Audio wave form imitator circuit

Thread Starter

LikeTheSandwich

Joined Feb 22, 2021
41
Ok, so I have an application where I want to imitate a microphone like beeping or buzzing input, doesn't really matter, just need a general audio waveform without using an actual mic and audio input. I was thinking maybe with a low-voltage zener diode, capacitor, and resistor might do it, but I'm not educated enough to know how to do this. The next best option I can think of is using a cheap chip like a Seeeduino Xiao which has an analog output, so I could program some waveform on it and have it output that, but those are a few dollars, like $6, and the size of a quarter, so I'm hoping there's some simple, easy, cheap, compact way to get something like that. I've searched the web a bit but I couldn't find anything. I would imagine someone has done something like this, but I can't seem to find it.
 

Yaakov

Joined Jan 27, 2019
2,442
Could you explain the application for this? Your description isn’t enough to help. If you could describe what you want to do instead how we can probably help.
 

Thread Starter

LikeTheSandwich

Joined Feb 22, 2021
41
Could you explain the application for this? Your description isn’t enough to help. If you could describe what you want to do instead how we can probably help.
I'm going to use two walkie-talkies, one as tx and one as rx. The rx will be used to control a remote shut-off. As long as it is receiving a signal, it will output over the "speaker" output which is fed into an Arduino-type controller, and it will keep a relay active. When the signal is discontinued or out of range, it will stop outputting, and the controller will shut off power to the motors. Although as I'm saying this, I'm realizing that I may not need an audio wave form, a constant voltage may work just fine as I don't need to receive an audio waveform, receiving a constant voltage signal would work just fine for my applications. So maybe just a resistor from the positive voltage to the positive contact from the mic input might work just fine.
 

Yaakov

Joined Jan 27, 2019
2,442
F
I'm going to use two walkie-talkies, one as tx and one as rx. The rx will be used to control a remote shut-off. As long as it is receiving a signal, it will output over the "speaker" output which is fed into an Arduino-type controller, and it will keep a relay active. When the signal is discontinued or out of range, it will stop outputting, and the controller will shut off power to the motors. Although as I'm saying this, I'm realizing that I may not need an audio wave form, a constant voltage may work just fine as I don't need to receive an audio waveform, receiving a constant voltage signal would work just fine for my applications. So maybe just a resistor from the positive voltage to the positive contact from the mic input might work just fine.
For many reasons, this is a terrible idea. Your first version is bad and the new version won’t work at all. In addition, it is almost certainly illegal, not matter where you are located.

1. Does this have anything at all to do with safety?
2. What is the range you need?
3. What are the motors? (size, function)
4. How are you controlling the motors?

and, why do you want to do this? What is the scenario?
 

Thread Starter

LikeTheSandwich

Joined Feb 22, 2021
41
F


For many reasons, this is a terrible idea. Your first version is bad and the new version won’t work at all. In addition, it is almost certainly illegal, not matter where you are located.

1. Does this have anything at all to do with safety?
2. What is the range you need?
3. What are the motors? (size, function)
4. How are you controlling the motors?

and, why do you want to do this? What is the scenario?
I have a small, custom-built electric go-kart. According to my local authorities, it is completely legal to have it, and even legal to use on the roads around my town. I want a remote kill switch so that while my niece or nephew is using it, I can remotely kill power if they are going too far or are heading into trouble, pretty much like this video, except my walkie-talkies work a bit differently.

I can imagine no reason why this would NOT be legal, can you explain how it possibly could be?

Range only needs to be a minimum of probably 300ft or so.

Roughly 1kW drive motor, brushless DC.

A BLDC motor controller.
 

Yaakov

Joined Jan 27, 2019
2,442
OK, so it is safety oriented which means that it must be designed inherently failsafe. The idea of turning off in the absence of a signal is a very good start, but you must be sure the receiver is immune to interference that would make the go kart continue to operate even if you stop transmitting.

It is not the go kart that is illegal, it is the use of walkie talkies for remote control. You have two problems:

1. The bands on which those devices operate are not allocated for this purpose.
2. Modifying type accepted equipment makes it illegal to operate on its intended frequencies.

