# Connect HX711 to a three wire load cell

I have the following load cells:

I'm using the following HX711 amplifier

Every guide i can find on the internet describes how to connect a load cell with four wires to the amplifier. How can i connect a load cell with three wires to the amplifier?

• Is there a datasheet for those load cells? – MarkU Nov 6 '15 at 22:17
• My first guess would be that it's single-ended, omitting one of the middle terminals of the bridge. – MarkU Nov 6 '15 at 22:25
• The next question is, do you expect to use two of these parts or four? There are different configurations for each option. – Nedd Nov 7 '15 at 5:12
• @Need I expect to use four of these parts, but if you can provide options for both two and four load cell, it would be very nice! – David Junker Nov 7 '15 at 11:42
• @David Junker: Here is a prior answer with similar 3-wire load cells. Follow the answer that has the pencil drawn circuit to connect up four load cells. (The 3 wire colors are shown as RWB and the outputs marked S+ and S- go to the A+ and A- of the HX711 board). See - electronics.stackexchange.com/questions/102164/… – Nedd Nov 9 '15 at 9:15

The load cell parts you have are most likely arranged as shown in the first diagram below. (This is a typical half bridge.) You should use an ohm meter to confirm the wire colors. In some low cost load cells one resistor (strain gage) will be active (changes with stress), and the other a reference (which can also help temperature compensate the other). Which resistor is which may be hard to determine, (though in some similar parts I've seen that the white wire is normally connected to the active resistor).

simulate this circuit – Schematic created using CircuitLab

To create a full bridge you want to connect at least two of these load cell parts to form a 4-wire connection:

simulate this circuit

The above diagram is made from the more popular wire color arrangements I've seen. (As above use an ohm meter to verify the wire connections of your parts.) Also from most specifications the Red wire is the "output". So if this is correct one Red wire would go to the A+, and the other to the A-, (of the Hx711 board). The connection of the other wires would be White and Black going to E+, and the other White and Black going to E-. The resistors shown as "Active" should be the ones that increase in value as the load cell is stressed. If the board happens to show a negative reading then swap the connections of the A+ and A- (Red wires).

To use four load cells it becomes just a bit more complicated. A typical connection diagram is shown in this SE answer, (see the pencil diagram): 3-wire load cells and wheatstone bridges from a bathroom scale

• can the 1K reference just be 1k resistors? This way you can use one load cell. – Vitaly Babiy Dec 18 '15 at 16:10
• If you were to use only one load cell you could substitute one side of the bridge (left or right) with two equal value precision resistors. Do note that with such an arrangement the total output will be reduced. (With this arrangement you could also replace one of the resistors with a potentiometer that would allow zero balancing of the bridge.) – Nedd Dec 24 '15 at 23:25
• The elements you have as 'Reference 1K' are likely matched strain gauges which are physically applied in the opposite sense (in compression while the other is in tension) On the lower clip in the OP's picture, if you push down on the little inside tabs, it compresses the gauge under the white epoxy, and stretches the matching gauge under the wires. – Dave X Mar 1 '16 at 20:50
• @Dave X: Yes, I agree, that is how the professionally designed part is intended to work. But this is only when the force is applied in the correct manner. Trouble is that most hobby users do not apply the force correctly so the second gage responds very weakly. – Nedd Mar 18 '16 at 13:23
• Responding weakly due to incorrect loading is a different issue than identifying an active vs inactive resistance. Your answer says that on some strain gauges one resistance is a reference is insensitive to stress and is provided for temperature compensation. Tuning a resistor to 0.1% to match the active gauge seems much more complex than printing/etching a center tap in the same material on the same substrate. Could you point to such a device? – Dave X Mar 18 '16 at 15:29

Your load cells are intended for use as a differential pair. They should be hooked up so that one sees tension while the other sees compression. Then both reds are connected together to V+, both blacks to ground, and the two whites form a pair of inputs.

EDIT

simulate this circuit – Schematic created using CircuitLab

• ... and if nothing seems to happen when load is applied swap red and black on one load cell only. – Andy aka Nov 6 '15 at 23:25
• Also take a look at this question electronics.stackexchange.com/questions/122811/… – user2578666 Nov 7 '15 at 1:56
• Some of these parts have a differential pair of strain gages already build into the center. Others low cost versions may have one active strain gage and one reference strain gage. Unfortunately without the exact manufacturer wiring diagram the user may need to follow the link (from user2578666) and take some measurements to figure out what they have and how to wire up a 4 wire bridge. – Nedd Nov 7 '15 at 8:02
• @WhatRoughBeast Could you make a simple schematic for me? How do i connect them exactly, do i connect two or four pairs into one amplifier? Thanks in advance. – David Junker Nov 7 '15 at 11:43
• @DavidJunker - See edit – WhatRoughBeast Nov 7 '15 at 15:24