Very simple electronics question

Dr Lobster*
Posts: 2013
Joined: Sat 30 Aug, 2003 20.14

Hello everyone, just wondering if anybody knows a bit more about electronics than me (which is very little!)

basically, i converted a mains lamp to run off a usb power bank because mrs lobster wanted a little table thing for our keys etc in the hallway, sadly, there is no mains power nearby and to go through a wall would mean running a cable across the lounge, so i thought this was the easiest way.

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all i've done is replaced the mains flex with a usb cable and bastardised the lamp holder.

as you can see, the light itself is pretty effective and because you can buy those little usb night lights which plug into a phone charger for pence off ebay, i thought it was quite a neat solution.

however... trying a couple of power banks (one higher capacity i purchased for this project and one i had kicking around the kids use)... it won't fire up the lamp.

these power banks need the load of a phone to come out of power saving mode, so despite going to the effort of wiring an inline switch, i need to connect my phone to the power bank with a fly lead, wait for it to come on and then connect the lamp.

my thinking for a quick and dirty solution, is that i could mount within the the inline switch a secondary small push button switch which is wired across the + and - terminals with a fairly high value resistor.... to fire up the lamp i turn the inline switch to on, depress the inline switch for a second and the powerbank thinks a load is connected.

question is, what value resistor do i need? or is there a better way to solve this problem?
Alexia
Posts: 2972
Joined: Sat 01 Oct, 2005 17.50

Sounds like a job for Big Clive.
bilky asko
Posts: 1171
Joined: Sat 08 Nov, 2008 19.48

Wouldn't a transformer of some sort do the trick?

P.S. Another Big Clive fan here.
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WillPS
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bilky asko wrote:Wouldn't a transformer of some sort do the trick?

P.S. Another Big Clive fan here.
You'd have to plug that in somewhere though, or else there would be nothing to transform.
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bilky asko
Posts: 1171
Joined: Sat 08 Nov, 2008 19.48

WillPS wrote:
bilky asko wrote:Wouldn't a transformer of some sort do the trick?

P.S. Another Big Clive fan here.
You'd have to plug that in somewhere though, or else there would be nothing to transform.
I didn't mean a plug in transformer.
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cwathen
Posts: 1149
Joined: Fri 15 Aug, 2003 17.28

Big Clive would no doubt sort this quickly...but as USB is 5 V and the least a charging device would draw is 500mA, then with R = V/I you need only 10 ohms? In which case pretty much any resistor would work?

Edit: actually that can't be right. The LED would already have a larger resistor in front of it and it won't switch with that. I'll leave it to Clive
bilky asko
Posts: 1171
Joined: Sat 08 Nov, 2008 19.48

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