I have a load sensor for measuring weights , it has for wires labelled E+,E-,S+,S-. I assume S means sensor and E means excitation and the power is sent into E+ and E-. The output of any load sensor would be the order of typically millivolts if it takes a load (depending on the weight of the load of course) so I guess amplifying it with a darlington would make its output useful for an interface to an arduino. What would be a generic circuit using a darlington transistor (e.g. a TIP 120) that can make the load sensor output useful for building an interface to an arduino with the minimal number of components? Ans this circuit is to be as simple as possible. Im trying to build a circuit with the components I have which does include a tip 120 and I have a lot of other common components.

  • You'd be better of with an opamp, if you have that.
    – Gerben
    Commented Aug 15, 2016 at 18:56
  • First decide if you are trying to weigh something, design a product, understand load cells, or "golf" a TIP120 solution. For the first, order an HX711 module, for the second or third do more research, and for the fourth see if you can find a meter stick and some string to go with your TIP120. Commented Aug 15, 2016 at 19:21
  • @Gerben Gerben I do have an lm358 opamp but isnt the gain of a tip120 sufficient? I dont have an hx711 module chris. Im trying to make a circuit that can weight betwteen 0 and 3000 grams. But I think there should be a generic circuit to do this.
    – RS2322016
    Commented Aug 15, 2016 at 19:24
  • 2
    Chris my understanding of load cells is its simply a variation of a wheatstone bridge which is of course a configuration of four resistors. But it differs from the standard wheatstone bridge as the resistors are replaced by variable ones which detect strain converting to changes in rresistance-though the change is so small it needs suitable amplification. There's more to it than that of course but that basically how the sensor works.
    – RS2322016
    Commented Aug 15, 2016 at 19:30

1 Answer 1


What you have there is (or sounds like) a full wheatstone bridge.

You require either a special amplifier (such as the HX711) or a highly sensitive and accurate differential ADC (such as the MCP3428 - (on a shield)).

  • Ok Ill try and get the HX711 or MC3428 if I can. Yes I think it is a full wheat stone bridge too. But cant you build it without special parts? Even just to detect rough weight measurements? I have lm358, tip120 to build it?
    – RS2322016
    Commented Aug 15, 2016 at 19:39
  • The best you could hope for is to build an instrumentation amplifier which consists of (at least) three op-amps. Yes, you could build an op-amp out of transistors and resistors, but it's far from easy (I wouldn't like to try it). Just get a HX711, it's the simplest option and there's plenty of tutorials.
    – Majenko
    Commented Aug 15, 2016 at 19:40
  • I mean, this is the schematic for the internals of the LM741 - and you'd need three of those, and it would still be inferior to an HX711. i.sstatic.net/JmQjc.png
    – Majenko
    Commented Aug 15, 2016 at 19:42
  • I agree with you I would not build an op amp from scratch-I already have a few lm358's-what I had in mind it could be done with a few transistors to amplify the mill volts output by the load sensor and use a simple circuit to convert them to useful voltages. It just seemed like you could make simple circuit from 20 components maximum.
    – RS2322016
    Commented Aug 15, 2016 at 19:45

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