Using Photodiode 50mV output to turn on circuit

Thread Starter

davedmw7

Joined Aug 21, 2008
2
Is there a circuit or device that can be used to switch on a low power circuit without itself using power? I am trying to take the output (min. 30-50mV) from a photodiode and use it to turn on a circuit that is connected to a +5V power source (4-AAA batteries). However, the switch itself can not draw any power.

I have tried using an NPN BJT. But it took 735mV to turn on the circuit and I can only use about 30-50mV. Thank you for any help.

Dave
 

mik3

Joined Feb 4, 2008
4,843
Is there a circuit or device that can be used to switch on a low power circuit without itself using power? I am trying to take the output (min. 30-50mV) from a photodiode and use it to turn on a circuit that is connected to a +5V power source (4-AAA batteries). However, the switch itself can not draw any power.

I have tried using an NPN BJT. But it took 735mV to turn on the circuit and I can only use about 30-50mV. Thank you for any help.

Dave
No you have to take a small amount of power from the 5 volt source to make it work otherwise you have to use another voltage source to power the control circuit.
 

kubeek

Joined Sep 20, 2005
5,731
You could use some FET and have the diode as a part of a resistor divider, but that would still consume some current all the time.

Maybe you could use a phototransistor?
 

Thread Starter

davedmw7

Joined Aug 21, 2008
2
kubeek, thanks for the phototransistor idea, I will look into that. I'm not sure if a JFET would work. From what I understand it controls current and I need a way to control voltage, however I could be wrong. I guess another possibility is trying a MOSFET like the IRL510, but I have yet to figure that out.

I actually did try a solar cell, one from a calculator, and it did work. But what I forgot to mention is that the reaction time needs to be in the microseconds range and unfortunately the solar cell was too slow. I also looked into photoresistors but they were too slow as well.

Is there a way to possibly store voltage in a capacitor and then "knock it out" using a photodiode?
 
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