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

Thread Starter

thatsmessedup

Joined Feb 15, 2018
59
Yesterday the batteries died on my key fob and I didn't have the appropriate batteries to replace them. I thought I could use my power supply to operate the key fob to open the car door but it did not work. I thought that it may be caused by a noisy power supply at the higher frequency range of 432MHz but when I got my hackRF to investigate the waveform with the battery and then with the power supply, I was shocked to find no signal at all. I have since replaced the batter and everything works great, but as a recently graduated EE I am at odds with what is going on. Can anyone help me figure out this mystery. Here is a youtube link of the issue
 

Thread Starter

thatsmessedup

Joined Feb 15, 2018
59
Maybe you applied the wrong voltage to the fob. What type were the original batteries?
The batteries are two CR2016 stacked in series. They are each 3[V] so 6[V] in total. 6[V] is what the power supply is set to and I also double checked that with the multimeter.
 

Thread Starter

thatsmessedup

Joined Feb 15, 2018
59
That's what I'm thinking, but I did expect to get something out of it. I scanned up and down the frequency range from 300-400 Mhz looking for a signal thinking it maybe shifted or something but nothing.
 

Thread Starter

thatsmessedup

Joined Feb 15, 2018
59
Hello there :)Check your power supply settings. From the video could not see the power supply as you pressed the momentary push button switch. If your power supply is on constant voltage, turn your current knob fully clockwise. If it has that capability. Quite frankly!The only thing retained from your video is LED has nothing to do with indicating data is being transmitted,that you can tell from the video, it lights either way.;)
Check your power supply power Leads I always make my own I know exactly what the voltage drop is, because of the lead resistance itself.
I double checked that the current knob was cranked up and also tried with another adjustable power supply I have. Both failed to produce a signal. I tested the voltage at the end of the leads and it was correct. Also there is negligible voltage drop as the unit doesn't use much power.
 

Delta Prime

Joined Nov 15, 2019
1,311
I double checked that the current knob was cranked up and also tried with another adjustable power supply I have. Both failed to produce a signal. I tested the voltage at the end of the leads and it was correct. Also there is negligible voltage drop as the unit doesn't use much power.
Very well. Make, model ,of FOB. :)
Edit: that's messed up
 

Thread Starter

thatsmessedup

Joined Feb 15, 2018
59
I also posted this on the physics forum and a user over there had some great insight. They noticed that there were not very big buffer capacitors. This would be fine for battery use, but given that I am sourcing power from a supply over long wires, the inductance wont allow the fast pulses of current for the transmission. Here is the quote

"The thing has no sufficient buffer caps. Through a battery this is not a problem, it can support some silly big current peaks on short term: but a PSU wiring will prevent going through any HF current, so no power for transmit.
Add some 100uF tantal (or: ceramic - a miracle that you can have these in ceramic!) to the battery terminals when it's with a PSU. I think it'll work with that." (https://www.physicsforums.com/threa...on-battery-and-not-power-supply-what.1006402/)

This seems very logical. Thanks everyone for you input! I dont have any tantal capacitors to try this but possibly I can pick some up or find some from other equipment.
 
Last edited:

Wolframore

Joined Jan 21, 2019
2,579
I agree with the capacitors but I doubt it will need to be so large. Ordinary electrolytic should be fine and a smaller ceramic parallel. I will have to try a similar test to see if my bench power supply acts the same way. Usually when we test RF circuits we bypass them but never tested a fob designed for battery use.

it is not a lot of current but it needs to be quick and inductance would slow it down due to di/dt.
 

MisterBill2

Joined Jan 23, 2018
13,824
At that high a frequency every part on the PCB is part of the circuit and so opening the case and connecting wires changes everything. So of course it would not function from an external supply.
In an emergency those button cells can be recharged a bit, but it is very easy to burst them by overdoing it. JUst a bit more than the rated cell voltage and only for a few seconds at a time. then use a meter to verify that the cell is recharged.
And be sure to wear eye protection while applying the recharge power.
 

Thread Starter

thatsmessedup

Joined Feb 15, 2018
59
At that high a frequency every part on the PCB is part of the circuit and so opening the case and connecting wires changes everything. So of course it would not function from an external supply.
In an emergency those button cells can be recharged a bit, but it is very easy to burst them by overdoing it. JUst a bit more than the rated cell voltage and only for a few seconds at a time. then use a meter to verify that the cell is recharged.
And be sure to wear eye protection while applying the recharge power.
THANKS!!
 

Delta Prime

Joined Nov 15, 2019
1,311
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.
 

Wolframore

Joined Jan 21, 2019
2,579
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
59
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
13,824
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.
 
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