A friend of mine is making an Escape Room and is looking to have an electronic "torch" which is essentially a flickering LED flashlight based on the LED products you can buy on amazon and other resellers. Customers that would have this "lit" should be able to touch other hand held or wall based torch to light it. The trick is that the handheld one would need to be on, and it would need to be close enough to be sensed, similar to how you would with an actual flame. The trick (as I see it) is how to trigger when very close, but not a specific spot. He doesn't want it to trigger when 1 foot away obviously.

He has electronics friends that can build it based on some suggestions.

Another part to this he wants to do (which is pretty easy) is when a wall based torch is light, ambient light would come on in the room.

I was thinking about RFID sensing or hall effect sensors. Any other suggestions, or anything off the shelf?

 

A possibility would be ultrasonics transmitter and sensor. These could allow you to actually measure the distance.

Have not worked out exactly how to do it, but it would require two-way communication. Maybe something like this.

When the master is on, it sends out pulses, save every 100 msec or so. When a slave one sees (hears?) a pulse, it responds, and when the master hears it, it sends out a second pulse. The receiver can then compute the distance from the time before it hears the second pulse.

One drawback is that the slave needs to be listening always, so it must use very little power.

 

rfid reader should do nicely if you can find something with suitable range and minimal effort (using ready made and low cost parts). typical sensing distance is about 1 inch for low frequency tags. alternatively (a bit more DIY) inductive coupling should work and allow easy tuning to get distance right. lit torch would drive own coil so it would be able to light other torches. if this is close enough to another coil, induced voltage in its coil could be sensed to turn on that torch... DIP meters etc used this for very long time.

 

I disagree. The power an RFID reader comes from the reader. It would have to be transmitting all the time, using too much power.

 

I think a bipolar Hall effect device could work.

ak

 

What comes to mind is using a simple brute-force type of electrical method:

Embed an oscillator into the "torch" unit that emits a low voltage signal ~ 100 khz or so.
Couple this signal to the outside via two large capacitive electrodes, (foil on the inside of a plastic case?) signal and Ground.

The receiving end could be a small electrode coupled to a high-impedance amplifier that is referenced to earth Ground.

If you hold this sender in your hand, enough of this signal will couple capacitively to the amplifier input to trigger an output.
A band pass filter will help to make it reliable.

Needs some experimenting, but would probably work nicely.

 

I disagree. The power an RFID reader comes from the reader. It would have to be transmitting all the time, using too much power.

This is a good point, but I think it could be worked around.

Since only the presence of a tag needs to be sensed a simple tank circuit at 13.56MHz (120KHz) is needed. The tag will load modulate the signal and that will be present at the tank circuit to detect. The tank could be energized with a slow, low duty cycle square wave to save power. The whole arrangement could be very low power and simple.

Since data integrity and rate are not an issue, a relatively poor PCB antenna would probably work, and using SMD components, the whole thing could be made a few millimeters square.

The advantage of using RFID/NFC tags is they are cheaply available as self-adhesive stickers and could be incorporated in more conventional ways in other escape house puzzles.

 

Since only the presence of a tag needs to be sensed a simple tank circuit at 13.56MHz (120KHz) is needed.

If you aren't going to use available tags and RFID reader, then turn it around — put the transmitter in the master and the receiver in the slave. Then the slave can be in very low power receiver mode and detect a nearby very low power transmitter to wake up. This would use far less power than the RFID or my original suggestion. And it is quite simple since no data need be transmitted. Controlling the distance if operation would be difficult though since the orientation of the transmitter and receiver would be variable.

 

If you aren't going to use available tags and RFID reader, then turn it around — put the transmitter in the master and the receiver in the slave. Then the slave can be in very low power receiver mode and detect a nearby very low power transmitter to wake up. This would use far less power than the RFID or my original suggestion. And it is quite simple since no data need be transmitted. Controlling the distance if operation would be difficult though since the orientation of the transmitter and receiver would be variable.

Possibly, but the utility of actually reading the tag in some cases seems potentially very high for the sort of stuff that is done for escape rooms, and I was thinking about how to maximize the potential. Also, if you want to reverse things, you can still use a tag—or at least the chip from one. Several of the very tiny tag chips offer "wake on field" functionality.

 

I also like the hall sensor idea/, it might be a practical challenge depending on exactly what behavior is desired, but it is simple, cheap, and seem like it could be made reliable.

 

Also, if you want to reverse things, you can still use a tag—or at least the chip from one. Several of the very tiny tag chips offer "wake on field" functionality.

That sounds fine, assuming “wake in field” means it has an output that it asserts when it is read.

 

That sounds fine, assuming “wake in field” means it has an output that it asserts when it is read.

Passive Wake-Up

AS3955 is able to operate NFC tag operations standalone and
fully powered by the RF field. The connected MCU can remain
is standby/sleep mode as long as its intervention is not required
by the application, in order to save power. AS3955 can be
configured to notify the MCU through a wake-up interrupt.

A number of triggering events can be selected, e.g.:

• Power up
• SELECTED state entered
• Reception of SLP_REQ command
• NFC device has updated memory content

 

Thanks for all the replies.

If anyone is able to produce a prototype of this, my friend is willing to pay you for it. DM me if you want me to send you his contact information.