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I am making a tachometer to measure speed of a wheel using a LED and Laser.

The phenomena is that when Ray of Laser is shown on LED , 0.5 V are produced. I will connect a Schmitt trigger (made using LM741). The output of the trigger will be read using the interrupt pin of Uno. And using some calculation i will measure the rpm.

The problem is that when i connect the Led in Pull up configuration as shown in figure. The LED voltage does not change with laser. I am using Arduino Uno to Read Analog Values.

    ////           + GROUND                                 +5 V   DC       
    ////           |                                        |  
    ////           <                                        < 
    ////           > 220 ohm resistor                       > 100K ohm resistor
    ////           <                                        <      
    ////           |                                        |  
    ////           |                                        |------Analog Read
    ////         -----                                    -----
    ////          / \    >>Laser >>>>>>>>>>>>>>>>>>>>>>>>  / \   LED 
    ////         -----                                    -----
    ////           |                                        |
    ////           |                                        |
    ////           + +5V DC                                  +  Ground

I think that this method of LEd and laser is non-practical. Is their any other way that can help me to make tachometer?

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    \$\begingroup\$ You could try using an actual photosensor rather than trying to use an LED as a photosensor. LEDs make pretty horrible sensors. If you insist, look up transimpedance amplifiers used for PD front ends (anything written by Phil Hobbs is good). \$\endgroup\$ Oct 24, 2016 at 15:58
  • \$\begingroup\$ something like this circuitstoday.com/tachometer-using-arduino \$\endgroup\$ Oct 24, 2016 at 16:28
  • \$\begingroup\$ 1.) wrong "color" laser and LED? 2.) not enough photocurrent; Use your DMM to measure the short circuit current from the LED. (I'd guess something in the uA range... what sort of voltage drop will you get?... perhaps a bigger resistor.) \$\endgroup\$ Oct 24, 2016 at 18:33
  • \$\begingroup\$ After realizing that LED as horrible sensor. I have decided to measure RMP using a Hall Sensor A3144. \$\endgroup\$ Oct 27, 2016 at 13:49
  • \$\begingroup\$ @GeorgeHerold The current is so low that when i connect it with Op-Amp, the LED output voltage has no effect in the circuit. The current in uA is totally dissipated. \$\endgroup\$ Oct 27, 2016 at 13:51

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