A customer recently asked me to optimize a lid open/close detection mechanism for a consumer product. To avoid relying on potentially unreliable electrical contacts, magnets or expensive optical solutions, I suggested the LDC0851 differential inductive switch.
Today I want to show you how to use an inductive switch for this purpose and how a threshold adjustment feature can help set the switching distance.
How can an inductive switch be used for lid open/close detection?
Typical inductive proximity-sensing applications use a small metal target such as a piece of copper foil or even a screw to detect the position of a moving part such as the lid or a door of a consumer product, as shown in Figure 1.
Figure 1: Lid open/close detection application
To determine if metal is present, the switch compares the inductance of a sense coil to the reference coil. The outer coil diameter determines the maximum switching threshold.
I suggested a stacked coil approach, in which printed circuit board (PCB) layers 1 and 2 contain the sense coil and layers 3 and 4 contain the reference coil. Figure 2 shows a system block diagram.
Figure 2: System block diagram
When should I use threshold adjust mode?
In proximity applications, it is important to determine if the LDC0851 should operate in basic operation mode or threshold adjustment mode:
Figure 3: Different ADJ codes affect the switching threshold
Configuring the LDC0851 for threshold adjust mode requires setting a resistor divider on the ADJ pin. The ADJ pin has 16 levels (level 0 disables the threshold adjust feature, and is equivalent to basic operation mode). The LDC0851 data sheet contains the recommended resistor values for each level. Figure 4 shows how the lid height and ADJ code affect the adjusted coil inductance.
Figure 4: Lid height vs. coil inductance and ADJ code
With the LDC0851, I was able to help the customer with a simple and reliable solution. Not all inductive proximity-switch applications have a metal target near the reference coil to set the switching point, however. In applications without a reference target, the threshold adjust mode ensures that switching occurs at the desired target distance. What’s your experience designing with inductive switches? Login and leave a comment below.
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