This transmitter and receiver is listed for sale in Core Electronics. It is actually a separate transmitter and receiver not a transceiver.
I have a programmed atmega 328p which sends a 5v signal to a random phase triac optocoupler every 10ms. The timing of the signal is for phase control of a 240vac mains appliance. At present the coupling is wired.
I want to use the 5v for rf connection to a remote opto. How can I do this with the wireless transmitter? I don’t want to have to use a MCU for this to keep the circuit simple. The 5v output has to be replicated at the remote site for phase control of the mains at the remote site.
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Hi Peter
You can’t send the 5VDC over radio. It has to be provided locally at the remote site then a controlling signal can be sent via radio to switch this local 5V.
Cheers Bob
PS There is a finite time involved here. Particularly with any switching method at the remote site. For instance a relay will take something like 10 mSec to operate which is half of one mains cycle so any thoughts of accurately switching on a phase related basis would probably go out the window. You mention an Opto Coupled triac with which you may stand a chance and if you can use electronic switching throughout you could get any overall delay delay down to microseconds. You will have to power any receiver locally at the remote site so this is a problem that needs to be overcome anyway so if you can keep that delay down this project could be doable.
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Hi Bob, The mains device, power supply for receiver, random phase optocoupler, triac are all at the remote site. I did not express myself clearly enough. I know you can’t send 5VDC over radio. What I meant was to switch a 5v on at the remote site from applying the 5v output of my MCU to the transmitter end.
The 5v is applied at different times in a 10ms half cycle of the 240vac mains at 50hz to get phase control. The 5v signal would only last micro seconds. That depends upon the digital out instruction of the MCU which in my case is an ATMEGA 328p. During that short period you would get a continuous signal at the receiver end. This has to switch the 5v to the optocoupler.
Here there is no complicated modulating at the transmitter to send a coded instruction to the receiver. The receiver either receives a transmission or not if you are using ASK mode of transmission.
Then how do you use this transmission to switch 5v on at the remote optocoupler?
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Hi Peter
Pretty easy. Will do a sketch of a circuit to-morrow evening, no time now.
Cheers Bob
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Hi Peter.
A quick sketch of a circuit that should be OK.
I could not find much pertinent information on this TX/RX pair. Particularly the drive capability of “Data Out”. It just indicates CMOS which suggests a low current capability is It would not drive an Opto LED directly so here I have used a small N channel Mosfet as a low side switch for the Opto LED. The 2N7000 is fairly common I think. There are others of course that will do the job but Core stock this one, CE 07139.
The 1kΩ gate resistor is only a guess as I can’t find the current drive capability of the RX Data output. This will limit the instantaneous gate current to 5mA. This is required as the gate is a short circuit to source at the instant of switch on due to the gate source capacitance. It is not much here (27pF) but it exists. This resistor should be as small as can be tolerated as if too large will delay switch on too much.
The series resistor to the Opto will have to be calculated as I don’t know how much current the LED requires. This resistor is absolutely necessary.
The 10kΩ resistor from gate to ground makes sure the Mosfet is off until driven on by the Data pulse. If the gate / source capacitance is not discharged either by the Data out drive or this resistor the Mosfet will stay on. Could be indefinitely if something is not done about it.
I think this circuit will work and I hope this helps. There will be some delay which would be measurable but should be only nanoseconds so should not be much concern.
Let us know how this works out.
Cheers Bob
PS. The correct length for the antennas at 433MHz is 165mm. A stiff length of wire soldered to the antenna terminal arranged to be vertical when mounted will be best if possible.
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Looks good Bob. I use an opto that has 5ma maximum. So 1k here. I wouldn’t have known about the 1k resistor to the mosfet gate as the dc input resistance of a power mosfet is in excess of 10^12 ohms. Would the short circuit possibly damage the receiver output or maybe nullify the data out spike so as not to be picked up by the mosfet gate.
Have not got the part yet. So may be a week or two before I can test the circuit. I will let you know how it goes.
