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Drive remote dual coil latch relay with single RF

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neazoi

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Hi,
I have a dual coil latch relay (NEC EA2-5T) and I would like to find a way to switch it on and off using a single RF signal from the same coaxial.
I cannot use DC fed into the coaxial for it, I can only use RF power remotely rectified into DC at the relay side.
One way I was thinking was the time delay like this
https://www.sentex.ca/~mec1995/tutorial/xtor/xtor3/xtor3.html in figure 7.
So when I send an RF pulse one coil is energized instantly, but when I send a longer pulse the other coil is energized.
I have to figure out how I will block the first (already energized) coil in that second case.

Any solutions or other ideas would be helpful.

Just bear in mind that I can only use the transmitter RF for control, nothing else.
 

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Does the rectified RF signal provide enough power to make the relay latch? Does it provide enough power if both coils are to be feed?
 
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Does the rectified RF signal provide enough power to make the relay latch? Does it provide enough power if both coils are to be feed?
I do not know if the relay can be switched using just the RF power, I have not tested it, but it seems a bit optimistic, even for these tiny relays.
In my post #1 I forgot to say that there is already a 3V battery at the remote end (battery powered active antenna).
So I could use a transistor with the relay as the load, which will be connected to the 3v. Then I could switch the transistor base with the rectified RF. As the link states a darlington pair could be better.
However, I am not sure if the transistor will draw any power (even tiny amounts) even when off (not perfect switch?), so gradually discharging the batteries and losing the advantage of using a latch relay for the shake of zero battery discharge when the circuit is off?
 

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Hi,

maybe you can control it with two different frequencies.
Needs two bandpass filters with different frequencies. rectify the bandpass outputs independently and feed it to the two coils.

Applying one frequencie activates coil1, activating the other frequency activates coil2.

***
Battery powered antenna...is it useful? Whant´s the battery livetime?

Klaus
 
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neazoi

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Hi,

maybe you can control it with two different frequencies.
Needs two bandpass filters with different frequencies. rectify the bandpass outputs independently and feed it to the two coils.

Applying one frequencie activates coil1, activating the other frequency activates coil2.

***
Battery powered antenna...is it useful? Whant´s the battery livetime?

Klaus

The antenna is an active rubber antenna for those V/U TX portables that have HF RX coverage. But it can be applied to any remote antenna. It will include it's own recheargable batteries and I would like to switch it on/off using only a signal from the handheld, and the only signal available is TX at VHF or UHF.

Yes the frequency way is a very good thought indeed, this is the other way I was thinking of, it will work I think and it is straight forward. My only problem on this is that I do not know how to design a simple BPF for VHF or UHV. Additionally, it is more "unclever" in the sense that you have to change frequency to switch the antenna on/off. The delay mechanism is more familiar to consumers and it does not depend on the transciever TX capability (if this is a dual band V/H handheld or not). You simply press the ptt for a short time to switch the antenna on and then you hold it down for longer time to switch it off. It is better than changing frequency.
 

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Hi,

does it need to be VHF or UHF? Why not 10kHz and 100kHz?

with one frequency you don´t have a feedback, and thus you can´t be sure if it is on or off.
With two independent frequencies you don´t have a feedback either, but maybe it is more reliable to switch to the state you want.

Btw:
If you want it to save battery power you may use a timer to switch it off, in case someone forgot...
And switch it ON with a long press, and switch it OFF with a short press.
Maybe it is possible to use a FET to switch On and OFF, then you don´t need to transmit that much power...
Or if it is UHF/VHF... is it possible to add filters, so you can supply it from remote? Transmitt all the needed power with DC. I have a DVBT antenna working this way.

Klaus
 
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Hi,

does it need to be VHF or UHF? Why not 10kHz and 100kHz?

with one frequency you don´t have a feedback, and thus you can´t be sure if it is on or off.
With two independent frequencies you don´t have a feedback either, but maybe it is more reliable to switch to the state you want.

Btw:
If you want it to save battery power you may use a timer to switch it off, in case someone forgot...
And switch it ON with a long press, and switch it OFF with a short press.
Maybe it is possible to use a FET to switch On and OFF, then you don´t need to transmit that much power...
Or if it is UHF/VHF... is it possible to add filters, so you can supply it from remote? Transmitt all the needed power with DC. I have a DVBT antenna working this way.

Klaus

The switch must operate from VHF/UHF frequencies, because this is the only transmit capability of the handheld. This is the only signal that can send to the antenna to switch it on/off.

In the noisy HF environment you can easily ensure that the antenna is switched on or off by listenning to the speaker noise. There is no need for an indicator led, that draws prescious battery power). But you have to remember to switch off the antenna when not operating, else battery will empty. But this is the case with a notmal mechanical switch as well.

A timer is a good approach, but how would you detect the user's will to not use the antenna any more?

A FET and possibly a dual gate mosfet (propose any in SMD package?) will be ideal, however consider the case where a thunder strikes near by. The FET may be too sensitive to the static and even to the static of the fingers near by, so that it is accidentally turned on.

