Author Topic: Making sense of this obscure RF module  (Read 2448 times)

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Offline igendelTopic starter

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Making sense of this obscure RF module
« on: June 09, 2023, 09:58:14 am »
Hi all,

Someone gave me a box with a ~100 of these modules. No explanation or background were provided, and I have no Analog/RF background myself. The modules were probably in use, around 20 years ago.
To my highly un-trained eye it looks similar to those simple Chinese OOK rf receivers. I bet you know more and can shed light on this.

Here are my observations so far:

- To remove all doubt, the board has only two layers
- The three ground pins are easy enough to spot by looking at the back side. The pin farthest from the IC isn't connected to anything.
- All the the deep-green surfaces with the straight, blackened scratches are trimmed resistors, right?
- The IC is a TI TLC272 Dual OpAmp, so the pins from 1 to 8 are 1OUT, 1IN-, 1IN+, GND, 2IN+, 2IN-, 2OUT and Vdd.
https://www.ti.com/lit/ds/symlink/tlc272.pdf

I removed one to see what's underneath.
The module's rightmost pins are therefore Vdd and Output (from 2OUT). What do the next three do? Are they used for threshold or some gain control by the carrier board?

- What does the third-from-left pin do? Above it are a capacitor(?) and then a transistor. Is this thing a transmitter too?

Thanks, I hope this post belongs here and not in the Complete Noob forum  ;D
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Online PA0PBZ

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Re: Making sense of this obscure RF module
« Reply #1 on: June 09, 2023, 10:38:58 am »
It looks like there is no RF oscillator on that board, I think pin 3 is the RF input which is then amplified by the 2 transistors (BF17p?). It seems that the supply voltage for the RF part would be connected to pin 4 but there is a component missing? Looks like an AM receiver, probably 433MHz.

Wait... Have a look at this  :) https://www.gotronic.fr/art-recepteur-am-433-92-ac-rx2-cs-27868.htm



I'd say that's very close.
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Offline igendelTopic starter

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Re: Making sense of this obscure RF module
« Reply #2 on: June 09, 2023, 11:30:58 am »
It seems that the supply voltage for the RF part would be connected to pin 4 but there is a component missing? Looks like an AM receiver, probably 433MHz.

All the boards have two unpopulated components, one right above the little copper coil, and one connecting the ground plane of that single pin.


Wait... Have a look at this  :) https://www.gotronic.fr/art-recepteur-am-433-92-ac-rx2-cs-27868.htm
I'd say that's very close.

Yes, it looks right. I have some cheap 433MHz transmitters, I'll try to hook then up and see if I can receive anything. Thanks!
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Offline radiolistener

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Re: Making sense of this obscure RF module
« Reply #3 on: June 09, 2023, 11:52:43 am »
Yes, it looks right. I have some cheap 433MHz transmitters

according to description it's 433 MHz receiver  :)
And your modules looks a little bit different
« Last Edit: June 09, 2023, 11:54:15 am by radiolistener »
 

Online PA0PBZ

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Re: Making sense of this obscure RF module
« Reply #4 on: June 09, 2023, 12:31:23 pm »
It seems that the supply voltage for the RF part would be connected to pin 4 but there is a component missing? Looks like an AM receiver, probably 433MHz.

All the boards have two unpopulated components, one right above the little copper coil, and one connecting the ground plane of that single pin.


