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| Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Garage door opener puzzle
Trying to find out why operation is erratic, I measured the voltage
across the hard-wired pushbutton in the garage. The button is illuminated by a red LED, which is in series with a 1k resistor across the switch. The LED is always on. Pressing the switch shorts the resistor-LED pair and operates the door. The puzzle is that when I disconnect the LED-switch assembly and measure the voltage across the wires from the opener unit, I get only a couple hundred millivolts DC, and less AC. How is it possible for the LED to light with only 200 mV across it? Isaac |
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#2
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Garage door opener puzzle
isw wrote:
Trying to find out why operation is erratic, I measured the voltage across the hard-wired pushbutton in the garage. The button is illuminated by a red LED, which is in series with a 1k resistor across the switch. The LED is always on. Pressing the switch shorts the resistor-LED pair and operates the door. The puzzle is that when I disconnect the LED-switch assembly and measure the voltage across the wires from the opener unit, I get only a couple hundred millivolts DC, and less AC. How is it possible for the LED to light with only 200 mV across it? it's a pulsed waveform that you can't measure with a regular DVM maybe? |
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#3
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Garage door opener puzzle
In article , frank
wrote: isw wrote: Trying to find out why operation is erratic, I measured the voltage across the hard-wired pushbutton in the garage. The button is illuminated by a red LED, which is in series with a 1k resistor across the switch. The LED is always on. Pressing the switch shorts the resistor-LED pair and operates the door. The puzzle is that when I disconnect the LED-switch assembly and measure the voltage across the wires from the opener unit, I get only a couple hundred millivolts DC, and less AC. How is it possible for the LED to light with only 200 mV across it? it's a pulsed waveform that you can't measure with a regular DVM maybe? Hmm. Could be. Time to limber up the ol' wiggle tube. Isaac |
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#4
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Garage door opener puzzle
On Tuesday, August 2, 2016 at 1:52:56 PM UTC-4, isw wrote:
Trying to find out why operation is erratic, I measured the voltage across the hard-wired pushbutton in the garage. The button is illuminated by a red LED, which is in series with a 1k resistor across the switch. The LED is always on. Pressing the switch shorts the resistor-LED pair and operates the door. The puzzle is that when I disconnect the LED-switch assembly and measure the voltage across the wires from the opener unit, I get only a couple hundred millivolts DC, and less AC. How is it possible for the LED to light with only 200 mV across it? Isaac Reconnect the LED/resistor and remeasure. |
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