BTW I am not sure if a bjt will draw any power when switched off (0v on base), does anyone know? I am talking about battery saving here, so even a few mA could be bad. My preamplifier draws 8-10mA @ 3v. If the bjts draw no power when switched off then I could avoid the relay and use a bistable electronic circuit https://www.electronics-tutorials.ws/waveforms/tim17.gif?81223b
 
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Hi,

a bjt draws no power when switched off.

Maybe protected high side FETs are an option.

I´m guessing there is an antenna cable. Where does it go to?

Klaus
 
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Hi,

a bjt draws no power when switched off.

Maybe protected high side FETs are an option.

I´m guessing there is an antenna cable. Where does it go to?

Klaus

There is no antenna cable since the antenna is clipped directly onto the transceiver. However a cable extension could be used.

Since you say a bjt draws absolutely no power when switched off then such a circuit could be used and triggered by a single pulse for on/off? https://www.electronics-tutorials.ws/waveforms/tim17.gif?81223b But a second look reveals that when tr1 is on, tr2 is off so one transistor will always draw some power from the battery, even if the circuit is supposed to switch off the power to the antenna. That is why I was thinking of the latch relay.
 

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How long time period are you intending the relays to stay active?
Considering ONLY the output power issue, you could have a diode to charge a cap, and then use the cap to feed current to the relay coils. That way you should be able to keep the relay active, at least for some seconds before the cap decharges.
 
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How long time period are you intending the relays to stay active?
The question is about dual coil latch relay, so it need to be activated only for some ten milliseconds. Or do you suggest a non-latching relays instead?

But a second look reveals that when tr1 is on, tr2 is off so one transistor will always draw some power from the battery, even if the circuit is supposed to switch off the power to the antenna. That is why I was thinking of the latch relay.
If you want a bistable (memory) circuit with zero quiescent current, you'll use a CMOS latch or FF.

- - - Updated - - -

It's a bit annoying that you are prescribing some application parameters (e.g. no DC can be feed through the antenna cable) without any reasoning. Not knowing the full story isn't particularly motivating...
 
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yes a latching type relay would be much smarter in a battery operated situation. Also a FET based switch might make much more sense than an RF relay, as a FET switch draws no current when in one state or the other.

You could pretty easily modulate the RF signal, maybe with FM, and have a simple frequency discriminator at the antenna end to sense the deviation of the RF, and choose one path or the other depending on the modulation frequency.
 
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The question is about dual coil latch relay, so it need to be activated only for some ten milliseconds. Or do you suggest a non-latching relays instead?

If you want a bistable (memory) circuit with zero quiescent current, you'll use a CMOS latch or FF.

- - - Updated - - -

It's a bit annoying that you are prescribing some application parameters (e.g. no DC can be feed through the antenna cable) without any reasoning. Not knowing the full story isn't particularly motivating...

Ok to make it more clear, I present you the cirquit I have thought of yesterday, after considerations from this thread. The preamplifier is shown at the left hand side and it is a 3v type (I use 3.6v hoping not do degrade performance) designed by a German radio amateur (pdf attached). Unfortunatelly this is not my native language and it was really difficult to translate the details of it, especially the transformer construction.
I have the same transceiver as the author and I have made some changes (based on the replies in this thread) to make the amplifier swithed on/off by the transceiver signal (middle and right hand side). I have finally used the frequency control method as I found it more straight forward.
This is how I think the circuit should work:
Transceiver is connected at X2.
To switch the amplifier on, transmit for a short period at 433MHz.
To switch it off, transmit on 144MHz.
These two frequencies can be stored in memories with names "Active HF ANT on/off" or something similar, feeling like a virtual menu to the user.
The 30MHz LPF is used to pass the amplifier rf from the antenna (100khz-30mhz) but it also ensures that the TX power (144/433MHz) won't affect to the vt2 when on.
Because I am also thinking to bypass the amplifier when it is switched off, with a second relay, switched in parallel to the first one, this LPF would also ensure that no 144/433MHz power would be transmitted, by the antenna, during the control of the amplifier.
I thought it would be better to use a BPF instead for 144MHz, because it sits between the HF and UHF and possible strong HF signals will affect the 144input if just an LPF was used. For 433MHz a simpler HPF is used and this is because the input is already limited by the 30MHz LPF.
I am not sure if the 2222A transistors would be needed, or if these miniature relays (NEC EA2 series) could be directly driven by the charge pump. This is something I need to try.

Going handheld, presents many challenges, one of which is the low voltage operation, low power consumption and small size.
Any comments are really appreciated, to find the optimum solution.

- - - Updated - - -

...and here is the version with the automatic bypass switch. It bypasses the amplifier when it is switched off.
The windings of a second relay are connected in parallel with the first one, so I think a second diode across the second relay contacts is not needed.

- - - Updated - - -

Plus more details about the mechanical construction of the active antenna. The copper body is used as a capacitive antenna (mini whip) and there is an option for adding extra capacitance using an external antenna.
 