A additional problem is the pervasive use of FRS band radios makes interference with your safety application.

A different approach is to use one of the many 433MHz modules intended for remote control. They are cheap and since they are digital, they are (more) immune to interference. These modules are available from many sources. Some have multiple channels.

Here's an example: https://www.amazon.com/Transmitter-Receiver-Motorcycles-Sutomobile-Anti-Theft/dp/B08YMYWFN4

With these modules, as long as you are transmitting, it will send a digital signal to that channels output on the receiver.

I strongly encourage you to look in this direction rather than your current one which has a lot of problems.
 

Thread Starter

LikeTheSandwich

Joined Feb 22, 2021
41
1. The bands on which those devices operate are not allocated for this purpose.
2. Modifying type accepted equipment makes it illegal to operate on its intended frequencies.
My understanding is that there are certain exceptions for low-power applications, like how anyone can operate a "pirate" radio station on any FM signal, as long as it's under a certain power rating. I've just tried googling "modifying walkie talkies," and "modifying type accepted equipment" and I couldn't find anything to indicate that it's illegal. Can you give me some links or keywords to google search so I can look further into this?


A additional problem is the pervasive use of FRS band radios makes interference with your safety application.
This would indeed be a consideration. I'm not terribly worried about it, as there are up to 15 channels to use (just researched and found out that a license is needed for 16-22), but it is a consideration. However, since I'll be broadcasting something very loud and obnoxious on whatever channel I'm using, then most likely someone else would want to switch channels, rather than interfere.

A different approach is to use one of the many 433MHz modules intended for remote control. They are cheap and since they are digital, they are (more) immune to interference. These modules are available from many sources. Some have multiple channels.

Here's an example: https://www.amazon.com/Transmitter-Receiver-Motorcycles-Sutomobile-Anti-Theft/dp/B08YMYWFN4

With these modules, as long as you are transmitting, it will send a digital signal to that channels output on the receiver.

I strongly encourage you to look in this direction rather than your current one which has a lot of problems.
A good suggestion, I will look into it, thank you.
 

Yaakov

Joined Jan 27, 2019
2,442
My understanding is that there are certain exceptions for low-power applications, like how anyone can operate a "pirate" radio station on any FM signal, as long as it's under a certain power rating. I've just tried googling "modifying walkie talkies," and "modifying type accepted equipment" and I couldn't find anything to indicate that it's illegal. Can you give me some links or keywords to google search so I can look further into this?



This would indeed be a consideration. I'm not terribly worried about it, as there are up to 15 channels to use (just researched and found out that a license is needed for 16-22), but it is a consideration. However, since I'll be broadcasting something very loud and obnoxious on whatever channel I'm using, then most likely someone else would want to switch channels, rather than interfere.


A good suggestion, I will look into it, thank you.
Unfortunately, your understanding of unlicensed operation is flat wrong. The actual FCC regulations strictly limit the use of the unlicensed bands including output power. Transmitting a low power signal isn't a special exception across the board. The only band where dual use includes low power transmissions is the broadcast band.

It is precisely using the frequency in a way that makes it unavailable to others which is illegal. FRS is a shared band and it is not allocated for remote control. There are bands allocated for remote control but FRS is not one of them.

Permitted uses of FRS: https://www.law.cornell.edu/cfr/text/47/95.531
Prohibited uses of FRS: https://www.law.cornell.edu/cfr/text/47/95.533

"§ 95.533 Prohibited FRS uses.
FRS units must not be used for one-way communications other than those listed in § 95.531(b). Initial transmissions to establish two-way communications and data transmissions listed in § 95.531(a) are not considered to be one-way communications for the purposes of this section."

Part 95C remote control band(s):

Permitted uses: https://www.law.cornell.edu/cfr/text/47/95.731
Certification exception in the 11m band:

"§ 95.735 RCRS equipment certification exception.
Notwithstanding the general requirement of § 95.335, a non-certified RCRS transmitterthat transmits only in the 26-28 MHz band and complies with the applicable technical requirements in this subpart may be operated in the RCRS for the purpose of controlling a remote device."

