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Test probe light

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Bill

04-02-2003 19:31:16




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This has got my curiosity up. I'm installing a remote start unit and it came with an installation kit. In the kit is a probe for checking the wiring harness. In the probe are two led. One red and one green. I know how leds work, but I'm wondering about the way this is constructed and I don't want to ruin it by cutting it open. There are two leads that come out of the handle, one is conected to a positive source and the other to ground (negative). THe probe then has a sharp point that you use to pierce the wire being tested. One leg of each led is conected to each one of the wires coming out of the probe handle. The other leg of each led is tied together and attached to the sharp point of the probe. When the two leads are connected as indicated above each led glows dimly. When a wire is probed if it's a ground wire the red led goes out and the green lights brightly, if it's a hot lead the red lights brightly and yhe green goes out. Question is why do both glow in the beginning? Secondly does anyone know if there is a resistor connected to each led? What voltage led do you think their using.

Thanks in advance
Bill

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T_Bone

04-03-2003 00:36:40




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 Re: Test probe light in reply to Bill, 04-02-2003 19:31:16  
Hi Bill,

Look up a electronic circuit called J-K flip/flop circuit. This is a very popular control circuit diagram.

If no body else chimes in too explain how this circuit works, I'll try latter on.

T_Bone



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Ben in KY

04-03-2003 04:59:27




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 Re: Re: Test probe light in reply to T_Bone, 04-03-2003 00:36:40  
Tbone,
Is that an edge triggered JK flip flop?
Although I think the tester probably just has 2 diodes, 2 resistors and a red and green led.
The value of the resistors would depend on the voltage being tested. LED's need about 20 MA of current. ie for 13 volts the resistors would be about 650 ohms.



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Bill

04-04-2003 08:49:43




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 Re: Re: Re: Test probe light in reply to Ben in KY, 04-03-2003 04:59:27  
Tbone; Why would you need to add 2 diodes to the setup as a LED is a diode in itself.

Thanks..
Bill

ps. It looks like there could possibly be one resistor on one of the leads on the LED, but not sure as the connection between the led and wire is covered with heat shrink tubing.



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