# Sense an LED running at .5 VDC

Thread Starter

#### thomhole

Joined Feb 20, 2008
1
I have a propane boiler under the house that has a status panel with LEDs that are only running at .5 (one half) volt DC. I would like to tie the indicators into my whole house automation system. I have the capability to sense "Dry Contact" on-off conditions. Any help with a very low voltage sensing circuit??

Thanks, Thom in beautifull Fuquay, NC

#### Papabravo

Joined Feb 24, 2006
12,702
That is a very unusual LED. Do I understand that you are measuring the voltage across the LED with a voltmeter and concluding that since the volt meter measures 0.5 Volts DC that the LED is operating at 0.5 Volts? Most LEDs have a forward voltage drop in the neighborhood of 1.6 - 2.2 volts for Red, Green, and Yellow/Amber. The drop is substantially higher for Blue and White. Have you considered the possibility that rather than having a fixed voltage across the LED it is really a pulse width modulated square wave, that your voltmeter is conveniently integrating for you?

Homework Question: What is the integral over one period of a square wave that goes between 0 VDC and 5 VDC with a duty cycle of 10%. That is 0 VDC for 90% of the cycle and 5 VDC for 10% of the cycle.

If this is what is happening then a simple compartor like an LM393 or LM339 will be more than sufficient for your puposes.

#### Audioguru

Joined Dec 20, 2007
11,251
I think the bi-metallic temperature sensor for the pilot light has an output of 0.5V and is used to drive the low voltage relay for the gas valve like in my natural gas furnace and also drives a transistor that turns on the 2.3V green LED that has a few volts for its supply.

#### mrmeval

Joined Jun 30, 2006
833
Use a phototransistor and put it on the LED. It's safer and more reliable.

#### SgtWookie

Joined Jul 17, 2007
22,210
Hmm - but if the LED is being driven by a square wave or PWM circuit and you use a phototransistor, it'll also have a square wave output. Perhaps a CdS would have a slow enough response to not "see" the modulation - or perhaps put a low-pass filter in the circuit.

#### Audioguru

Joined Dec 20, 2007
11,251
It is a propane boiler. It is turned on to heat the water or it is turned off when the water is hot enough. It doesn't use PWM.

#### SgtWookie

Joined Jul 17, 2007
22,210
It may be using something like an LM3909 IC as a charge pump to fire the LED, getting source voltage from a thermocouple.

I do not know offhand of an LED that can light up at less than a standard red LED (around 1.6-2.1V)

#### RiJoRI

Joined Aug 15, 2007
536
Hmm - but if the LED is being driven by a square wave or PWM circuit and you use a phototransistor, it'll also have a square wave output. Perhaps a CdS would have a slow enough response to not "see" the modulation - or perhaps put a low-pass filter in the circuit.
Or use a retriggerable astable multivibrator? I believe a 555 can be configured like that, though I'd want to prototype it...

--Rich