As far as type acceptance goes, every service has type acceptance criteria. The devices must pass certification tests to be legally operated in the respective services. Modifications to the equipment would require recertification before operating.
 

Thread Starter

LikeTheSandwich

Joined Feb 22, 2021
41
Interesting. Well that's inconvenient for me and is therefore stupid (not really). But those transmitters you linked me to are very inexpensive, as well as they appear to be quite simple and functional, very low power. Although they don't list a range, they should be fine. They also appear to be somewhat programmable, and so won't interfere with each other.
 

Yaakov

Joined Jan 27, 2019
2,442
Interesting. Well that's inconvenient for me and is therefore stupid (not really). But those transmitters you linked me to are very inexpensive, as well as they appear to be quite simple and functional, very low power. Although they don't list a range, they should be fine. They also appear to be somewhat programmable, and so won't interfere with each other.
Yes, probably a good choice and quite cheap. They are very easy to apply. If you try, please let me know. Sounds like a fun project.
 

Yaakov

Joined Jan 27, 2019
2,442
Will do, and thanks so much for all your help!
By the way, there are four channels, I would suggest you consider one as the dead man switch you propose and a different one as a positive disconnect "emergency stop" button. Alternatively, you could make it an actual dead man switch and have a button that you have to hold down for the kart to go.
 

Thread Starter

LikeTheSandwich

Joined Feb 22, 2021
41
By the way, there are four channels, I would suggest you consider one as the dead man switch you propose and a different one as a positive disconnect "emergency stop" button. Alternatively, you could make it an actual dead man switch and have a button that you have to hold down for the kart to go.
I'm just planning on a single dead-man, since it will be interpreted through an Arduino-type controller.
 

Reloadron

Joined Jan 15, 2015
5,888
Years ago Forrest M Mims III Engineers Notebook II did a few circuits using a CEX-4000 Tone Module in conjunction with a few walkie talkies. A Google of Touch Tone DTMF Encoder and DTMF Decoder should bring up some pads on Amazon or other seller. Just like a telephone keypad it gives you 12 DTMF tones in the example circuit. See page 109 of this link. Also see page 108. The concept is also popular with ham radio operators (licensed) who run a phone patch using a 2 meter handie talkie. Again, for those you need a license. So anyway you may want to Google DTMF Encoders and DTMF Decoders. If you just want to experiment producing DTMF tones on your home computer Audacity is a free audio program which will produce DTMF and about any other tones you could want. If you just want to decode a few tones it is easily done using a 567 tone decoder IC.

Ron
 

Audioguru again

Joined Oct 21, 2019
2,875
The circuit from the internet uses a 53 years old 741 opamp that will stop working when the 9V battery voltage drops a little.
Use a modern opamp that is guaranteed to work when the battery voltage drops to 5V or less.
 

Reloadron

Joined Jan 15, 2015
5,888
The circuit from the internet uses a 53 years old 741 opamp that will stop working when the 9V battery voltage drops a little.
Use a modern opamp that is guaranteed to work when the battery voltage drops to 5V or less.
Yeah it dates back to the early 80s. So find a newer op amp but as shown it works. I suggested it because using DTMF is simple and suggested Google for links to new DTMF encoder and decoder modules. So do you have a current circuit to suggest or are you just critiquing others today?

Ron
 

Audioguru again

Joined Oct 21, 2019
2,875
Yeah it dates back to the early 80s. So find a newer op amp but as shown it works. I suggested it because using DTMF is simple and suggested Google for links to new DTMF encoder and decoder modules. So do you have a current circuit to suggest or are you just critiquing others today?

Ron
Nope, the lousy old 741 opamp was already 17 years old in 1985. It was designed for a +/-15V supply and you are lucky to find one that still works when its total supply is 10V. Most do not work when the 9V battery has dropped to 6V.
I am an audio guy so I use OPA134 single, OPA2134 dual and OPA4134 quad low noise audio that work fine when the supply has dropped to 5V.
 

djsfantasi

Joined Apr 11, 2010
7,561
And I suspect that DTMF, op amp selection, etc... may be beyond the TS pay grade, given the presentation of the original design.

The TX/RX 433MHz modules are an excellent suggestion.
 
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