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Hi Peter
More likely damage the RX output. The time that it is a short circuit is extremely small but the damage is cumulative so may not be apparent for some time.
Not quite correct. You need to consider the forward voltage drop of the LED which could be more than 2V. When calculating the resistor to allow a specific current this has to be subtracted from the supply. Thus assuming 2V forward drop 5V - 2V = 3V so the correct resistor to allow for 5mA would be 3(V)/.005(A) = 600Ω. The forward voltage drop of your Opto LED will be in the Data sheet.
Regarding the gate / source capacitance. In this case (2N7000) this is only 27pF but in some of the larger power Mosfets this can be well over 1000pF or 1nF which can cause a driver problem as the instantaneous current can exceed 1A which most logic drivers will not tolerate. The same current appears in the discharge cycle. In these cases the common approach is to drive the gate with a NPN/PNP pair of transistors to charge and discharge or there are dedicated “Mosfet Driver” ICs available which can source and sink 2A or so for a very short period.
Another item to remember about Mosfets is the “body diode”. This exists and is normally reversed biased when voltage in the “normal” sense is applied. That is Drain positive wrt Source for N channel devices. However if voltage is applied in reverse this diode will present a short circuit the same as any diode would.
Cheers Bob
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Hi Peter and Bob,
Thanks for pointing this out, I’ve added a note to get this fixed up!
Thanks for helping out here Bob, the circuit is great, and mentioning the workings behind it ought to help anyone in the future adapt it.
Liam
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The 1k works Bob. I use it with both zero crossing and random phase triac optocouplers. Still you are right.
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Hi Peter
Yes you did say 5mA Max so 1k would be about 3mA which would be adequate.
Cheers Bob
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Hi Bob, Thanks for the advice on the led in the opto. I was having a problem with the moc3043 zero crossing optos. I thought they were faulty. It was probably because 3ma wasn’t enough. They were driving me nuts there for a while because I had used them before and now they weren’t working. I tried a different brand. A tlp3043 and this worked. Probably because it could work at the lower 3ma. Scrapped the moc3043 optos unfortunately.
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3mA borderline maybe ??? Have you checked the forward voltage drop from the data sheet I only used 2V as an example. If it is greater than 2V the current will be less than 3mA. Unfortunately at these relatively low voltages all these voltage drops become critical as you soon run out of puff. Using 3.3V supply is even more critical (one reason to dislike 3.3V). If the info is not at hand measure it. Either under working conditions or with the diode test function of your DMM. This drop will be much the same over a wide range of current.
Cheers Bob
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Typical 1.25v drop. Max 1.5 from data sheet Bob. Usually measure 1.25 so its quite low. Maybe didn’t account for the failure. I think I stressed one moc3043 at one stage. The other one had been squashed and all the pins were bent. I straightened them out but very likely one or more pins could have become disconnected.
Connections in a circuit account for most of the headaches as you would know. I used a 12k dropping resistor in a power supply circuit recently. The circuit wasn’t working. Meter showed connection. Problem was the single wire from the resistor which was not connecting properly to the screw down terminal. Soldered a multicore wire to the resistor then the multicore to the terminal. No more problem. Even the 24v power supply checked out. It was intermittent connection from the resistor causing the problem.
It took me several days to find that problem.
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Hi Peter.
I did have a quick look at the data sheet. It is a bit hard to fathom as there seem to be several variants each with a different suffix letter and different parameters. The one version that mentions 5mA reads to me as that 5mA is the trigger point so anything less would be a bit iffy. With 1kΩ resistor that would allow 3.75mA, probably a bit borderline. 5mA would need 750Ω resistor which is a preferred value in the 1% metal film range.
In the commercial world this would be put down to carelessness and you wouldn’t be working long in this field if it happened too often. After 40+ years in the commercial engineering environment this would be a very dirty word indeed. Just can’t be afforded.
