HT12D turning on with no signal?

[adamthole]

Okay, I finished my wireless pager system and everything was working great...at first. Now weird stuff it happening. If I turn on the receiver (which consists of the receiver, an HT12D, a 555 timer, and a dc motor) it should do nothing until it receives a signal from the transmitter. Which it does, for a while. However if I let it sit on a table for an hour or so, when I come back it is usually vibrating. How? No transmitter in on to transmit anything. I didn't think it was likely that it would pick up the turn on signal out of noise, but I guess it is possible. Is that what is happening, or did I do something wrong? One thing that I did not do is set any of the Address pins. I didn't set them on the receiver or the transmitter. Could this be part of the problem?

Another problem: If I have the transmitter on and the receiver on and a laser shining into the photo transistor, everything works fine. I can take the laser and shine it into and out of the phototransistor and the receiver will be turned off and on respectively. However, if I leave them both on for about an hour, and come back and try it, it will turn the receiver on when I move the laser away, but not off when I shine the laser back into the phototransistor. This seems very odd to me. The dc vibrating motor cannot be causing any interference in this case, because it is on a 555 timer and turns on and off.

Any ideas? I have enclosed the schematic to both my transmitter and receiver. I realize that the phototransistor is in bacwards on my current transmitter, but I haven't fixed it yet because it still works the way I have it. Or is that the cause?

Thanks!

 

[zachtheterrible]

Hey Adam, one thing that I have noticed with the HT12D is that it will turn on sometimes when you give power to the circuit if there is no TE signal from the HT12E. I don't know if that has anything to do with it, but I doubt it.

I would try setting an address pin, who knows it might work.

Also, although extremely unlikely, it could be noise. Try disconnecting the receiver after you turn it on, this will tell you if you're getting noise from the receiver.

 

[Nigel Goodwin]

You should use 38KHz modulation on the transmitter, and an IR receiver IC on the receiver end - Holtek make chips specifically for that, and doing it properly should solve all your problems. As it is you're feeding the decoder random noise ALL the time, so it may well trigger randomly.

 

[adamthole]

How would I use a 38 KHz modulation on the transmitter? What I think you mean on that is to change my resistor value for the oscillator frequency of the HT12E? The Highest the chart goes up to is 7KHz. Just as an FYI, I am currently running it at 3 KHz.

And if I could use 38KHz modulation (which I hope I can, but am just missing) how would it cut down on the noise? Or are the IR chips made differently than the 12D?

Thanks

 

[Nigel Goodwin]

How would I use a 38 KHz modulation on the transmitter? What I think you mean on that is to change my resistor value for the oscillator frequency of the HT12E? The Highest the chart goes up to is 7KHz. Just as an FYI, I am currently running it at 3 KHz.

And if I could use 38KHz modulation (which I hope I can, but am just missing) how would it cut down on the noise? Or are the IR chips made differently than the 12D?

You would need to change the transmitter IC, you've got the wrong one for IR - they make a specific HT12 series chip that outputs the required 38KHz modulation to feed an IR LED.

However?, looking at your rather confusing circuits, I'm not quite sure what you're trying to do (or how?) - you seem to show both photo-transistors and RF modules?.

So are you trying to make a radio system?, or an IR system?, and what's with bits of both on the circuit?.

 

[audioguru]

Your 2N3906 transistor connected to the backwards phototransistor doesn't have a 100k resistor between its base and emitter to turn it off.
When it and/or the phototransistor heats-up a little in about 1 hour the transistor's tiny leakage current makes it conduct.
With it cold, test its temperature sensitivity by holding your warm finger on it. It will conduct in a few seconds to prove it. :lol:

 

[adamthole]

Nigel, I am not doing an IR system. I am using 315 MHz RF modules. Does that mean that I don't need to change the chip?

I will try a new transmitter first I guess and see where that gets me...I'll post a schem before I make it. Thanks.

 

[Nigel Goodwin]

Nigel, I am not doing an IR system. I am using 315 MHz RF modules. Does that mean that I don't need to change the chip?

What's the photo-transistor for? - as Audioguru has pointed out, that part of the circuit is very poorly 'designed' - if you're using that to trigger the encoder chip, it could well be doing so at the wrong time. If that's so?, remove all those components and replace it by a simple switch and resistor - then see if a simple button press version is reliable.

Far easier (and more sensible) than changing the radio modules.

