Change the frequency of the output from op amp.

Thread Starter

Chefslot

Joined Sep 24, 2018
37
How would I go about changing the frequency output given out by an op amp? I've looked into 555 timers and also schmitt Trigger Oscillators. I have an AD8232 op amp and I'm wanting to change the output frequency to around 20 khz. When I plug it into a speaker it is rather low, I'm lost on if I need to use a 555 timer or use a piezo electric speaker rated at that frequency or both and how would a schematic look.
Regards
 

MrChips

Joined Oct 2, 2009
23,515
If you have a musical note in the form of a sine wave signal, it's frequency (musical pitch) is a fundamental property of the note. You cannot change the frequency with an op-amp.

Frequency shifting requires much more complex schemes.

Tell us what you are trying to do in the big picture.
 

Thread Starter

Chefslot

Joined Sep 24, 2018
37
Trying to get my Ad8232 Single Lead Heart Rate Monitor to output a higher frequency of 18500 to 19500 hz so that a mobile app can pick it up. The AD8232 is a crude ecg machine, there is a app and some hardware that will pick up the output of the opamp but at higher frequencies of the latter. The hardware uses a AD8504 opamp, a LTC6990 vco and a Piezo.
 

Thread Starter

Chefslot

Joined Sep 24, 2018
37
So what i am trying to aim for is using the 555 chip as a voltage controlled oscillator as the price of shipping for one LTC6990 from the chip shops is kind of pricey! Regards
 

Wolframore

Joined Jan 21, 2019
2,273
you want to modulate your signal??? I'm unclear how you're turning lead into gold. What kind of signal? If it's a square wave you can divide it to get higher frequency... we need more detail.
 

Thread Starter

Chefslot

Joined Sep 24, 2018
37
you want to modulate your signal??? I'm unclear how you're turning lead into gold. What kind of signal? If it's a square wave you can divide it to get higher frequency... we need more detail.
He possibly explains it a little better in the video, but it is picking up small electrical signals from your heart. I wouldn't like to speculate what sort of wave form it mimics but maybe more of a saw tooth wave form.
 

Papabravo

Joined Feb 24, 2006
16,107
So your saying buy the LTC6990 then? Is there like a group buying page on here or facebook etc So one person buys lots of little things that people want and then posts them off?
No, I am saying that the 555 is a very old design with limitations. The LTC6990 is a more recent design, in a small package with, with greater flexibility in selecting frequency and range. If you are a hobbyist/experimenter then learning how to use a 555 is a valuable investment of time and energy. If you are designing a product that requires a feature or features the 555 lacks then you go with an alternative.
 

Audioguru again

Joined Oct 21, 2019
3,173
How would I go about changing the frequency output given out by an op amp?
I have an AD8232 op amp and I'm wanting to change the output frequency to around 20 khz. When I plug it into a speaker it is rather low.
The AS8232 is designed to detect and amplify the extremely low frequency of a human heartbeat. Its datasheet shows that it has a filter that cuts its gain above 1kHz. A speaker is usually 4 ohms or 8 ohms but the AD8232 is designed to drive 50 thousand ohms or higher. It is an amplifier, not an oscillator.

Most speakers cannot produce 20kHz but some tweeters can. Some piezo squeakers can produce 20kHz.

I'm lost on if I need to use a 555 timer or use a piezo electric speaker rated at that frequency or both and how would a schematic look.
The datasheet of a 555 show how to make a 20kHz astable oscillator. It can drive a tweeter that has a resistor in series with it or drive a piezo squeaker directly.

Why do you want 20kHz? Only kids and dogs can hear it.
 

Thread Starter

Chefslot

Joined Sep 24, 2018
37
The AS8232 is designed to detect and amplify the extremely low frequency of a human heartbeat. Its datasheet shows that it has a filter that cuts its gain above 1kHz. A speaker is usually 4 ohms or 8 ohms but the AD8232 is designed to drive 50 thousand ohms or higher. It is an amplifier, not an oscillator.

Most speakers cannot produce 20kHz but some tweeters can. Some piezo squeakers can produce 20kHz.


The datasheet of a 555 show how to make a 20kHz astable oscillator. It can drive a tweeter that has a resistor in series with it or drive a piezo squeaker directly.

Why do you want 20kHz? Only kids and dogs can hear it.
Yes as stated earlier the AD8232 to produce the amplified sound wave from electrical pulses from the body, then a VCO to turn the signal to around (just below) the 20khz range, allowing the app on the phone to pick it up.

Wouldnt the stable oscillator only produce a set frequency, whereas a VCO would give a sweep on that frequency of and around 18500 to 19500 Hz.
Regards
 

Audioguru again

Joined Oct 21, 2019
3,173
The datasheet of a 555 shows a Pulse-Position-Modulator which is a VCO.
Why vary the frequency? Doesn't your heart actually beat with a pulse for each beat? Then why not simply activate the high frequency oscillator for each beat? Oh, maybe the app on the phone must detect when the signal stops because it is out-of-range and not that the heartbeat has stopped.
Phones are designed to use voice frequencies below 3kHz so aren't there many brands of phones that will have trouble "hearing" 19kHz? Many FM stereo radios produce the 19kHz stereo pilot tone.

You did not say the reason.
I guess you want to send heartbeats to a distant doctor in a phone call, or record the heartbeats with a phone? Then to avoid bothering everybody who can still hear 19kHz you are using the high frequency that old people cannot hear?

Did you notice that heartbeats are heard beeping a normal sound frequency in hospitals all the time without bothering anybody? The nurses need to hear the beeps.
 

MisterBill2

Joined Jan 23, 2018
8,743
So what i am trying to aim for is using the 555 chip as a voltage controlled oscillator as the price of shipping for one LTC6990 from the chip shops is kind of pricey! Regards
There is abetter alternative to that 555, which is to use the VCO in a CD4046 PLL IC. it uses fewer components and the VCO is intended to be a linearly voltage controlled oscillator. Even better, with a bit of work it might work as a PLL frequency multiplier, providing an exact multiplication of frequency.
 

wa3tfs

Joined Mar 14, 2017
8
I agree the PLL idea is the best way to duplicate the incoming signal at a higher frequency, however if the thought is to transfer 20 kHz over a phone line, that probably would not work as typically pone lines do not have performance at that frequency.
 

MisterBill2

Joined Jan 23, 2018
8,743
A cheaper PLL IC is the CD4046, which includes both the VCO and phase detectors. There is no spec on the minimum frequency, but the top end is a few megahertz. Since it is a mature CMOS device it is also available from a number of sources. It would bbe quite simple to control the VCO with the output of that heartbeat sensor IC and get whatever frequency span you want. And it is, or at least was, cheaper than many other VCO only ICs
 
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