The only reason that should happen is if you had another wire in there that was larger than the resistor wire or maybe you were using a terminal which was not fitted with wire protectors and the wire went in alongside instead of under the screw. Lesson. NEVER use terminal blocks with just a screw and no wire protector unless you use a ferrule and NEVER put 2 single or a single and stranded wire into a terminal without first inserting BOTH into a ferrule. 2 stranded wires are usually OK (with wire protectors). Personally I will never use a terminal block that has a screw straight onto wires without protectors. Rising Clamp terminals are OK but the comments about multiple and mixed wires still applies.
Cheers Bob
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Hi Robert, Don’t know what data sheet you were looking at. Not the same as mine apparently. 5ma is the maximum current it takes to trigger the moc3043 triac opto. Data sheet attached. This would read as ‘if it takes more than 5ma to trigger it is not a moc3043’.
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Hi Peter
That is how I read it too. 5mA is the maximum needed to trigger, that is it should trigger at 5mA or less. It does not mean that 5mA is the max allowed current (I think), it does mean however that there is no guarantee it will trigger at 3.75mA. It is a bit of a funny way to put it, I would have thought this would have been quoted as a minimum guaranteed trigger current. I guess there is more than one way of interpreting that entry. I got the impression you were saying earlier that 5mA was the max allowable LED current. Misinterpretation all around I think. Anyway it seems if you calculate for 5mA and a 1.25V LED voltage drop you should not go far wrong.
Still an interesting project, keep us up to speed.
If you are experiencing any funnies with the radio link let us know straight away. I have no experience with this type of modulation at 433MHz. Used to be called CW, the carrier is switched on and off with the modulating pulse. I have had experience with this at HF (2 to 30MHz) and 10 and 25kW where the modulation pulse has to be modified to prevent an overly large number of sidebands which is wasted power and will cause interference. And yes low (read Very low by modern standards) speed data is transmitted at these frequencies using CW.
By the way, don’t take offence at my comments regarding your connector problems. That criticism is meant to be constructive and educational. Just some of the precautions taken as a matter of routine when dealing with a project which could have 50 or 100 thousand or more connections. Imagine the problems if we came up with your scenario too often. And yes, things like screwing a connection down on the PVC insulation and things like that DO happen, fortunately very rarely with experienced installers. A lot is avoided in documenting properly the specs and procedures in the first place.
Cheers Bob
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Hi Bob, I should have the part soon. It may not be as simple as it looks. If there is a good PLL at the receiver everything may be ok. Otherwise interference may be a problem. I will let you know if I am having any problems with it.
These things usually use code so other transmitters cannot turn them on. There is none of that in this setup. The range being short should help.
I have just received a quite fascinating device. It is called a Thermpro. It reads temperatures remotely. Its called a “wireless dual probe meat and BBQ thermometer”. I may have need of it to test some heat transfer oil which probably is ok but best to be on the safe side and test it remotely. This appears to be very accurate and measures temperatures from 0c to 300c.
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Hi Peter
It is not interference with your receiver I was referring to. It is interference to others. We had quite a tight specification to meet regarding harmonics and spurious transmissions. I forget what the harmonic spec was now but spurious was measured in Pico Watts. I think that was the main reason for the “curbing” as it was called. I don’t this sort of spec applies to this 433MHz band so don’t worry unless you get some strange things happening. If you are interfering with someone else you may have to do something about it. These things are very low power for that reason so you should be OK in that regard.
By the way. I think in a photo I saw, the receiver antenna seems to be coiled up. I don’t know why, it would work better straight and as I said the length for 433MHz is 164mm.
Cheers Bob
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Hi Peter
Just noticed this bit.
The range of your set up has nothing to do with external interference. It is the range from your RX to any interfering TX on the same frequency. The down side to your simple non coded system is if another carrier is detected it will probably trigger your opto as you are just switching the carrier at the TX end and receiving this at the RX end to switch. If there is an interfering signal on the same frequency your RX will not care which TX sends it, it will respond accordingly.
Cheers Bob
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