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I would like to design this circuit shown in the picture. I have simulated it in Multisim. It functions as follows: When the switch is open, the phototransistor is high impedance, when the switch is closed it is low impedance. I wonder how to input this into Multisim. I would like to adjust current on phototransistor according to distance between photodiode and phototransistor. For example: if the distance is 1 metre, current on phototransistor is 10 mA; if distance is 2 m current on phototransistor is 4 mA. I have not found out how to do this in Multisim. Do you know how to do it in Multisim or do you know other software where this is possible?

  • \$\begingroup\$ "Do you suggest any software to simulate or Does Multisim any function to do" No software, except for some extremely specialized software with in-depth configuration, will be able to simulate dust, dispersion, mechanical noise in the direction of the light emitter and phototransistor, and so on. If you want to "simulate" this to a high level of fidelity, first do preliminary calculations using the datasheets of the devices, select device models, and construct a physical prototype to test with. \$\endgroup\$
    – nanofarad
    Jul 29, 2016 at 11:48
  • \$\begingroup\$ I edited your question rather heavily, could you look at the edit and check if I got it right semantically? \$\endgroup\$
    – caconyrn
    Jul 29, 2016 at 12:59
  • \$\begingroup\$ @caconyrn thanks for your edition. I think you have information about this issue. Can you help me please ? \$\endgroup\$
    – user114800
    Jul 29, 2016 at 13:32
  • \$\begingroup\$ When you say "photodiode" do you mean LED? The model of the phototransistor probably doesn't have the effect of the photocurrent, but you might be able to get some insight into the circuit with a current source into the base of the phototransistor (and just ignore the LED) \$\endgroup\$
    – BobU
    Jul 29, 2016 at 16:09
  • \$\begingroup\$ No, I obviously do not know how to do this or I would have written an answer. By the way, it is considered redundant to specifically ask for help/attention, people will gladly help without that. \$\endgroup\$
    – caconyrn
    Jul 29, 2016 at 19:18

1 Answer 1


You cannot use a photodiode to trigger a phototransistor.

A photodiode is another kind of light detector, with a similar function to a phototransistor. It cannot be used to generate light (without damaging it). A diode that emits light is rather unimaginatively called a light-emitting diode or LED.

  • \$\begingroup\$ No. You can use photodioede to trigger a phototransistor. Can you look this datasheet please. optekinc.com/datasheets/OPB100EZ.pdf \$\endgroup\$
    – user114800
    Aug 1, 2016 at 6:04
  • \$\begingroup\$ The datasheet talks about an LED and a phototransistor. The word photodiode doesn't appear anywhere in it. \$\endgroup\$
    – The Photon
    Aug 1, 2016 at 14:41
  • \$\begingroup\$ Anyway I couldn't use LED to trigger a phototransistor in Multisim. You can be right. I think I must set circuit on breadboard. Then I must test according to distance of between two component. Because it is not possible set in multisim to analyzing. By the way thank you so much for coorperation. \$\endgroup\$
    – user114800
    Aug 2, 2016 at 8:15
  • \$\begingroup\$ @user114800, you can model the phototransistor or photodiode as a CCCS, possibly with a parallel capacitance. The LED is just a diode as far as the electrical circuit is concerned, but the parameters are pretty far off from a silicon diode, so it takes some knowledge to tune up an LED model to give realistic results. \$\endgroup\$
    – The Photon
    Aug 3, 2016 at 15:56

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