For the on-off-on switch, the MSP-3 and MSP-4 operate as follows:
Figure 1 – Operation of MSP-3 and MSP-4 switches.
||Terminal 2 to Terminal 3
||Terminal 2 to open circuit
||Terminal 2 to Terminal 1
Here is an alternative way to connect the SPDT to the MCU which is useful when running out of pins because it only requires one input pin. It does however require more complicated software to interpret the input e.g., hysteresis and debounce could be implemented with thresholds such as:
- > 90% Vcc for > 50 ms means high.
- < 10% Vcc for > 50 ms means low.
- > 45% Vcc and < 55% Vcc for > 50 ms means OFF.
- anywhere in between means no change.
Figure 2 – Connecting SPDT switch to MCU.
Throw vs Stop vs Contact
In normal parlance, a throw describes a physical action that moves a mechanism to a new mechanical stop; and throws are distinct from stops and contacts. As such, a switch is a transducer that converts physical movements (throws) to an electrical signal via a system of mechanical stops and electrical contacts.
From Figure 1, the totals are 2 throws, 3 positions, 2 contacts and 1 pole.
However, matters have been confused by a minority of people erroneously metamorphosing the term “throw” to mean “mechanical stop” or “electrical contact.” As you can see in this example—as is often the case—the number of throws equals the number of contacts, but that is not always the case. For example, if there were a centre contact for the centre stop, there would be three contacts, but still only two throws. Perhaps the fact that throws and contacts are often equal in number has led some people to conflate and confuse the separate entities. And perhaps that the number of stops can sometimes equal the number of contacts has led some people to conflate and confuse them too. For this kind of switch, the number of throws is always one less than the number of stops.
Matters are not helped by the E-Switch 200MSP4T1B1M1QE datasheet showing an incorrect schematic for the MSP-3, 4 and 5 that omits the centre mechanical stop that should be labelled OFF.
I’ve re-annotated the datasheet, Figure 3, and modified the following prose to comply with the common-sense approach of Figure 1, being mindful that a minority of people use “throw” to mean “contact,” and added the correct schematic for MSP-3, 4 and 5. (The original annotated version and prose was an attempt to use the datasheet’s own erroneous nomenclature and schematic.)
Figure 3 – Annotated datasheet.
MSP-1, 2 and 6 are actually single-throw switches despite what the datasheet implies which conflates its description of its SPST with its SPDT switches (which caused some confusion in the comments about the intent and variety of the switch).
The switch required for the above example is the MSP-3 on-off-on. Or to simulate momentary button presses, the MSP-4 (on)-off-(on). Both of these are single-pole double-throw switches (SPDT), not because they have 2 electrical contacts, but because they have 2 physical throws.
In the context of the erroneous datasheet, throws and contacts are erroneously synonymous. In normal parlance, neither throws nor positions are electrical contacts/terminals/pins (though the datasheet has conflated them as well). However, each mechanical stop does have an associated electrical connection/disconnection that should be explained in the datasheet. Unfortunately, in this case, the table in the datasheet has conflated SPST with SPDT and used the term “NONE” to indicate that a position doesn’t exist for the SPST types (MSP-1, MSP-2, and MSP-6). For MSP-3, 4, and 5, notice how POS 2 doesn’t have an associated contact/terminal/pin in the green schematic, Figure 3.
From experience, the only way to be sure is to buzz out the switch with a multimeter (continuity test). So, keep Figure 1 in your mind and a multimeter in your hand, and you won’t go far wrong.
- Throws are physical.
- Stops are mechanical.
- Contacts are electrical.
- Claims such as, “My contact is a throw.” are delusional.
- Conclusions can be lyrical.
Phantom Scroll-By Downvoter
This answer has a attracted a phantom downvote with no comment expressing why this is warranted.
Does the phantom think:
- the alternative circuit is wrong?
- the thresholds are wrong?
- the errant use of the English language and dodgy schematics in the datasheet are acceptable?
Bear in mind that the terms “throw,” “stop,” and “contact” (their meanings and usage) are taught by schoolteachers to millions of school children every year; and used by billions of English speakers every day. Is the global education system (which isn’t perfect) wrong on this particular point? On what grounds and exceptional circumstances is a datasheet allowed to use such terms in an inappropriate manner?
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