Digital output sensors, like other signal-driven actuators, are commonly used in various industrial applications. We can easily find various types of digital output sensors, including temperature, flow, pressure, speed, etc., which have digital signal outputs in various formats.
A digital sensor is a type of sensor that only produces binary output. Compared to analog output sensors that produce continuously varying values within a certain range, digital output is only “0” or “1”. The simplest example of a digital sensor is a contact switch. A typical contact switch is an open circuit with infinite resistance, which becomes a circuit with zero impedance when the switch is pressed.
1. Dry Contact (On/Off Signal)
Dry contact, as opposed to wet contact, is a type of passive switch that has two states of closing and opening, with no polarity between the two nodes, allowing interchangeability. Common dry contact signals include:
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Various switches such as limit switches, travel switches, foot switches, rotary switches, temperature switches, liquid level switches, etc.;
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Various buttons;
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Outputs from various sensors, such as sensors in environmental power monitoring: water immersion sensors, fire alarm sensors, glass breakage, vibration, smoke, and condensation sensors;
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Outputs from relays and reed switches.
2. Wet Contact (Voltage Signal)
Wet contact, in contrast to dry contact, is a type of active switch that has two states of energized and de-energized, with polarity between the two contacts, which cannot be reversed. In industrial control, the common voltage range for wet contacts is DC0-30V, with DC24V being fairly standard. AC110-220V outputs can also be wet contacts, but this is less common. Common wet contact signals include:
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If a dry contact signal is connected to a power source, with the other end connected to the power source, it becomes a wet contact signal;
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NPN transistor collector output and VCC;
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Darlington transistor collector output and VCC;
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Outputs from infrared reflection sensors and through-beam sensors;
3. Source Input
Source input is used to connect drain output devices, as shown in Figure 1.
Figure 1 Source Input Schematic
Drain output devices provide a current path to ground. The NPN collector open circuit shown in Figure 2 is a typical drain output device. When a low level output is required, the transistor is in saturation, equivalent to the output being connected to ground; when a high level output is required, the transistor is in cutoff, equivalent to the output being disconnected from ground (output floating).
Figure 2 Drain Output Schematic
The signal source for source input devices (drain output) can only provide driving capability to ground.
4. Drain Input
Drain input is used to connect source output devices, as shown in Figure 3.
Figure 3 Drain Input Schematic
Source output devices provide power or positive voltage, equivalent to a switch connected to the power supply, as shown in Figure 4. When the output is “logic 1”, the switch is on.
Figure 4 Source Output Schematic
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