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Hopefully simple led question

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hp1729

hp1729

Joined Nov 23, 2015
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Elrikos said:
Hi there
Thanks for the swift replies to this. To answer a couple of questions raised. The existing opto detectors are quite narrow in the range of frequencies they accept, visible leds don't seem to work and I'm loathe to change all the detectors to suit the transmitters. The specs of the ir led supply are, 12v supply, 1.5v forward voltage and 40ma current.
Thanks for the suggestion hp1729. What's the method of operation for the current sensing circuit? I see the transistor q1's Base is connected past a resistor. How would that sense current flow rather than just see the voltage and allow current flow through collected and emitter? If the ir led were to fail short wouldn't that still light d4?
Sorry for the simple questions, I'm still quite new to all this and just learning.
Thanks again for the help
Rich
Click to expand...
Re: the schematic
R4 is chosen so that at the working current 600 mV is dropped across it. At 600 mV the transistor turns on, turning on the monitoring LED. So as long as the IR LED is on the monitoring LED is on. Typically R4 is a pretty low value. At 100 mA this would by like 6.2 ohms. So this probably wont throw off your IR current by very much.
Not knowing specifics of your circuit I can't suggest specific values.
 
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