Fast wireless trigger method without using processor?

T Chap

I am exploring an idea that requires a non software option of a wireless trigger event. The source event is a contact closure, and receiver is just as simple, it can be an active low. The distance needs to only be 4'. There can be no processors on either end, since the system that this is feeding already has the Nordic and Propeller that will relay the information to other systems, thus the lag time is already maxed out with the Nordic and Propeller time lost. The goal is a small PCB, with a miniature battery of some sort. The battery only turns on when the contact closure event occurs, thereby maintaining a long battery life.

I have seen some devices that are similar used on camera hotshoes for remote flash triggering. They are tiny modules that sit on the camera, the camera closes a contact when the button is pressed to take the photo( closing a contact on the hotshoe ). There is a receiver module on the other end. I took these apart and the guts are ultra simple, a single unidentified DIP IC.

Any suggestions on a method? The goal is to lose only a few uS, but I don't have a set max time.

Duane C. Johnson

Hi T;
The photo flash circuit is ultra simple.
All it is is a light activated photo SCR. Maybe a resister to adjust sensitivity.
The master photo flash activates the slave flashes.

Duane J

T Chap

Duane, the versions I am speaking about are radio, they take the contact closure from the hot shoe as a trigger, send that to a receiver, and close a contact on that end.

kwinn

The simplest would be an aimed IR led/phototransistor if that is physically possible. Slightly more complex would be an RF transmitter/receiver. Transmitter could be as simple as having the switch connect a charged capacitor to a coil to produce a brief RF pulse.

T Chap

IR is not an option. RF is the only way to do what I need. I just need a direction to look for this type of what I call "non data" trigger method.

Duane Degn

T Chap wrote: »

Duane, the versions I am speaking about are radio, they take the contact closure from the hot shoe as a trigger, send that to a receiver, and close a contact on that end.

Are you sure?

As soon as I read Duane's reply, I almost slapped my head remembering that's how slave flashes generally work.

You could of course test Duane's theory by blocking light to the slave flash to see if it's still triggered.

T Chap

I am positive, there are RF versions and light triggered versions.

Duane Degn

T Chap wrote: »

I am positive, there are RF versions and light triggered versions.

Okay, there's another thing I didn't know. Good luck with your search.

kwinn

@T Chap

I used a 555 timer and a 24" wire to discourage kids from blasting their boom box behind my house several years ago. I am sure sending a pulse to an LC circuit would produce an RF pulse that could be picked up by an AM, FM or other receiver circuit nearby. The square wave from the timer has many higher frequency harmonics.

Phil Pilgrim (PhiPi)

Take a look at the Micrel's RF chip offerings:

http://www.micrel.com/index.php/en/products/rf-wireless-ics.html

-Phil

RDL2004

There just isn't much need to fire a flash where you also can't see it, so the vast majority of remote flash triggers are optical. They can be made small and cheap. There are a few RF type but those are more complex and expensive. Have you considered modifying those little remote controls that are used to unlock the doors or turn off the alarm systems on automobiles? You can find those kind of cheap.

http://www.ebay.com/bhp/wireless-on-off-switch

T Chap

What I am wanting to trigger is not a flash, but a remote device that cannot be IR, it must be radio as it may be in your pocket. The devices Phil linked may be an option. A lot of keyfobs are sending a data string so that means the time to send the packet and parse the packet on the other end would take too much time, I need something much faster than the FOBs, The other part of this is that I have to build the PCB for both the xmit and receiver, so I am looking for the method to do it with a small footprint, not an off the shelf system to hack.

Thanks for the suggestions.

rosco_pc

What about this: http://www.talkingelectronics.com/projects/WirelessDoorbell/WirelessDoorbell.html ??

Phil Pilgrim (PhiPi)

The problem with RF is that speed and reliability are inversely related. For best reliability and freedom from false triggering, you want to respond to a coded data stream. Without that, the receiver may be going off all the time due to RF noise from natural and artificial sources.

-Phil

phatallica

T Chap wrote: »

IR is not an option. RF is the only way to do what I need. I just need a direction to look for this type of what I call "non data" trigger method.

So, are you looking for functionality like this?

