COTO TECHNOLOGY | tel: (401) 943.2686 | cotorelay.com

5

rev. 02122015

SAMPLE APPLICATIONS CIRCUITS

GENERAL APPLICATIONS NOTES

AND INFORMATION

1. Conversion to Single Pole Single Throw Normally

Closed Operation

In some cases, it is desirable for the switch to open in the pres-

ence of a magnetic field rather than to close. This functionality

can be achieved with the circuit shown below, which incorporates

a P-channel enhancement-mode MOSFET. R2 can be selected

in the range of 100 Megohms to several thousand Megohms

depending on the characteristics of the MOSFET. When the

RedRock switch is open, the gate is negative biased relative to the

source, allowing battery current to flow to the load. Conversely,

when the switch is closed, the MOSFET turns off. Note that

a small quiescent current drained from the battery through R2

while the switch is closed. Bear this in mind when selecting the

battery type and value of R2.

1. Choosing magnets

The RR100 switch can be operated by any permanent magnet

that generates a magnetic field at the required operating distance

that is greater than the lower end of the switch’s nominal closure

sensitivity range. For example, a RR100-JWTR switch will close

when immersed in a field that exceeds 20 mT, when it is angled

towards the pole of the magnet at the angle shown in Figure 1.

This is the minimum required field; for optimum contact life, a

higher field is desirable, and there is no maximum field that will

damage the switch. Bear this in mind when designing a switch-

magnet system.

2. mT vs A/m, Gauss and Oe

Coto specifies switch closure sensitivity in mT (millitesla). One

mT is equivalent to exactly 10 Gauss or approximately 1.257 A/m

(Amps per meter). One Oe (Oersted) = 1 Gauss in air.

3. Magnetic overdrive

Magnetic overdrive is any excess magnetic field above the mini-

mum level needed to just close the switch. For example, if the

nominal closure field is 20 mT and the switch is immersed in a

field of 40 mT, 100% overdrive is being applied. Overdrive is

desirable, since higher forces are applied to the contacts, and the

contact resistance is reduced. There is no level of overdrive

2. Interfacing RedRock with Microcontrollers

Most low power microcontrollers, have one or more pins des-

ignated as interrupt or “wake-up” pins. Suitably programmed,

the microcontroller recognizes a positive or negative logic level

transition on the designated pin and changes from a sleep mode

where very little power is consumed to full operational mode.

The RR100 switch shown in Figure 7 is normally closed by a

magnet; when it opens, the wake-up pin goes low, waking up

the microcontroller. R2 should be chosen to be as large a value

as possible, but no higher than one-tenth the input impedance

of the wake-up pin. This will minimize the quiescent current

draw through R1 and R2 while the switch is closed. Consult the

microcontroller documentation for further details.

Fig. 6: Conversion to normally closed (N/C) functionality, using

P-channel, enhancement mode MOSFET.

Fig. 7: Typical method of interfacing an RR100 switch to a low-

power microcontroller. When RR100 switch opens, wake-up pin

goes logic low, bringing microcontroller out of sleep mode. R2

should be no greater than one-tenth of the microcontroller’s input

pin impedance.

RR100

switch

R1

LOAD

1 Meg

R2

Bat

PMOS

M1

microcontroller

wake-up pin

RR100

switch

R1

10K

R2

see note

Vcc