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I'm adding STM2 switch monitors to my layout, which has Tortoise switch machines controlled by (a) LCS layout software (love this interface for controlling switches; it's visual and there's no need to memorize switch numbers or scrolling to find the switch), (b) CAB2 remote via ASC2, and (c) DCS remote via AIU.  I use momentary contacts along with relays to switch polarity on the tortoise switch motors. Works flawlessly. All of the relays/ASC's/AIU's/STM's are on a centralized board with only power wires heading out to the tortoise machines.

There's a couple of things I would like advice on:

First I noticed that my 12v DC power supply is really outputting closer to 13.5v.  The tortoise machines have not had issues to date, but do i need to switch power sources to one with perhaps a pot to trim this output back to 10-12v range?  I'm concerned that I may be shortening the life of my switch machines by supplying more than 12v.

Second, the STM2 requires an input to sense the thrown direction of the switch. It needs 2.5-5v dc. So the question is, what's the best way to do this? Will a simple resistor suffice, since the number of switches on the circuit will not change (ie, relatively constant current consumption), or do I need to invest in some kind of power step-down buck to get the output voltage from the switch (which is currently 13.5v or 0v depending on throw of switch) down to the 5v range?  ...or is there another option??

I did try adding a 4.5MΩ resistor for a trial of the device and it gave desired voltage range and the STM responded as expected, showing switch changes issued by remote on the LCS display.  However, is this risky?



switch-monitor-addition



-Scott

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Last edited by ScottV
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I've been looking at buck converters to get the voltage down to 5v for the STM2 inputs for every switch. Found these. I have 24 switches, so these are certainly more economical than most that are going for 5$ a piece.

amazon.com/Regulator-Module-Voltage-Reducer-Adjustable

The question remains, is it necessary? (vs. the alternative of just inserting a resistor to lower the voltage)

@ScottV posted:

I've been looking at buck converters to get the voltage down to 5v for the STM2 inputs for every switch. Found these. I have 24 switches, so these are certainly more economical than most that are going for 5$ a piece.

amazon.com/Regulator-Module-Voltage-Reducer-Adjustable

The question remains, is it necessary? (vs. the alternative of just inserting a resistor to lower the voltage)

Scott, you get 20 converters for $14.99.  That’s $0.75 each.

Last edited by Danr

@Danr  yes! certainly more economical than most i buck converters I have seen (and have purchased in the past that are in the 3-5$ a piece range). I'm sure there's some difference between them to warrant the cost difference ...maybe handling loads better and/or having better heat dissipation?  

In this case, I think I'll go with the 75 cent converters since the load of the STM sensing circuit is practically zero.

For the 5V circuit, you could use a voltage regulator like below.  This will take anything between ~7V and 30V and regulate to 5V and has a maximum current rating of 3A.  I'm not sure what kind of load the STM sensing circuit would present.

https://www.mouser.com/Product...rZlaERTj280krQ%3D%3D

For the 13.5V Tortoise circuit, have you tried measuring the voltage at the Tortoise machine?  My guess is there will be enough of a voltage drop over that distance to put you in the 12V rating of the switch.  Just a hunch, but worth measuring before you spend money on a problem you may not even have.

Last edited by rplst8
@ScottV posted:

@Danr  yes! certainly more economical than most i buck converters I have seen (and have purchased in the past that are in the 3-5$ a piece range). I'm sure there's some difference between them to warrant the cost difference ...maybe handling loads better and/or having better heat dissipation?  

In this case, I think I'll go with the 75 cent converters since the load of the STM sensing circuit is practically zero.

I have used a very similar product on a smoking caboose project.  I used a small smoke unit with one of these to control the resistor and another to control the fan.  Both fit inside the cupola.  They can handle a max load of 3 amps.

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