Modulated LED

170

02:50:41

Linear Modulated LED  the led current follows the input signal but inverted the current goes from   4mA to 12mA  the input gain is around 4 

Not tried this but you could input a audio signal and the LED would fade in sink with the audio 

Or you could send the led output down a  fiber optic cable and use a photo transistor to demodulate the signal some distance away 

Will experiment with this circuit and post 

Modified version by hurz  thank you 

Further simplified by hurz

published 11 years ago

hurz

11 years ago

For my taste there are 3 transistors to much. Removed current mirror and output one. Reduce 39k emitter resistor to 220ohm and connect LED directly to second one. Working fine with 2 transistors ;-)

jamilfaultfinder

11 years ago

Some times you don't see it and you follow on ending up a with complex design i usely double check but this one got past me reposted with your modifications thank you

hurz

11 years ago

Could even further simplified ;*)

jamilfaultfinder

11 years ago

Take out the current sink Put led in the collector off the first transistor and change the emmiter resistor

hurz

11 years ago

Only disadvatge, less linear and needs higher input voltage swing

hurz

11 years ago

From top to down and left to right, 5.6k,1k,39 I think is a good combination ;-)

jamilfaultfinder

11 years ago

Simulated with 2 volts peak it is linear current goes from 2mA to 20mA have to change bias network as well

hurz

11 years ago

ahhh, and reduce the voltage again to 330mV ;-)

jamilfaultfinder

11 years ago

Don't know how calculate gain for this circuit RC/RE = gain here we have LED for RC could i use 2/0.02 = collector load = 100 /330 gives a gain off 0.3 ?

jamilfaultfinder

11 years ago

Don't need gain the whole purpose is modulate the the LED going to build this circuit and hook it up to my mp3 player see how it works

hurz

11 years ago

See it as current sink where Rc can have almost any value. Given a base voltage - 0.7 is Ure which the transistor will keep constant. If Ure is constant Ic is constant.

jamilfaultfinder

11 years ago

OK but you have Led in collector so Max IC = 20 mA the voltage drop across Led is 2 volts this divided by IC Max is RC / RE = Gain ?

hurz

11 years ago

No, the gain is changing because the resistance of a diode is not constant and does change.

jamilfaultfinder

11 years ago

OK seen your version if you remove input to base then you have 1.26 volts on base bias - 0.8 / Re gives around 11 mA if you add 0.3 volts to the input to you have 1.26 + 0.3 = 1.56 - 0.8 gives 0.76 volts on Re this divided by 39 ohms gives 19 mA - 11 mA is 8 you have three LEDs total votage drop is 6 volts this divded by 19 mA = 315 / 39 = 8 so Voltage to Current Gain is 8 ?

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