 

[zachtheterrible]

Hey Adam, maybe you could provide an explantion of your circuit, cuz i am a bit confused as well. I think you're using the phototransistor to trigger the HT12E, is that correct?

If that's so, then what audio said about it heating up is probably right on the money, because that would make it transmit.

 

[audioguru]

Another problem with having the phototransistor backwards is that its base-emitter junction is reverse-biased with more than its absolute max limit of about 5V. Therefore it has avalanche breakdown like a zener forcing the 2N3906 to conduct. Note that the collector base junction would be forward-biased in series with the "zener". :lol:

 

[adamthole]

Okay, all poorly "designed" transmitters aside, why is this thing turning on when no signals are being sent? Let's pretend I do not have a transmitter for the time being. I turn on my receiver, and after a while it turns on. Could random noise be turning this on as discussed earlier, or were we on the wrong track because I'm not using IR?

I don't know if this is a problem with my design, or something in the chip itself.

Perhaps I should just do the whole thing in PIC?

Thanks!

BTW: I won't be able to work on this for a while because I am vatationing to California. Just so no one thinks I gave up Your opinion is still wanted.

 

[audioguru]

why is this thing turning on when no signals are being sent?

Didn't you understand my explanations about what is probably happening?

 

[adamthole]

Yes, I understood your explanation (for the most part) but it is not relevant to what I am talking about. Let me rephrase:

If I turn the power switch to my pager ON the HT12D, 555 Timer, and 315 MHz receiver (as well as anything else on the board) all receive power. However the DC motor that is hooked up to M+ and M- in my schematic stays off. This is correct operation. The DC motor should not turn on unless pin 10 of the HT12D goes low, which it should only do if it receives the correct code from the receiver. If I put the pager on my desk, for example, and leave for a while when I come back the receiver is often vibrating, which means that pin 10 of the HT12D is low, which theoretically means that the RF Receiver received the correct code to change that pin to low. This would normally mean that the transmitter has transmitted that proper sequence to the receiver. However, in this case the transmitter is not even on. And when I say it's not on, I mean that it is laying in a trash can in my room, unsoldered, and without power.

Why is the HT12D turning on?

 

[audioguru]

Sorry Adam,
When you said the receiver was turning on, I thought you meant the IR receiver was turning on.
Maybe the RF receiver is picking up a signal from someone else.

Happy holidays. :lol:

 

[Nigel Goodwin]

RF modules sometimes have a signal strength output, you should use that to mute the connection between the RF receiver and decoder chip. Otherwise it's likely that in the absence of signal the decoder will be fed random white noise - it's possible that this could eventually trigger the decoder?.

 

[zachtheterrible]

RF modules sometimes have a signal strength output, you should use that to mute the connection between the RF receiver and decoder chip. Otherwise it's likely that in the absence of signal the decoder will be fed random white noise - it's possible that this could eventually trigger the decoder?.

Yeah, Ive looked at the output of my 433MHZ receiver. Since it is an AM receiver, it gets ALL kinds of noise coming through it. I am doing my project using the HT12D and HT12E chips, I should check to see if I get your same problem.

 

[adamthole]

I charged the batteries up fully and let it on all night last night. This morning it was on. However, I changed HT12D chips and I've had it on all day with it not turning itself on at all, yet. So, I will hope for the best!

 

[zachtheterrible]

Hey adam. I tried leaving it on for a couple hours with the receiver connected and nothing happened. It must be a bad uchip.

Maybe the ambient temperature is too hot for the chip?

Have you noticed with the HT12Ds, that if you turn them on without there being any signal from the HT12E, that it will latch its output on until you give it a signal from the HT12E, even if its just turning on the TE pin? Then its fine afterward.

 

[adamthole]

I think so. When you turn on your receiver all of the output pins on the HT12D are high until you use a transmitter to make them low? That is because the active state on the HT12D is low. I don't notice it on my pager device because it operates on an active low, but if I made a device that operates on an active high (probably what your device is), it would be that way. I don't think it matters which way you do it though. I did think it was weird when I realized that they were normally high though.

Happy 4th!

 

[zachtheterrible]

No no, you're misunderstanding, i guess I wasn't that clear.

Whenever I give power to my HT12D, all of the output pins go low (thats what i shouldve said :lol: ) until you give it a signal from the HT12E. Then it is fine afterward.

In other words, whatever devices you have connected to the HT12D will stay on until the HT12E gives it a signal. Maybe thats just the case with my circuit, Ill have to test it a bit more.