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 Ideas needed for a low current indicator.. 
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Post Ideas needed for a low current indicator..
Here's the story...
The controller for my MFJ Mag-Loop failed a while ago..(unable to source a circuit diagram from MFJ) so I,m building a manual controller......

The motorised variable capacitor is driven by a 12V reversible DC motor over a 2 wire circuit....motor draws around 25mA....

The capacitor is fitted with micro switches that disconnect the 12V DC at the end of travel in both directions....

I want to incorporate an indicator of some sort (preferably an LED ) which would extinguish when the micro switches disconnect the power...

I,m stumped for ideas and would appreciate any tips that members might have.
I want to keep it to a 2 wire circuit if possible.

Thanks Roger G0IUW


Sun Feb 28, 2016 5:14 pm
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 Ideas needed for a low current indicator.. 
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Post Re: Ideas needed for a low current indicator..
Silly question but assuming the microswitches are in a normally closed state, when the capacitor reaches the end of travel it disconnects supply, could you not simply put an LED in series with each microswitch and increase the supply voltage to compensate for the small voltage drop of the LED?


Sun Feb 28, 2016 5:29 pm
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 Ideas needed for a low current indicator.. 
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Post Re: Ideas needed for a low current indicator..
Not sure that an LED would appreciate 25mA thru it...

2E1IIP wrote:
Silly question but assuming the microswitches are in a normally closed state, when the capacitor reaches the end of travel it disconnects supply, could you not simply put an LED in series with each microswitch and increase the supply voltage to compensate for the small voltage drop of the LED?


Sun Feb 28, 2016 5:39 pm
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 Ideas needed for a low current indicator.. 
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Post Re: Ideas needed for a low current indicator..
Fair point standard LESs are rated about 15mA max, ok replace the microswitches with DPST types and use the second pair of contacts to switch the LED off when end of travel is reached.


Sun Feb 28, 2016 5:52 pm
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 Ideas needed for a low current indicator.. 
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Post Re: Ideas needed for a low current indicator..
How would you achieve that over a 2 wire circuit?

2E1IIP wrote:
Fair point standard LESs are rated about 15mA max, ok replace the microswitches with DPST types and use the second pair of contacts to switch the LED off when end of travel is reached.


Sun Feb 28, 2016 6:20 pm
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 Ideas needed for a low current indicator.. 
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Post Re: Ideas needed for a low current indicator..
You could put an LED in series with a 1.5k current limiting resisistor across the switch contacts. That way it would only light up when contacts are open.


Sun Feb 28, 2016 6:26 pm
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 Ideas needed for a low current indicator.. 
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Post Re: Ideas needed for a low current indicator..
The only drawback with that is...the controller is in the shack....the switches are down the garden....

G4NYC wrote:
You could put an LED in series with a 1.5k current limiting resisistor across the switch contacts. That way it would only light up when contacts are open.


Sun Feb 28, 2016 6:31 pm
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 Ideas needed for a low current indicator.. 
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Post Re: Ideas needed for a low current indicator..
The only other way I can think of would be using an op-amp circuit at the shack end as a comparator that will sense when current is drawn by the motor. When current is not being drawn i.e micro switch open, the o/p of op amp will drive led.


Sun Feb 28, 2016 6:41 pm
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 Ideas needed for a low current indicator.. 
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Post Re: Ideas needed for a low current indicator..
A low value resistor in series with each lead to the motor with an LED across the resistor, while the motor is being supplied with power there will be a voltage drop across the resistor and the LED will light, when the micro-switch opens no more current through the resistor and hence no voltage drop and the LED goes out.

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Sun Feb 28, 2016 6:51 pm
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 Ideas needed for a low current indicator.. 
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Post Re: Ideas needed for a low current indicator..
Good idea. The simple ideas are usually the best.


Sun Feb 28, 2016 7:01 pm
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Post Re: Ideas needed for a low current indicator..
A more elegant way would be to put an opamp as a comparator across the resistor to drive the LED to achieve the same result.

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Sun Feb 28, 2016 7:05 pm
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Post Re: Ideas needed for a low current indicator..
Something like this, R1 is the dropper resistor, the other values need to be worked out to make the opamp turn on and off correctly.


Attachment:
opamp.jpg

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Sun Feb 28, 2016 7:36 pm
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 Ideas needed for a low current indicator.. 
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Post Re: Ideas needed for a low current indicator..
I'm assuming being a 2 wire circuit that the voltages are reversed to turn the motor either way?
That might scupper some of the previous suggestions, unless built twice.
How about a centre zero milliamp meter with a shunt across it in series with one of the lines?

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Sun Feb 28, 2016 10:18 pm
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 Ideas needed for a low current indicator.. 
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Post Re: Ideas needed for a low current indicator..
You could feed it through a bridge rectifier then it would not effect the polarity.


Mon Feb 29, 2016 9:44 am
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 Ideas needed for a low current indicator.. 
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Post Re: Ideas needed for a low current indicator..
On second thoughts that would not work.the previous post using a center ammeter is the easiest solution to the problem.

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Mon Feb 29, 2016 11:47 am
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