My best estimate is that response time is closer to 1msec, rather than a few usec. However, it is about as simple and reliable as you can get for a contact closure triggered TX/RX.

tonyp12

Like Phil said, you could use the Micrel 113
http://www.mouser.com/ProductDetail/Micrel/MICRF113YM6-TR/?qs=%2fha2pyFaduiPPrdazS6z1lEiGl17TgOMdH3hBi%2fBuxBRBc%2fvVjWdyQ%3d%3d

But sending just a burst of RF with no type of data will make the Receiver susceptible to false triggering.
If you could clock in some type of 10101110, but the circuit to do that will probably be as hard as to add a cheap msp430 etc.

With its minimum 50uS *8bits = 0.4mS before receiver could determine that the data is correct.

But if just clocking in 50uS 101010101......
And receiver simple times first two bits that they are within +-2uS,
when it could trigger at the start of 3rd bit, so as little as 105uS is possible but false triggering could happen as that type of signal appear all around us.

T Chap

I can get 297uS and high reliability (including 32 bytes of data) with a Trigger>Prop>Nordic [air] Nordic>Prop>Pin But this is too slow for the application. On these cheap Chinese flash triggers shown, there is no processor or data, and I have never seen a false trigger after many hours of use plus the battery life is incredible. I can't hook these up at the moment but there are very fast. The chip on the xmit is a HS153SN and when googled says it is a HIGH EFFICIENCY RECTIFIERS(1.0A,50-400V). The receiver is about as simple but they covered the IC near the antenna is covered in epoxy. The other 14 pin SOIC has the part number ground off.

With the Micrel, that part will have to be ON all the time, not in stanby, which may work for battery life but I haven't done the math. If since it has calibrate the antenna at bootup it would be way too slow unless always powered. But it may be a solution if the battery life is good if left On all the time or with a power switch added. The Chinese modules shown do not have a power switch and I don't remember ever changing the battery on the xmit side, maybe the receiver batt was changed though.

Edit: I see power on this modules IC all the time, so it must be active and never turns off.

Thanks for the additional info guys.

Tracy Allen

There are low frequency wake-up receivers such as the ones from Austria Microsystems (AMS). E.g. AS9532. TI also makes some. They work on microamps, at low frequency, 125kHz. Their intended purpose is to wake up a much higher power active RFID tag, while conserving battery power. They have a pin that goes active as soon as it detects a 125kHz carrier, and that is followed by a pattern string that has to be matched to set an assured wakeup pin, and after that it can receive further encoded data. Sophisticated devices. The response on the order of milliseconds, due to the low frequency. The initial carrier burst is around 0.5ms, and the carrier detection pin goes high fairly early in the burst, and it rejects bursts that are too long. Digikey has a fairly nice writeup on the role of low frequency wakeup in active RFID systems.

RDL2004

I wonder what the response time of one of those cheap RF flash triggers actually is. Most cameras sync at 1/125 sec at best. Seems like they wouldn't have to be all that fast to work. Triggering within a couple of milliseconds is probably good enough. I could be wrong though.

henrytj

I kind of know what you're talking about. Having done some photography, I have used the IR triggers and have several. The problem in group photography shoots is that we were setting off each others flash units. Hence the utility of the RF flash triggers which usually have a selection of several channels for just such multiple photography.

But how fast do you want it? Even the RF triggers have a lag. Most photography done with the can't sync much faster that 1/100 sec. THere are some that I think can go 1/250, but I'm guessing those go hundreds of $'s. Which at one time was why I was looking to make my own.

I thought I had a solution with "transmitter on a chip" and "receiver on a chip" Very small, but like said you need some kind of discrimination on it will trigger with almost any nearby RF source of a similar frequency. So I thought I'd add a garage encoder/decoder chips, one on each end set the same. Worked in principle, but wasn't fast enough. Couldn't sync faster than 1/10 sec. (100 ms).

I used to see phone touch tone encoders/decoders using the phone system dual tone system. But never got them to try. Not sure how fast that would be. But that lead me to the idea of a maybes similar analog RC tone circuit that might do the trick and reduce false triggering. Two tones I imagine would be more secure than one tone, but never got around to trying to build it.

So I did manage to tinker something together but it was only 1/10 sec sync, which isn't good enough for photography. THat was years ago, that project is still in some storage box somewhere.

Now the Chinese flash triggers used to be found cheaper tens of $'s, but from what I read, they use frequencies that technically violate FCC standards. Used to find those on ebay. They were not sold in photography stores for the wrong frequency issue.

Good luck. Not sure if any of the above will help.

H.