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Help with circuit

Discussion in 'General Electronics Discussion' started by MiZZi, Sep 14, 2014.

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  1. MiZZi


    Sep 14, 2014
    Hi Guys! I am trying to build a sensor such as this one:


    I tried to simulate it in Multisim, but why is the voltage level in the indicator very small? It doesn't vary with the potentiometer. I substituted the photocell with a potentiometer. By the way, is the opamp package correct? Can someone help me? I have only a little background in electronics. Thanks.

    This is my multisim simulation:

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Change R5 to a much larger value (200k at least) A photocell has a very high resistance (often 500k or higher) in darkness, falling to somewhere between hundreds and a few thousands of ohms in bright light.

    Also beware that an SCR will latch on once it has been triggered. If you use an indicator lamp, it will turn on and not turn off.

    Presumably the "see text" reference says that the bell must be such that its current draw drops to zero from time to time.

    I'm a little surprised at R2 and R3. I think they should be the other way around, i.e. R2 should be 1k and R3 27k

    8V is quite a bit on the low side for a 741, but it's not being asked to do an awful lot, so it might be OK. A 12V power supply would be better (of course it depends on your bell too).

    If the bell latches on, you could replace S1 with an N channel mosfet or possibly an NPN transistor.
    MiZZi likes this.
  3. Gryd3


    Jun 25, 2014
    Please post what you are finding. 'Very small' leaves it up to the imagination.

    R4 will not vary the voltage of the indicator. R4 is setting a 'tigger' so that when R5 changes, the voltage on the inverting side of the opamp will allow the output to change. The opamp should be operating digitally with the output either on or off.

    Can you please check the voltage on the line between the opamp and R2?
    As you modify the value of R5(Your substitute light sensor) what happens to the voltage on the line between the opamp and R2?
    MiZZi likes this.
  4. MiZZi


    Sep 14, 2014
    @(*steve*) Actually, this is a challenge question for us in school. I am really interested in grabbing the challenge that is why I am trying my best to learn as well. Hehehe. Well, we are required to make a circuit as long as an Actuator, Opamp, Sensor, and Thyristor is present. Im planning to change the bell with an actuator. Thus, I couldn't really change the devices there. Also, I still don't have that knowledge to come up with my own design. My teacher still accepts schematics which are designed from others though. We are just tasked to simulate a working circuit that has the 4 devices. Thanks!

    Anyway, here is what you have suggested. Unfortunately, I could not vary the voltage in the indicator:

    It is stuck at 35.021 uV despite varying R5. Could you help me? Thanks


    Here is what you have suggested:

    Here is the voltage drop for interchange values of R3 & R2:

    Thank you so much for your help. I do hope I could finish this one. Thanks!
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    1. As Steve said, R5 is far too low. Something like 100 kΩ or 500 kΩ would be better.

    2. The schematic does not make it clear whether the wire from pin 2 of the 741 to the wiper of R5 is supposed to connect to the junction of R5 and R6 but it IS supposed to connect there.

    3. R3 and R4 are not an appropriate combination of values; the SCR gate will only see 1/28th of the voltage on pin 6 of the 741 and this will not be enough to trigger it. Change R3 to something between 3k3 and 6k8.

    4. If a bell is used as the load, the SCR may not stay latched on, because a bell repeatedly interrupts its own current. An indicator such as a light bulb or an LED (assuming the current is high enough) will be OK.

    5. This circuit will not behave cleanly around the transition point. There are multiple factors that will affect its behaviour. When a physical quantity is being compared in this way (i.e. ambient light compared with a threshold), the proper way to do it is using positive feedback, also called hysteresis.

    This characteristic causes the threshold to change depending on the output state, so once the threshold is crossed in one direction (for example, ambient light increases past the "bright" threshold), the quantity must go further back in the other direction (ambient light must drop below a different threshold that is lower than the "bright" threshold) before the circuit will return to the opposite state. This creates a "deadband" within which no switching occurs, and ensures quick, clean switching at the threshold points.

    Hysteresis is used in practial devices such as thermostats - the ON and OFF thresholds are different - because it ensures switching is clean and doesn't happen too often.

    Hysteresis can be implemented easily in that circuit by adding a resistor between pins 6 and 3 of the 741. A value of around 100k would be a good starting point. Increase this resistance to reduce the hysteresis (bring the rising and falling trigger points closer together) or reduce it to make the trigger points further apart.
    Arouse1973 likes this.
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