On a sunny day (Fri, 13 Oct 2017 12:58:52 -0500) it happened Peabody wrote in :
You could perhaps improve it by making the first opamp stage a bandpass for the 'tone', only needs some resistors and caps.
The biasing of the inverting amplifier does not work without a negative power supply or virtual ground at the non-inverting input.
You should have some DC block in the input line, to prevent possible DC component from the input from messing with the amplifier.
My guess is that you're seeing some kind of pass-through instead of proper amplification (which may not be necessary).
--
-TV
As Jan Panteltje mentioned, you'll need some kind of tone filter to prevent falsing. Someone talking on the same channel and tone will trigger your circuit. A squelch burst, heterodyne beat notes[1], and tone signaling on adjacent channels, could also trigger the circuit. Single tone decoders still have a "talk falsing" problem but it's better than having the decoder accept anything in the audio bandpass. I suggest you abandon your scheme and look into touch tone encoders and decoders. The decoder could be some kind of lock system, where you punch in the correct 3 or more digit code to activate the door. Something like this: Don't forget a time delay on release to allow the person entering enough time to open the door.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
More and probably cheaper DTMF decoders: That receive audio processing is built in so you don't need to build an audio bandpass filter or DTMF band seperation filters.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
The call tones are not single tones. They bounce around among several tones, and I think are in the same general range as the spoken voice. I'm not sure what would be accomplished by trying to filter for them.
It's a CMOS op amp, and the common mode input voltage extends to the negative rail minus 0.2V according to the data sheet. It is specifically designed to work well using a single supply. I have built the circuit, and it DOES work with the + input at ground. My scope shows the inverting amp output to be a rail-to-rail almost-square wave at the same frequency as the input signal. Presumably that's the first .2V of negative input multiplied by the
-100 gain, with all the positive portion of the input signal coming out at ground.
The scope shows that the walkie tallkie output has no DC offset. But if it did, I would use a cap to remove it.
No, as described above, my scope shows it's amplifying as I expected.
The radios work quite well in enforcing the channel and CTCSS/DCS "subcode" selections. Testing this with the other radio transmitting on another channel, or another CTCSS/DCS selection on the same channel, at close range, I've found that nothing at all gets through to the receiver unless both the channel and subcode match. So I don't see any evidence of falsing or any of the other things you mention producing any audio output at the receiving radio.
There are 2662 (22 x 121) possible combinations of these two settings, or 1826 combinations if I limit the subcode selection to the DCS codes, which require a digital code to be transmitted before anything gets through. And both the channel and subcode can be changed for each meeting. While someone else could be using the same channel and subcode within the range of the receiver, only someone standing in front of the door and pulling on it would be in a position to take advantage of that coincidence. And of course the transmitting radio will also be in listening mode when it's not sending the calI tone, so all of us in the meeting will hear any traffic that might come in, and someone can go down and secure the door if needed, or change the channel/subcode.
I think the channel and subcode selectivity is plenty good enough for this purpose.
Abandon? Such negativity for a design that has been built, scoped and tested, and works perfectly! Not to mention that it uses only stuff I already had. :-)
That's built into my circuit. Capacitor C1 charges through the 47K R3, but discharges only through the 1 Meg resistor R4. Takes about 5 seconds for the relay to turn off after the call tone ends.
Walkie talkies don't have numeric keypads. So I would also need to develop a way to generate the DTMF tones. I think I'll stick with what I have.
FRS walkie talkies don't have keypads. Some GMRS radio do have keypads.
You don't need to generate every possible keypad sequence. You only need a recording of the correct digits that need to be sent on the keypad. I have a few long repeater control sequences saved as an MP3 file on my smartphone voice recorder. If I need to send one of these sequences, I find the appropriate file, and play it through the smartphone speaker and into the microphone of the walkie talkie.
Or, perhaps just use a DTMF generator app:
or perhaps a digital voice recorder:
or perhaps a talking greeting card:
Well, good luck.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Looks to me like they copied a Bluetooth 2.4 GHz antenna. I wonder if they even made an effort to match whatever impedance at this frequency. Do you really think they spent any time in a lab with expensive test gear to design this thing? I think they built one, tested it at 50 feet and said, "Ship it!"
