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schematic

simulate this circuit – Schematic created using CircuitLab

I'm trying to get a phototransistor to saturate a BJT so that when a small amount of light is present, the BJT is on. At the moment, there needs to be a lot more light than I would like, so I've been looking for a way to change how much light I need. One thing I've tried is to put two of the phototransistors in series along with a resistor, and found that by pointing one away from the light, I can get a large enough voltage across one of them to saturate the BJT with a small enough amount of light. However, put the BJT B-E in parallel, and the voltage jumps down to below what is needed. I've tried using a resistor in series with the B-E junction, but it still requires the same amount of light as before. Any suggestions? Sorry about the poor schematic. I couldn't find any phototransistors, so I used BJT + arrows instead. Also, it doesn't need to be a bjt, I could get something else if it would work better.

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    \$\begingroup\$ Draw a circuit of what you tried. \$\endgroup\$ – Andy aka Feb 10 '18 at 17:40
  • \$\begingroup\$ WHat do you have for a collector resistor \$\endgroup\$ – Trevor_G Feb 10 '18 at 17:50
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Try it this way instead.....

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ With the phototransistor backwards? \$\endgroup\$ – JoeP Feb 10 '18 at 18:30
  • \$\begingroup\$ @JoeP photo-diode..yes. Light causes it to leak. \$\endgroup\$ – Trevor_G Feb 10 '18 at 18:33
  • \$\begingroup\$ Works perfectly! But it's a phototransistor - the schematic says diode because I couldn't find a phototransistor. I don't understand why it works either - how is the phototransistor supposed to behave backwards? \$\endgroup\$ – JoeP Feb 10 '18 at 18:44
  • \$\begingroup\$ @JoeP perhaps I am confused by your description. try redrawing it using regular transistors with a couple of arrows to indicate its optical. \$\endgroup\$ – Trevor_G Feb 10 '18 at 18:55
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    \$\begingroup\$ Done. Well, very curious, but at least it's put a happy end to a very stressful few hours. \$\endgroup\$ – JoeP Feb 10 '18 at 19:54

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