Right, I didn't notice the pin was connected to ground. So the VCC must be coming from pin 5 then and the unpopulated part is a decoupling cap. I didn't expect the RF part to be supplied via a printed resistor but so be it.
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Offline igendelTopic starter

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Re: Making sense of this obscure RF module
« Reply #5 on: June 09, 2023, 12:59:54 pm »
Yes, it looks right. I have some cheap 433MHz transmitters

according to description it's 433 MHz receiver  :)
And your modules looks a little bit different

No, I meant - these mystery modules are receivers, and I have (unrelated) transmitters with about the same frequency, so I'll try to make them work together  :)
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Offline igendelTopic starter

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Re: Making sense of this obscure RF module
« Reply #6 on: June 09, 2023, 05:38:29 pm »
So, first round of testing:

Part 1: I picked a unit at random, soldered a few breadboard-friendly pins for Vdd, GND and OUT, and watched the output with a scope while nearby transmitter is silent or working. Output is always maximum, no change (not even noise). I took a closer look and discovered that for this specific unit, the PnP machine was drunk - one big cap is missing and the other two are 45 degrees from their proper pads  :palm:

Part 2: Picked another unit, inspected it visually, repeated the test. Output is always minimum. Added a wire for what seems to be the "ANTENNA" pin, no change.
Looking at it now, I really see no path from Vdd to the RF part. The pin assignment in the above linked product must be irrelevant to this module, despite the physical similarity.
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Online PA0PBZ

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Re: Making sense of this obscure RF module
« Reply #7 on: June 09, 2023, 09:41:28 pm »
Antenna pin is #3 obviously, what other connections did you make?
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Offline igendelTopic starter

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Re: Making sense of this obscure RF module
« Reply #8 on: June 10, 2023, 04:28:23 pm »
Antenna pin is #3 obviously, what other connections did you make?

Third from the left, looking from the component side, correct? That's the one I tried for ANT.
I connected the rightmost and the next one to Vdd and Out respectively,  and the fourth from the right to GND. All the ground pins are connected to each other through the hatched plane at the back.
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Offline MarkMLl

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Re: Making sense of this obscure RF module
« Reply #9 on: June 10, 2023, 09:20:24 pm »
I took a closer look and discovered that for this specific unit, the PnP machine was drunk - one big cap is missing and the other two are 45 degrees from their proper pads  :palm:

Eyeball all units. If there's substantial variation you might have a box of manufacturer discards.

That's a relatively-expensive thick-film module on a ceramic substrate, it's reasonable to expect that fabrication and assembly took more stages than we're used to with bulk SMD boards, and in that case it's also reasonable that there might have been inspection/testing between each stage rather than just on completion.

I definitely recognise the outline with the sparse DIP pinning, but in practice all that means is that for that type of functionality there was a vague expectation that that's what the module would look like. By analogy, almost all cars these days have four wheels but that doesn't mean that they take the same size tyres or that engine parts are interchangeable.
 

Offline igendelTopic starter

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Re: Making sense of this obscure RF module
« Reply #10 on: June 11, 2023, 01:07:42 pm »
Eyeball all units. If there's substantial variation you might have a box of manufacturer discards.

Interesting idea, but no - I checked dozens and they all look ok. Just my luck, to pick the one defective unit out of 100 for testing  :-DD

Many of the modules' pins, however, are a bit bent and have solder traces on them. They must have been pulled.

I'll keep trying to make them work, when time allows; any other suggestions towards this goal are welcome  :)
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Offline Mr Simpleton

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Re: Making sense of this obscure RF module
« Reply #11 on: September 11, 2023, 06:03:10 pm »
It looks to me to be a regenerative receiver for  OOK (on-off keying) and I bet the frequency is 433 MHz. The missing op-amp acts as a noise-riding slicer, i.e. it will detect a  signal level above the noise (very smart design). The recieiver is wideband and will lock at any signal in the vincinity of 433 MHz. You should see  5v/0 logical level out matching the input signal puls train.
It's the first time I see a module like this having thick-film components on the pcb, nice  :-+ Looking at other similar modules you should be able to figure out how to power the module and where to get data out. But you need stimuli like a key fob or remote car key using 433 MHz and OOK modulation to verify it works.
 
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Offline MarkMLl

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Re: Making sense of this obscure RF module
« Reply #12 on: September 12, 2023, 07:21:32 am »
But you need stimuli like a key fob or remote car key using 433 MHz and OOK modulation to verify it works.

Every home should have one...

MarkMLl
 


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