That was supposed to be sarcasm. They all sell the same designs. I have seen a very basic circuit using two transistors for the receiver and a decoder chip. Then I have seen a superhetrodyne receiver chip used. I think a number of companies make a pin compatible 8 pin chip. Otherwise the difference all seems to be in the decoders with a number of pin compatible devices with small differences. Each variant seems to have a million sellers at a range of prices.
I have yet to figure out how they program the transmitter. Do you think they are setting different codes in all the transmitters they sell? I would be worried about others' remotes working with my receiver. That is no small part of the reason they came out with rolling codes.
-- Rick C Viewed the eclipse at Wintercrest Farms, on the centerline of totality since 1998
On a sunny day (Fri, 13 Oct 2017 19:55:20 -0500) it happened Peabody wrote in :
As long as always 1 of those is transmitted that matches your filter, then it should be enough to charge a cap quickly.
The whole thing can then be made much simpler:
The top one uses an LC in resonance driven from a low impedance speaker output, in resonance the voltage over C becomes very high, that is why the zener.
The second one has the same LC, but with a high impedance input, the PNP limits the voltage over the LC.
Have not tested those circuits, but I have some small 150 mH inductors that somebody handed me when I asked for 150 uH, kept those ;-)
It would not surprize me if version 1 could directly drive some relay without the power MOSFET....
The word is Q.
We Need No Opamps :-)
I never know how to assess the level of engineering that goes into products like these. My $24 scope kit had a lot of work behind it, and continues to get firmware updates. And the antenna on this one might be right, kinda, if the LC stuff in there ends up making an impedance match. Anyway, I've ordered another one from a different source, and if it works at all I'll do some antenna length experimenting with it.
My understanding is that the whole point of superheterodyne is to be able to tune in different frequencies. If you're only dealing with a fixed frequency, why wouldn't you directly demodulate that frequency? Is it a problem of dealing with components that work with higher frequencies? Or is this just another example of inappropriate engineering?
As far as these PT2262/2272 remotes are concerned, they all come with the same code. Pins 1-8 of both chips are dedicated to setting the code. On opposite sides of that row of holes there are Vcc and Gnd rail traces. The pins come unsoldered, and you're supposed to bend each one over to the rail you want and solder it there. It works with nothing selected because NC is also a legitimate selection. It's a true 3-state input. That's why you get 6561 possible settings from 8 pins (3^8). But as delievered, they all have the same code, which is 8 NCs.
On garage door openers this was done with dip switches, and I suspect they were encoded to something random at the factory. But everything is rolling code now. The dip switiches weren't very secure, but still led to lots of CS calls. Worst of both worlds.
Anyway, I suspect that most people who use these cheap remotes in projects aren't looking for security. They just want the thing to trigger something. The Youtube videos on them rarely even mention the code settings. One guy complains of poor quality as evidenced by the fact that 8 of the pins weren't even soldered to the board. Well, you know, these things don't come with instructions, so that kind of thing is to be expected.
Thanks very much for the examples. I'll study them.
On a sunny day (Sat, 14 Oct 2017 09:03:37 -0500) it happened Peabody wrote in :
Not exactly. In a super-het you can set the bandwidth (sometimes using a ceramic or mechanical filter) very precisely for whatever frequency you tune to. Also in normal cheap receivers you only have to tune 2 circuits, the antenna circuit and the local oscillator. In a comparable multi stage RF receiver you would need to tune each stage... then there is the feedback possibility into the antenna, getting a multistage RF amplifier stable is no small thing, lots of screening needed. And because the IF is usually lower than the received frequency, you can get more gain out of cheaper components than if you did amplify at RX frequency.. The list is longer....
Jan Panteltje wrote on 10/13/2017 2:13 PM:
Or add a proper filter and do something that will virtually eliminate false alarms like detecting DTMF. It will be a bit of work to generate the tones as you can't buy a $4 remote to do that. I know the OP doesn't want to turn this into a major project. But if he wants something that will work well... well, it can require some work.
-- Rick C Viewed the eclipse at Wintercrest Farms, on the centerline of totality since 1998
I don't know anything about a $24 oscilloscope, but a $4 remote transmitter has very little work put into it. A number of manufacturers make the same products using the same chips and sell them through channels that provide very little technical info. In other words, it's all junk, but will likely work for the typical customer.
