Battery powered cat flap.  - Page 1
Forum section: electronics / beginners

author: Sounduser
author_rank: Contributor
timestamp: March 21, 2020, 03:36:18 pm
content: Hi.Can I pick you guys brains?We've got a battery powered cat flap. But it uses lots of battery's.I soldered two wires to the battery connectors.4 x AA 1.65v = 6.6vEasy I thought. I wired a 2 amp boost converter to a USB charger. Didn't work. Not enough current I thought.Second I tried a way over sized power supply and a buck converter. Didn't work.So I gave up and connected a 6v lead acid battery. Worked fine.Connected a little trickle charge to the battery... Stopped working. Light flashes. But doesn't click.The thing really doesn't like being connected to the mains.Should I try two diodes on the charge cables?Cheers

author: AVGresponding
author_rank: Super Contributor
timestamp: March 21, 2020, 03:55:19 pm
content: I guess it's just about possible that if it uses an RFID tag on your cat's collar, the EMI from the mains might interfere with it...You could try to put some filter caps in parallel at the terminals.

author: Zero999
author_rank: Super Contributor
timestamp: March 21, 2020, 04:58:37 pm
content: If it's designed for four alkaline batteries in series, then it should work perfectly from 5V power supply, as long as it has a low enough impedance. The voltage of alkaline batteries falls, as they discharge and a properly designed product should work from as less than 1V per cell, giving a total of 4V, from four cells.Try the 5V USB supply again, but with a large capacitor, say 1000µF, in parallel.

author: Sounduser
author_rank: Contributor
timestamp: March 21, 2020, 05:51:32 pm
content: Thanks for the replys.I think it is some type of interference.Connected the charger. And press the bottom a few times to disable the lock. The motor clicked fine. But won't allow the cat in.Shame I dont habe a spare chip to play with.I'll try adding bypass caps tommorow.I did try a cap on the out put of the boost converter. 5v to 6.6v with a 4700 Mike and I tried a smaller one. But that didn't work.I'll try at the car flap terminals.Cheers.

author: Sounduser
author_rank: Contributor
timestamp: March 23, 2020, 06:44:28 pm
content: Not messed with caps yet.I connected a small solar panel today... Just a panel and diode. Didnt work... Might give up if caps don't work.... 😂

author: engrguy42
author_rank: Frequent Contributor
timestamp: March 23, 2020, 06:48:12 pm
content: Wow, talk about learning something new every day...I just could NOT figure out what a battery powered cat flap is. HUH???And after some searching apparently you can lock other cats and varmints out of your house by giving your cat a sensor-thingy and the flap/door only opens for your cat? Wow.Can't you just put a sign outside the door saying "THIS DOOR ONLY FOR MR. FUZZY !!"?

author: IDEngineer
author_rank: Super Contributor
timestamp: March 23, 2020, 08:16:30 pm
content: It's almost certain to be an inrush current problem.I've worked with some similar devices which required an amazing inrush current at startup. One in particular would work on a gelcell battery, but would trigger a 35A switching bench supply. The only supply that could operate it was an old linear 70A bench unit meant to power an amateur radio linear power amplifier. Amazed, I put a current probe on it and discovered it was hitting 65A+ peak currents for several milliseconds at startup, before the back EMF could build to begin limiting it.Your batteries don't have current limiting circuits so they just grunt through the near-dead-short. But your various other power supplies are probably trying to protect themselves.I agree with Zero999's suggestion to parallel a really big cap on the supply. Though the supply may balk at the initial charge current for a big cap, too.Perhaps what you need are rechargable batteries and an appropriate charger. The batteries would be able to survive the peak current requirement, then the charger would keep the batteries topped off. Think of the batteries as performing the same function as the recommended cap - a source of peak initial current. But batteries would not likely make the supply as unhappy as a really big cap, which can also appear as a near-dead-short to a power supply once the cap has discharged through the motor.This battery+charger scheme also addresses the question of whether the power supply has sufficient current to run the motor. Even beefy quality USB chargers generally max out at 2.4A, are you sure that's enough to make your motor happy?

author: Zero999
author_rank: Super Contributor
timestamp: March 23, 2020, 09:19:50 pm
content: Quote from: Sounduser on March 21, 2020, 05:51:32 pmYes, it sounds like electromagnetic interference from the switched mode power supply.Do you have any old fashioned low frequency transformer power supplies? They're generally heavier and more bulky than the modern type.Failing that, try a common mode choke, ferrite beads and series inductors, as well as some ceramic capacitors: 1nF to 100nF.

author: IDEngineer
author_rank: Super Contributor
timestamp: March 23, 2020, 10:20:44 pm
content: When activated, does the door just "unlock" (requiring the cat to push the door open) or does the door actually retract itself? I presumed the latter, which is why I emphasized inrush current. I thought the batteries were being asked to do substantial physical work. We have a kitchen trash can that has a top which opens under the power of six AA's and it chews through them like crazy too. Your original description reminded me of that trash can lid and its AA power source.

author: IDEngineer
author_rank: Super Contributor
timestamp: March 23, 2020, 10:22:35 pm
content: Quote from: Zero999 on March 23, 2020, 09:19:50 pmOr just some simple shielding. Longer wires. A scope on the power leads or signals within the circuitry. Anything. It feels like we're flying blind here and there are lots of simple A:B tests that would reveal a lot.

author: AVGresponding
author_rank: Super Contributor
timestamp: March 24, 2020, 12:35:30 am
content: AFAIK these devices have a mechanical interlock that prevents the door opening, and a small electromagnet releases the interlock when the correct RFID is sensed.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: March 26, 2020, 01:10:24 pm
content: How much current is needed to open the cat flap?To reduce interference, use a ferrite core. the bigger, the better, and make as many windings of your power supply wire as will fit in the center of the core. Also put it near the cat flap.Making a simple power supply from a transformer 4 diodes and a capacitor may also be better than those new fangled SMPS modules you see everywhere, but those generate RFI also.