Key fob only works on battery and not power supply. What's going on here?

Wolframore

Joined Jan 21, 2019
2,387
look for the trace antenna, looks like the power supply line is interfering with it.
can you do a Fast Fourier transform on your scope?
 

Thread Starter

thatsmessedup

Joined Feb 15, 2018
55
I cannot come up with options for all contingencies given the information in hand you're a recent graduate an electrical engineer that makes you my Superior. This is why I love AAC I learned something new everyday egos aside please keep us up to date to your findings so others as well as myself can acquire this knowledge! Thank you sir.
I will keep you guys updated! I think that adding the capacitor will fix everything. One last thing I tried was smaller leads but I think they were still to long. The wavelength at 432[MHz] is about 70[cm] so the leads would need to be much smaller than that to have a chance at sourcing power in a timely manner to be transmitted. Additionally, even a few [nH] of inductance will create a lot of reactance at these high frequencies. So, I'm hoping that a low inductance, close proximity source will operate the key fob. I've got some tantalum caps on the way... We will see.
 

MisterBill2

Joined Jan 23, 2018
9,802
Usually every part of that FOB PCB is actually part of the circuit. And the cell body is part of the antenna circuit as well. The result being that almost any addition will alter the performance, and usually not improve it.
 

Thread Starter

thatsmessedup

Joined Feb 15, 2018
55
Usually every part of that FOB PCB is actually part of the circuit. And the cell body is part of the antenna circuit as well. The result being that almost any addition will alter the performance, and usually not improve it.
looks like you are the correct answer to this post. So, how do they make these things... purely in software?
 

Thread Starter

thatsmessedup

Joined Feb 15, 2018
55
Do you think its a fair assumption that taking out the battery removed quite a bit of metal from inside the loop that is supporting the antennas resonance?
 

MisterBill2

Joined Jan 23, 2018
9,802
Do you think its a fair assumption that taking out the battery removed quite a bit of metal from inside the loop that is supporting the antennas resonance?
Certainly removing the battery reduced the capacitance in most of the circuit. And these devices are designed as a system, rather than as a collection of independent circuit elements. It is even possible that the case enters into the operation.
 

Wolframore

Joined Jan 21, 2019
2,387
yes, I believe its due to the magnetic permeability of the lithium ion cell near the antenna trace. We have the reverse of this issue with nearfield antennas for RFID… it is possible to retune the antenna to work without the battery…

would you mind trying an experiment? Place a dead battery in the compartment with positive terminal insulated from the battery and power it up with the lab supply. Would love to see the output frequency.
 
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