Magnetic reed switches are simple switches which change state in the presence of a magnetic field. Reed switches are available in a variety of shapes, sizes and configurations. Designers commonly specify Normally Open (NO), Normally Closed (NC), Single Pole Single Throw (SPST) and Single Pole Double Throw (SPDT) versions, and others are also available.
Unfortunately, reed switches have an important and familiar failing; they readily break. This is because the casing of the reed switch’s body is normally made of glass; it is very fragile.
A more robust alternative to the traditional reed switch is the solid state magnetic switch. In many applications, replacing a reed switch with a magnetic switch is simple and straightforward, and it offers several advantages over the reed switch:
- Reliability: a magnetic switch is much more robust than a reed switch,
because it is a solid state device. It has no moving parts, and its plastic package, unlike the reed switch’s glass package, is tolerant of shock and vibration.
- Speed: the frequency response of a magnetic switch is much higher than a reed switch and can be measured in nanoseconds.
- Size: a magnetic switch is much smaller than even the smallest reed switch. For instance, the CTSR420C-IS2 from Crocus Technology is housed in a 3 x 2.6mm SOT23 package.
- Cost: magnetic switches have lower manufacturing costs, and so are cheaper to buy than reed switches.
Operation of a Solid State Magnetic Switch
A solid state, magnetic switch, such as the CTSR420C-IS2, CTSR440-IS2 or CTSR460-IS2 from Crocus Technology, changes state in the presence of a magnetic field. This means that its functionality is very similar to that of a magnetic reed switch.
The output of a Crocus magnetic switch is a resistance: high (Rh)when it is not in the presence of a magnetic field of sufficient strength, and low (RI) when in the presence of a magnetic field.
Figure 1 shows the output resistance of the Crocus magnetic switch as a function of magnetic field strength (or flux density, in mT). The operating point (Bop) marks the threshold at which the output changes from Rh to RI. The required field strength is low; for instance, the operating point of the CTSR420C-IS2 is 5mT.
Magnetic Switch Circuit Implementation
The implementation of a circuit using a magnetic switch is almost identical to that of a circuit using a typical magnetic reed switch. Both devices are simple switches that require completion resistors to convert the switch resistance of the device to a voltage. This voltage is normally sensed by either a microcontroller or by discrete analog circuitry.
When an MCU is used to monitor the output voltage, the circuit shown in Figure 2 can be used. It is worth noting that the output voltage from the Crocus magnetic switch on the right-hand side in Figure 2 will not switch from rail-to-rail. The output of the magnetic switch will change from an Rh value of 20kΩ when there is no magnetic field present to an RI value of 10kΩ when a magnetic field is present.
If a microcontroller is not directly monitoring the output of the switch circuit, or if a rail-to-rail output voltage change is required, a comparator can be used to set the threshold of the circuit and to provide a full rail-to-rail swing.
The circuit in Figure 3 shows a generic comparator circuit interface to a sensor circuit. The threshold value can be provided by a constant voltage source or a variable source provided by a DAC, depending on the application. Many analog ICs have built-in reference voltages. These built-in voltage sources can also be used to set the threshold voltage.
The optional pull-up resistor, R5, should be used if an open-collector or open-drain comparator is used. Equally, a push-pull comparator may be used. An open-drain or open-collector comparator should be used if the output of the comparator is to be tied to another comparator’s output.