Key fob only works on battery and not power supply. What?

[hutchphd]

I assume that was what made the made it fail as an electromagnet.

I think it more likely for the electromagnets that the batteries were just more capable to produce high currents for short time intervals. Batteries are pretty impressive and an electromagnet is a current-hungry device. The half-wave sag will become more pronounced at high current too.

 

[rsk]

True, school power packs often state 1A max and flip off pretty quickly.

 

[CWatters]

Gave it a try with a 100[uF] tantalum capacitor on the terminals. Not a single blip from the HackRF.
View attachment 288192

When you remove the battery does the +ve battery terminal short the power supply by contacting the -ve terminal on the pcb? Especially if you have it the other way up to press the buttons. Try replacing the battery with cardboard when on the power supply.

 

[hutchphd]

When you remove the battery does the +ve battery terminal short the power supply by contacting the -ve terminal on the pcb? Especially if you have it the other way up to press the buttons. Try replacing the battery with cardboard when on the power supply.

The LED lights when the button is pushed.