I'm buying waterproof units so my selection is much smaller than yours. If I don't want to pay a lot, I'll take what I can get including the waterproofing. I can't even find a supplier who can tell me what "waterproof" means.
I think you misunderstand superheterodyning. My understanding is that it allows the required filtering to be done more simply and effectively. Filters are proportional to the frequency of operation. Build a filter at
315 MHz and it will have transition regions that are pretty wide. Shift that range of frequencies down to a lower frequency and the filter skirts become sharper allowing more spurious signals to be rejected more completely. Successive multiple stages at lower frequencies allow successively better filters to be used. So the superheterodyne receiver can work better.The part you are talking about is that the first mixer uses a variable oscillator to bring the signal of interest down to a fixed frequency IF which allows the subsequent filters to be of a fixed frequency while allowing the receiver to tune a range of frequencies with only the initial band select filters changing. No, this aspect is not so useful except that the chip which does the superheterodyning works with multiple RF frequencies.
I've not seen one of these cheap transmitters opened up yet. I'm surprised they have a DIP chip inside, they don't look that large. They typically use one of those 12 volt batteries and I didn't think there was much room left. BTW, they can use up to 11 pins for the ID code. They dual purpose three pins trading off code for data. The selection is made by using different versions of the chip. Is the transmitter you selected one button or more?
There are also versions that are "learning". That means it uses a "fixed" code, but that code can be changed by operating another fixed remote very close to the transmitter after pressing a particular button combination. I believe nearly all of the cheap transmitters I've seen were of this type. I'm not sure how you mod one if you aren't cloning an existing transmitter.
The fixed codes weren't intended to be secure. They were used to prevent your neighbor's remote from opening your door. There are sill *many* of these old units in use and most remotes sold via eBay or Aliexpress use the fixed code.
My needs aren't for security, but I don't want it accidentally operated and I don't want it jammed which can happen on some frequencies that are close to certain military freqs.
Yeah, but they just aren't thinking for the most part. You often don't want false triggering either.
-- Rick C Viewed the eclipse at Wintercrest Farms, on the centerline of totality since 1998
Someone posted a link to this months ago, so I got one. It works very well and it's nice to have one in the palm of your hand. A lot of thought went into the design, so it's surprising that it has a power connector but no internal battery holder. I just glued one to the back.
And now they have them in the US warehouse (and for $1 less than the Chinese warehouse?) so you don't have to wait a month.
I started to watch a review and it has a 200 kHz bandwidth meaning very limited application. I'd prefer it was an attached scope so I could actually see it. Those two things make $21 overpriced for me. But it was mentioned because it is an example of a low priced product where they are performing continuing engineering. I guess if they are still selling them they have some reason to improving the product and sharing firmware updates. The contrast is with a $4 remote control which works or doesn't and unless you are too far from your driveway, they work well enough.
-- Rick C Viewed the eclipse at Wintercrest Farms, on the centerline of totality since 1998
I received the other remote I ordered. I haven't had time to test it yet, but I notice that the receiver board does not have the little trace antenna the other one had. This one appears to be the real 315 MHz, whereas the other one was probably 433. Anyway, there does appear to be some variability in the designs, but of course no way to tell which is best. But I think this one will definitely need to have an antenna added to the receiver.
Yes, I was just thinking the local oscillator wouldn't be needed if you're only receiving one frequency. But as you say, all the filtering would be more effective if done on the lower IF even if you don't need to tune to a different station.
Plenty of room:
Also notice that pins 1-8 of the dip are unsoldered, with Vcc and GND pads above and below.
It's four buttons. The M4 version of the chips, which is momentary, 4 outputs. The also have a T4, which is toggle.
Yes, and my garage door is one of them. I recently added a keypad to the setup, and realized that someone could remove the keypad from the side of the house in the dead of night, copy the dipswitch settings from it, set their own clicker ($15 on Ebay) to the same settings, and open my garage door. No need to guess the pin. So I rewired the board on the keypad so the settings are not what they appear to be, and copying them to the clicker won't work. Still not really secure. I think you'd just have to record one clicker transmission and replay it to get in.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.