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neazoi

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...finally, there is a version that assumes that the relays can be switched without using transistor drivers. The 30MHz LPF the 433MHz HPF and a 144MHZ LPF (previously said to be a bpf) are also shown.
The V/U duplexer is from a tested circuit, but I have not tested the 30MHz LPF. However my simulations show 0.5db loss maximum at 30MHz and at 24MHz the loss is negligible. I have tried to vary input ant output impedances quite a lot ant the filter respponded ok.
 

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neazoi

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Hi, I did some tests yesterday.
I measured the current between the relay and the collector of the npn with a digital multimeter, set on the 2mA scale, when the B voltage was zero. I could not measure any current in this scale, so that is good news. It seems that the flow of electrons on C-E is zero when Vb is zero.
I was able to switch the relay from VHF low power setting (~100mW) but on UHF the transmitter outputs half that, so it could not be switched on. A voltage quadrupler did not help.
I was able to switch the relay from VHF medium power setting (~500mW) but on UHF it could not be switched on again.
I do not want to transmit full power (5w) to switch on the relays, but when I do so, the relays are activated in both VHF and UHF.
I am thinking that maybe the bjt is the problem.
I will use 3v coils relays when I have them available, so see if the problem will be corrected.

However, I was wondering if I could use a 2n7000 (already available in the lab) instead. Since they are enhancement mode fets, will they be triggered on positive voltage on the gate? How should the gate circuit be biased?
 

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This is not true. no active device is a perfect switch.

wow, for a guy that likes to hack ancient ham radio circuits and use regenerative receivers, you are getting all theoretical on us now?? For all intents and purposes, an "off" transistor has zero current draw.
 

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wow, for a guy that likes to hack ancient ham radio circuits and use regenerative receivers, you are getting all theoretical on us now?? For all intents and purposes, an "off" transistor has zero current draw.
So you are a guru :)
Anyway,I am sure the current of the switched-off transistor is more than a sleeping processor, which is a small part of 1uA. You cannot measure this with a DVM. I have not measured the off current of a transistor but I would be surprised if it is completely zero! I think it is more likely that it will also be drawing some very small current.

I think one good way to measure such a small current is the decay voltage of a capacitor. For example take a 10uF capacitor. Charge it to 5V. Measure it after 1 minute, using your DVM. Still 5V or very close to that. Now try connecting your switched off transistor across the 10uF capacitor, and again measure voltage after 1 minute. If it is 0V, then you need a bigger capacitor. If it is still 5V, then you need a smaller capacitor. Once you have a capacitor where the voltage drops a reasonable amount after 1 minute, then you are able to calculate the charge loss in the capacitor and from that, approximately the current drawn. I think this is one way, there are others I'm sure. But certainly a DVM on 2mA scale won't show it, not on an OFF transistor, nor on a sleeping processor.

It is this specific application that requires zero switch off current for the batteries to last for ever. Well ignoring the internal discharge. Although eneloop rechargables have low self discharge. I have used them after a year without charging them and they are good to 70% or so.
As said I have managed to switch 5v latch relays using about 500mW at vhf, but this is the limit. A voltage quadrupler did not improve things. Maybe 3v relays would better do.
 

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Hi,

I think one good way to measure such a small current is the decay voltage of a capacitor. For example take a 10uF capacitor. Charge it to 5V. Measure it after 1 minute, using your DVM. Still 5V or very close to that. Now try connecting your switched off transistor across the 10uF capacitor, and again measure voltage after 1 minute. If it is 0V, then you need a bigger capacitor. If it is still 5V, then you need a smaller capacitor. Once you have a capacitor where the voltage drops a reasonable amount after 1 minute, then you are able to calculate the charge loss in the capacitor and from that, approximately the current drawn. I think this is one way, there are others I'm sure. But certainly a DVM on 2mA scale won't show it, not on an OFF transistor, nor on a sleeping processor.

or just read a datasheet.

In your schematics you use MMBT2222.
The datasheet says: I_EB0 (Emitter Cutoff Current) = max. 10nA.
(that equals to 300 MegOhms resistor, that might be in the range of flux residuals)

So now you could say "It is not zero". You are right.

I for myself knew that in fact it is not zero. But for your application it is compareable with zero, because the self discharge current of your battery is for sure more than 100 times then bjt current.

I have to learn to give the information more precisely. Maybe with the word "about zero" or "no considerable current".

Klaus
 
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Hi,



or just read a datasheet.

In your schematics you use MMBT2222.
The datasheet says: I_EB0 (Emitter Cutoff Current) = max. 10nA.
(that equals to 300 MegOhms resistor, that might be in the range of flux residuals)

So now you could say "It is not zero". You are right.

I for myself knew that in fact it is not zero. But for your application it is compareable with zero, because the self discharge current of your battery is for sure more than 100 times then bjt current.

I have to learn to give the information more precisely. Maybe with the word "about zero" or "no considerable current".

Klaus

Thanks very much Klaus.
All the information in this thread are very valuable!
 

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