019, 12:06 pm   #5
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Default Re: Low frequency indicator?

A guitar tuner circuit may give you some ideas.
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Old 21st Mar 2019, 1:04 pm   #6
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Default Re: Low frequency indicator?

LM2917?

Still available from RS for not much currency.
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Old 21st Mar 2019, 4:45 pm   #7
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Default Re: Low frequency indicator?

@Herald1360 -- you beat me to it!

Use something like a 74LS121, configured as a retriggerable monostable producing constant-width and constant-amplitude output pulses, and just feed these into an analogue meter. The faster the pulses are coming, the greater the average energy er unit time will be.
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Old 22nd Mar 2019, 8:37 pm   #8
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Default Re: Low frequency indicator?

Thanks folks. Someone meanwhile has offered to give me an intro to the Arduino, which sounds like a nice opportunity - but I like the monostable idea, driving a meter with a much slower time constant. I think I might try that too - I'll post back here when I have something on a breadboard.
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Old 23rd Mar 2019, 8:36 am   #9
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Default Re: Low frequency indicator?

I've measured the output from the transducer I want to monitor - a noisy but more-or-less sinusoidal floating signal, 2v pk-pk at ~300Hz, rising to ~5v pk-pk at 1500Hz. I'm a bit confused by the max/min trigger voltages on the 74LS121 spec sheet - will I need a 1.8v zener (or something) across the input to stop me damaging the chip with the signal I have?

I hope I'll be able to 'calibrate' this indicator just by choosing R + C to set the pulse length to about 1/1500 - it's quite convenient as a 5v full-scale output would work nicely. Something in the back of my head is bothering me about whether the time integral of a square pulse is linear with pulse duration, but these days I'm more inclined to make it than try to calculate it! I guess I might need an emitter follower to drive the analogue voltmeter I want to use.

Thanks for putting up with my term 1 year 1 elec eng questions
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Old 23rd Mar 2019, 11:17 am   #10
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Default Re: Low frequency indicator?

The average value (as indicated by a moving coil meter) of a repetitive train of constant height square pulses is proportional to the pulse rate, with the limitation that the pulse rate has to be high enough for the needle droop between pulses not to be noticeable.

For 500-1500 Hz this will be fine.
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Old 23rd Mar 2019, 11:29 am   #11
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Default Re: Low frequency indicator?

TTL is not easy to drive direct from a transducer. The input drive needs to sink around 2mA to pull low so you usually need some buffering, usually a comparator. The 121 does have a Schmitt trigger input so that can help in some cases. A cmos version of the 555 timer may work with a simple RC low pass filter and protection diodes on the input. This needs negligible trigger current. Your signal level is a bit low at low freq though, it might need a boost as the trigger is related to supply voltage.

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Old 23rd Mar 2019, 6:37 pm   #12
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Default Re: Low frequency indicator?

I think I was concerned as not only the pulse rate, but the mark-space ratio will be changing, since the 'mark' length is fixed and the 'space' will just last until the next trigger event, which depends on the period of the signal coming out of the transducer. That doesn't sound like a recipe for a linear relationship between frequency and average output voltage to me. Well, we'll see when the bits arrive - maybe I'll have to go for really short 'marks' and then amplify the output to make it more linear.
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Old 24th Mar 2019, 7:41 pm   #13
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Default Re: Low frequency indicator?

I think I may have an original frequency to voltage converter project from Maplins based on the LM331 chip complete with data file info all ready built on a pcb somewhere in the garage if you are interested. It hasn't been used for about fifteen years but you are welcome to it FOC if it will solve your problem. (If I can find it.)
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Old 24th Mar 2019, 9:19 pm   #14
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Default Re: Low frequency indicator?

That sounds good Alan. I'll send you a PM, thanks, Mark
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Old 24th Mar 2019, 10:46 pm   #15
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Default Re: Low frequency indicator?

Quote:
Originally Posted by mark_in_manc View Post
I think I was concerned as not only the pulse rate, but the mark-space ratio will be changing, since the 'mark' length is fixed and the 'space' will just last until the next trigger event, which depends on the period of the signal coming out of the transducer. That doesn't sound like a recipe for a linear relationship between frequency and average output voltage to me. Well, we'll see when the bits arrive - maybe I'll have to go for really short 'marks' and then amplify the output to make it more linear.
The relevant point here is that the pulse height and width are fixed, so the average voltage of a train of them is proportional to the rate at which they happen.

If pulse height is 5V and width is 1ms and frequency is 100pps, average voltage will be 0.5V. Double the pulse rate and the average also doubles to 1V. Double it again gets 2V.

This is a linear relationship of 0.5V/100Hz. Obviously, for the 1ms pulse width, the relationship hits the endstop at 1kHz and 5V when the pulses run into each other and the output is a continuous 5V.

Pick your pulse height and width appropriately and theoretically you can have any range you want, though in practice, circuit non ideality will impose limits to what can be achieved.

This retriggerable monostable driving a moving coil meter technique was used for diy rev counters historically and no doubt other applications too.
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Old 25th Mar 2019, 10:58 pm   #16
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Default Re: Low frequency indicator?

Thanks for that Chris, I see it now. I guess the average of a square pulse with even mark-space will always be half the pk-pk value, whatever the rate - and the monostable sorts this out by fixing the mark - I should have seen it before, and now I do
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Old 26th Mar 2019, 3:09 am   #17
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Default Re: Low frequency indicator?

Quote:
Originally Posted by Herald1360 View Post
This retriggerable monostable driving a moving coil meter technique was used for diy rev counters historically and no doubt other applications too.
555 circuits for diy rev counters were quite popular too.

One example: http://www.555-timer-circuits.com/car-tachometer.html
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Old 26th Mar 2019, 5:28 am   #18
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Default Re: Low frequency indicator?

I'd go for a simple circuit that uses a zero crossing detector and generate a narrow pulse. This tends to eliminate noise and amplitude components which are unimportant. If the signal is distorted it doesn't matter and it can also be filtered too. (On this topic I made a link to the processing of the signals from audio tape for early computers like the Sol-20, these do a terrific job of what amounts to zero crossing detection and due to an agc works over a large range of input signal levels, but I can't attach it on on my mobile device easily)

Once you have your narrow pulse it's dead easy to convert it to a DC voltage proportional to frequency. All you do is feed it to a transistor which resets (discharges) a capacitor voltage to zero over the time of the narrow pulse. Then you charge the capacitor with a constant current source.Then the peak of that climbing ramp signal is merely peak detected by a diode and filter cap with a parallel resistor and that is buffered at the + input of an op amp. It could for example be made to vary from 1 to 9v depending on what you want and your supply voltages. You get a nice stable DC level between updates , it's a simple sample hold. The technique was used in some early servo systems in VCR's and it works very well and is simple for a frequency to voltage converter, either for a meter or a servo system.

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Old 26th Mar 2019, 4:00 pm   #19
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Default Re: Low frequency indicator?

Quote:
Originally Posted by Terry_VK5TM View Post
Quote:
Originally Posted by Herald1360 View Post
This retriggerable monostable driving a moving coil meter technique was used for diy rev counters historically and no doubt other applications too.
555 circuits for diy rev counters were quite popular too.

One example: http://www.555-timer-circuits.com/car-tachometer.html
Note of course the first line in that reference saying what the 555 is configured as......
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Old 27th Mar 2019, 12:18 am   #20
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Default Re: Low frequency indicator?

Yep , single IC solution to the problem
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