Simple RF 315MHz transmitter

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kstillson
 
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Simple RF 315MHz transmitter

Post by kstillson »

Hi folks...

The "Simple RF 315MHz receiver" boards that Adafruit sells for $5 are fantastic, and seem to work fine with the $7 key fob.

So the use-case where you have a human triggering the transmission and a device reacting is just fine. But in my use-case, I have a device deciding when to transmit (basically I have an alarm system run by a Raspberry PI and I want a remote indicator of which of it's 4 possible arming states it's in).

My thought was that I would pop the cover off the key fob transmitter, and just wire up some transistors to simulate pushing the buttons. This might be doable, but the contacts for the buttons are very small and would be quite difficult to solder to. And interestingly, neither side of the button contacts are connected to either ground or Vcc, which I suspect makes the circuit a little trickier, although I haven't experimented with that yet.

It's a shame that there doesn't seem to be a breakout board for transmission as there is for receiving, and it seems sorta surprising, so I figured I'd check-in for other ideas before I proceed with hacking my fob to death.. :-)

Any thoughts on whether there's an easier way? Thanks all!

- Ken

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adafruit_support_mike
 
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Re: Simple RF 315MHz transmitter

Post by adafruit_support_mike »

Other people have connected a transistor between the pads of a keyfob before, with good success. The ring-and-pad contacts for the switch aren't all that hard to reach.

civi
 
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Re: Simple RF 315MHz transmitter

Post by civi »

@kstillson: Hi, I am just wondering how the switch is connected in the Key fob transmitter like as you mentioned neither side of the switch is connected with VCC and GND so how it will provide trigger to the transmitter? And what are the connections to the switch of it is not connected with VCC and GND?

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kstillson
 
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Re: Simple RF 315MHz transmitter

Post by kstillson »

The circuit board wiring is really small, and it's dual sided, which makes it rather hard to trace where things are routed to. Presumably one end connects into the IC, but where the other end goes, I dunno. I tested both the inner circles and the outer rings with an ohm-meter to see if they're connected to Vcc or ground; if they are, it's via some significant resistance.

I've temporarily moved on to another project.. When I get back to it, my intention is to use a volt meter to determine whether the inner circle or the outer ring has a higher voltage, and that'll guide whether to use a NPN or PNP transistor for simulating the button push. It shouldn't matter than that the transistor is allowing through a signal that's neither Vcc nor ground, it just simplifies the concept in my head, but I don't think it makes any difference to the electronics..

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kstillson
 
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Re: Simple RF 315MHz transmitter

Post by kstillson »

Well, just wanted to say.. the "simple rf" break-out boards are so very close to really useful.

But you really need a break-out board transmitter that matches the receivers without needing to hack into the key-fobs, and it's such a shame that the existing addressing system in the 2262/2272 chips is made unavailable. If they could just be left as solder points... Then these would be perfect, and I'd be ordering dozens of them.

oh well.

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