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Author Topic: Sensing a .84 Volt light - - -- the easy way?  (Read 890 times)
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Honestly I should know how to do this already...but I`m having a brain cramp I guess.
There is a circuit that looks like this---

(+)volts--------(lightbulb)------/---transistor/---ground(-)

it is on a lamp matrix and the voltage won`t be passed to ground until the transistor base
is activated.  if the transistor base is activated then the light lights up. 
so it`s like a light on a switch. 
ok.  this light on a switch is not connected to my arduino in any way at this point. 
i want to be able to determine when the lightbulb is lighted and when it is off. 
i can`t seem to figure it out.  arggh. 
i build a little box of an infared LED and a reciever to find out the voltage isn`t enough to power
the darned LED...must have mis-measured.  aarrrggh.  then i tried to build another transistor switch triggered by the lamp "on" voltage that would turn the infared LED on HIGH and then in turn that would be read by the infared reciever.....then i thought....hey.   there must be an easier way to do this.
Anyone have any suggestions?
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What is the (+)volts in your circuit? I'm thinking maybe you can use the connection between lightbulb and transistor as a sense point. This voltage will be (+)volts when the lightbulb is off, and about 0.2V-0.3V when the transistor is on.

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perhaps that will work.  where would i put the wire?  and would i simply run that one wire to an analog input pin?  would that be all?
to answer your question..regarding what voltage i measure....it depends where i measure the (+) voltage.  for instance, when i measure it from the (+) side of the lightbulb and then to the machine ground i get between 6 and 15 volts.  when i measure from one side of the lightbulb to the other i get .8 volts.
« Last Edit: February 05, 2011, 02:35:23 pm by arduiYES » Logged

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Since you are getting a (+)volts between 6V-15V you have to bring this down to a 5V level for sensing with a digital input. There is no need to use an analog input pin for this. Connect the lightbulb-transistor node through a 10k series resistor to an Arduino digital pin. Then, add a 5V zener diode between this pin and ground (cathode at digital pin, anode at ground).

It might be easier to measure the voltage at the transistor base. Where is this coming from and what is the voltage?

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