Load cells in Wheatstone bridge to produce logic signal
Use an indirect current feedback instrumentation amplifier MAX or Analog Devices,...then subtract tare or threshold setpoint value. Make the REF input voltage such that you will have some well defined voltage in middle of supply voltage, so you can use some comparator with in-built reference voltage.



Note that AD needs an opamp buffer for REF voltage, while MAX has a built in buffer.









simulate this circuit &ndash; Schematic created using CircuitLab

Let we have a load cell with 2mV/V and you supply the cell with 5V, so it would output 10mV at full span, 10mV/50kg = 0.2mV/kg. You want to have an adjustment of 5-20kg, so delta mass is 15kg and delta V is 3mV. Let we amplify this with gain of 1000 and we get voltage swing of 3V. 

$$V_{out}=G(V_+-V_-)+V_{REF}$$
$$V_{out}=-200\dfrac{mV}{kg}\cdot m +V_{REF}$$
$$V_{out}[20kg]=-4V +V_{REF}$$
$$V_{out}[5kg]=-1.0V +V_{REF}$$
$$m&gt;20kg: V_{REF}-4V &lt; 0.4V ; V_{REF} = 4.4V$$
$$m&gt;5kg: V_{REF}-1.0V &lt; 0.4V ; V_{REF} = 1.4V$$
You invert the cell output, so that signal subtracts from V_ref.